describe the critical period hypothesis

Critical Period In Brain Development and Childhood Learning

Charlotte Nickerson

Research Assistant at Harvard University

Undergraduate at Harvard University

Charlotte Nickerson is a student at Harvard University obsessed with the intersection of mental health, productivity, and design.

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Editor-in-Chief for Simply Psychology

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Key Takeaways

• Critical period is an ethological term that refers to a fixed and crucial time during the early development of an organism when it can learn things that are essential to survival. These influences impact the development of processes such as hearing and vision, social bonding, and language learning.
• The term is most often experienced in the study of imprinting, where it is thought that young birds could only develop an attachment to the mother during a fixed time soon after hatching.
• Neurologically, critical periods are marked by high levels of plasticity in the brain before neural connections become more solidified and stable. In particular, critical periods tend to end when synapses that inhibit the neurotransmitter GABA mature.
• In contrast to critical periods, sensitive periods, otherwise known as “weak critical periods,” happen when an organism is more sensitive than usual to outside factors influencing behavior, but this influence is not necessarily restricted to the sensitive period.
• Scholars have debated the extent to which older organisms can develop certain skills, such as natively-accented foreign languages, after the critical period.

The critical period is a biologically determined stage of development where an organism is optimally ready to acquire some pattern of behavior that is part of typical development. This period, by definition, will not recur at a later stage.

If an organism does not receive exposure to the appropriate stimulus needed to learn a skill during a critical period, it may be difficult or even impossible for that organism to develop certain functions associated with that skill later in life.

This happens because a range of functional and structural elements prevent passive experiences from eliciting significant changes in the brain (Cisneros-Franco et al., 2020).

The first strong proponent of the theory of critical periods was Charles Stockhard (1921), a biologist who attempted to experiment with the effects of various chemicals on the development of fish embryos, though he gave credit to Dareste for originating the idea 30 years earlier (Scott, 1962).

Stockhard’s experiments showed that applying almost any chemical to fish embryos at a certain stage of development would result in one-eyed fish.

These experiments established that the most rapidly growing tissues in an embryo are the most sensitive to any change in conditions, leading to effects later in development (Scott, 1962).

Meanwhile, psychologist Sigmund Freud attempted to explain the origins of neurosis in human patients as the result of early experiences, implying that infants are particularly sensitive to influences at certain points in their lives.

Lorenz (1935) later emphasized the importance of critical periods in the formation of primary social bonds (otherwise known as imprinting) in birds, remarking that this psychological imprinting was similar to critical periods in the development of the embryo.

Soon thereafter, McGraw (1946) pointed out the existence of critical periods for the optimal learning of motor skills in human infants (Scott, 1962).

Example: Infant-Parent Attachment

The concept of critical or sensitive periods can also be found in the domain of social development, for example, in the formation of the infant-parent attachment relationship (Salkind, 2005).

Attachment describes the strong emotional ties between the infant and caregiver, a reciprocal relationship developing over the first year of the child’s life and particularly during the second six months of the first year.

During this attachment period , the infant’s social behavior becomes increasingly focused on the principal caregivers (Salkind, 2005).

The 20th-century English psychiatrist John Bowlby formulated and presented a comprehensive theory of attachment influenced by evolutionary theory.

Bowlby argued that the infant-parent attachment relationship develops because it is important to the survival of the infant and that the period from six to twenty-four months of age is a critical period of attachment.

This coincides with an infant’s increasing tendency to approach familiar caregivers and to be wary of unfamiliar adults. After this critical period, it is still possible for a first attachment relationship to develop, albeit with greater difficulty (Salkind, 2005).

This has brought into question, in a similar vein to language development, whether there is actually a critical development period for infant-caregiver attachment.

Sources debating this issue typically include cases of infants who did not experience consistent caregiving due to being raised in institutions prior to adoption (Salkind, 2005).

Early research into the critical period of attachment, published in the 1940s, reports consistently that children raised in orphanages subsequently showed unusual and maladaptive patterns of social behavior, difficulty in forming close relationships, and being indiscriminately friendly toward unfamiliar adults (Salkind, 2005).

Later, research from the 1990s indicated that adoptees were actually still able to form attachment relationships after the first year of life and also made developmental progress following adoption.

Nonetheless, these children had an overall increased risk of insecure or maladaptive attachment relationships with their adoptive parents. This evidence supports the notion of a sensitive period, but not a critical period, in the development of first attachment relationships (Salkind, 2005).

Mechanisms for Critical Periods

Both genetics and sensory experiences from outside the body shape the brain as it develops (Knudsen, 2004). However, the developmental stage that an organism is in significantly impacts how much the brain can change based on these experiences.

In scientific terms, the brain’s plasticity changes over the course of a lifespan. The brain is very plastic in the early stages of life before many key connections take root, but less so later.

This is why researchers have shown that early experience is crucial for the development of, say, language and musical abilities, and these skills are more challenging to take up in adulthood (Skoe and Kraus, 2013; White et al., 2013; Hartshorne et al., 2018).

As brains mature, the connections in them become more fixed. The brain’s transitions from a more plastic to a more fixed state advantageously allow it to retain new and complex processes, such as perceptual, motor, and cognitive functions (Piaget, 1962).

Children’s gestures, for example, pride and predict how they will acquire oral language skills (Colonnesi et al., 2010), which in turn are important for developing executive functions (Marcovitch and Zelazo, 2009).

However, this formation of stable connections in the brain can limit how the brain’s neural circuitry can be revised in the future. For example, if a young organism has abnormal sensory experiences during the critical period – such as auditory or visual deprivation – the brain may not wire itself in a way that processes future sensory inputs properly (Gallagher et al., 2020).

One illustration of this is the timing of cochlear implants – a prosthesis that restores hearing in some deaf people. Children who receive cochlear implants before two years of age are more likely to benefit from them than those who are implanted later in life (Kral and Eggermont, 2007; Gallagher et al., 2020).

Similarly, the visual deprivation caused by cataracts in infants can cause similar consequences. When cataracts are removed during early infancy, individuals can develop relatively normal vision; however, when the cataracts are not removed until adulthood, this results in substantially poorer vision (Martins Rosa et al., 2013).

After the critical period closes, abnormal sensory experiences have a less drastic effect on the brain and lead to – barring direct damage to the central nervous system – reversible changes (Gallagher et al., 2020). Much of what scientists know about critical periods derives from animal studies , as these allow researchers greater control over the variables that they are testing.

This research has found that different sensory systems, such as vision, auditory processing, and spatial hearing, have different critical periods (Gallagher et al., 2020).

The brain regulates when critical periods open and close by regulating how much the brain’s synapses take up neurotransmitters , which are chemical substances that affect the transmission of electrical signals between neurons.

In particular, over time, synapses decrease their uptake of gamma-aminobutyric acid, better known as GABA. At the beginning of the critical period, outside sources become more effective at influencing changes and growth in the brain.

Meanwhile, as the inhibitory circuits of the brain mature, the mature brain becomes less sensitive to sensory experiences (Gallagher et al., 2020).

Critical Periods vs Sensitive Periods

Critical periods are similar to sensitive periods, and scholars have, at times, used them interchangeably. However, they describe distinct but overlapping developmental processes.

A sensitive period is a developmental stage where sensory experiences have a greater impact on behavioral and brain development than usual; however, this influence is not exclusive to this time period (Knudsen, 2004; Gallagher, 2020). These sensitive periods are important for skills such as learning a language or instrument.

In contrast, A critical period is a special type of sensitive period – a window where sensory experience is necessary to shape the neural circuits involved in basic sensory processing, and when this window opens and closes is well-defined (Gallagher, 2020).

Researchers also refer to sensitive periods as weak critical periods. Some examples of strong critical periods include the development of vision and hearing, while weak critical periods include phenome tuning – how children learn how to organize sounds in a language, grammar processing, vocabulary acquisition, musical training, and sports training (Gallagher et al., 2020).

Critical Period Hypothesis

One of the most notable applications of the concept of a critical period is in linguistics. Scholars usually trace the origins of the debate around age in language acquisition to Penfield and Robert’s (2014) book Speech and Brain Mechanisms.

In the 1950s and 1960s, Penfield was a staunch advocate of early immersion education (Kroll and De Groot, 2009). Nonetheless, it was Lenneberg, in his book Biological Foundations of Language, who coined the term critical period (1967) in describing the language period.

Lennenberg (1967) described a critical period as a period of automatic acquisition from mere exposure” that “seems to disappear after this age.” Scovel (1969) later summarized and narrowed Penfield’s and Lenneberg’s view on the critical period hypothesis into three main claims:

• Adult native speakers can identify non-natives by their accents immediately and accurately.
• The loss of brain plasticity at about the age of puberty accounts for the emergence of foreign accents./li>
• The critical period hypothesis only holds for speech (whether or not someone has a native accent) and does not affect other areas of linguistic competence.

Linguists have since attempted to find evidence for whether or not scientific evidence actually supports the critical period hypothesis, if there is a critical period for acquiring accentless speech, for “morphosyntactic” competence, and if these are true, how age-related differences can be explained on the neurological level (Scovel, 2000).

The critical period hypothesis applies to both first and second-language learning. Until recently, research around the critical period’s role in first language acquisition revolved around findings about so-called “feral” children who had failed to acquire language at an older age after having been deprived of normal input during the critical period.

However, these case studies did not account for the extent to which social deprivation, and possibly food deprivation or sensory deprivation, may have confounded with language input deprivation (Kroll and De Groot, 2009).

More recently, researchers have focused more systematically on deaf children born to hearing parents who are therefore deprived of language input until at least elementary school.

These studies have found the effects of lack of language input without extreme social deprivation: the older the age of exposure to sign language is, the worse its ultimate attainment (Emmorey, Bellugi, Friederici, and Horn, 1995; Kroll and De Groot, 2009).

However, Kroll and De Groot argue that the critical period hypothesis does not apply to the rate of acquisition of language. Adults and adolescents can learn languages at the same rate or even faster than children in their initial stage of acquisition (Slavoff and Johnson, 1995).

However, adults tend to have a more limited ultimate attainment of language ability (Kroll and De Groot, 2009).

There has been a long lineage of empirical findings around the age of acquisition. The most fundamental of this research comes from a series of studies since the late 1970s documenting a negative correlation between age of acquisition and ultimate language mastery (Kroll and De Grott, 2009).

Nonetheless, different periods correspond to sensitivity to different aspects of language. For example, shortly after birth, infants can perceive and discriminate speech sounds from any language, including ones they have not been exposed to (Eimas et al., 1971; Gallagher et al., 2020).

Around six months of age, exposure to the primary language in the infant’s environment guides phonetic representations of language and, subsequently, the neural representations of speech sounds of the native language while weakening those of unused sounds (McClelland et al., 1999; Gallagher et al., 2020).

Vocabulary learning experiences rapid growth at about 18 months of age (Kuhl, 2010).

Critical Evaluation

More than any other area of applied linguistics, the critical period hypothesis has impacted how teachers teach languages. Consequently, researchers have critiqued how important the critical period is to language learning.

For example, several studies in early language acquisition research showed that children were not necessarily superior to older learners in acquiring a second language, even in the area of pronunciation (Olson and Samuels, 1973; Snow and Hoefnagel-Hohle, 1978; Scovel, 2000).

In fact, the majority of researchers at the time appeared to be skeptical about the existence of a critical period, with some explicitly denying its existence.

Counter to one of the primary tenets of Scovel’s (1969) critical period hypothesis, there have been several cases of people who have acquired a second language in adulthood speaking with native accents.

For example, Moyer’s study of highly proficient English-speaking learners of German suggested that at least one of the participants was judged to have native-like pronunciation in his second language (1999), and several participants in Bongaerts (1999) study of highly proficient Dutch speakers of French spoke with accents judged to be native (Scovel, 2000).

Bongaerts, T. (1999). Ultimate attainment in L2 pronunciation: The case of very advanced late L2 learners. Second language acquisition and the critical period hypothesis, 133-159.

Cisneros-Franco, J. M., Voss, P., Thomas, M. E., & de Villers-Sidani, E. (2020). Critical periods of brain development. In Handbook of Clinical Neurolog y (Vol. 173, pp. 75-88). Elsevier.

Colonnesi, C., Stams, G. J. J., Koster, I., & Noom, M. J. (2010). The relation between pointing and language development: A meta-analysis. Developmental Review, 30 (4), 352-366.

Eimas, P. D., Siqueland, E. R., Jusczyk, P., & Vigorito, J. (1971). Speech perception in infants. Science, 171 (3968), 303-306.

Emmorey, K., Bellugi, U., Friederici, A., & Horn, P. (1995). Effects of age of acquisition on grammatical sensitivity: Evidence from on-line and off-line tasks. Applied Psycholinguistics, 16 (1), 1-23.

Knudsen, E. I. (2004). Sensitive periods in the development of the brain and behavior. Journal of cognitive neuroscience, 16 (8), 1412-1425.

Hartshorne, J. K., Tenenbaum, J. B., & Pinker, S. (2018). A critical period for second language acquisition: Evidence from 2/3 million English speakers. Cognition, 177 , 263-277.

Kral, A., & Eggermont, J. J. (2007). What’s to lose and what’s to learn: development under auditory deprivation, cochlear implants and limits of cortical plasticity. Brain Research Reviews, 56(1), 259-269.

Kroll, J. F., & De Groot, A. M. (Eds.). (2009). Handbook of bilingualism: Psycholinguistic approaches . Oxford University Press.

Kuhl, P. K. (2010). Brain mechanisms in early language acquisition. Neuron, 67 (5), 713-727.

Lenneberg, E. H. (1967). The biological foundations of language. Hospital Practice, 2( 12), 59-67.

Lorenz, K. (1935). Der kumpan in der umwelt des vogels. Journal für Ornithologie, 83 (2), 137-213.

Marcovitch, S., & Zelazo, P. D. (2009). A hierarchical competing systems model of the emergence and early development of executive function. Developmental science, 12 (1), 1-18.

McClelland, J. L., Thomas, A. G., McCandliss, B. D., & Fiez, J. A. (1999). Understanding failures of learning: Hebbian learning, competition for representational space, and some preliminary experimental data. Progress in brain research, 121, 75-80.

McGraw, M. B. (1946). Maturation of behavior. In Manual of child psychology. (pp. 332-369). John Wiley & Sons Inc.

Moyer, A. (1999). Ultimate attainment in L2 phonology: The critical factors of age, motivation, and instruction. Studies in second language acquisition, 21 (1), 81-108.

Gallagher, A., Bulteau, C., Cohen, D., & Michaud, J. L. (2019). Neurocognitive Development: Normative Development. Elsevier.

Olson, L. L., & Jay Samuels, S. (1973). The relationship between age and accuracy of foreign language pronunciation. The Journal of Educational Research, 66 (6), 263-268.

Penfield, W., & Roberts, L. (2014). Speech and brain mechanisms. Princeton University Press.

Piaget, J. (1962). The stages of the intellectual development of the child. Bulletin of the Menninger Clinic, 26 (3), 120.

Rosa, A. M., Silva, M. F., Ferreira, S., Murta, J., & Castelo-Branco, M. (2013). Plasticity in the human visual cortex: an ophthalmology-based perspective. BioMed research international, 2013.

Salkind, N. J. (Ed.). (2005). Encyclopedia of human development . Sage Publications.

Scott, J. P. (1962). Critical periods in behavioral development. Science, 138 (3544), 949-958.

Scovel, T. (1969). Foreign accents, language acquisition, and cerebral dominance 1. Language learning, 19 (3‐4), 245-253.

Scovel, T. (2000). A critical review of the critical period research. Annual review of applied linguistics, 20 , 213-223.

Skoe, E., & Kraus, N. (2013). Musical training heightens auditory brainstem function during sensitive periods in development. Frontiers in Psychology, 4, 622.

Slavoff, G. R., & Johnson, J. S. (1995). The effects of age on the rate of learning a second language. Studies in Second Language Acquisition, 17 (1), 1-16.

Snow, C. E., & Hoefnagel-Höhle, M. (1978). The critical period for language acquisition: Evidence from second language learning. Child development, 1114-1128.

Stockard, C. R. (1921). Developmental rate and structural expression: an experimental study of twins,‘double monsters’ and single deformities, and the interaction among embryonic organs during their origin and development. American Journal of Anatomy, 28 (2), 115-277.

White, E. J., Hutka, S. A., Williams, L. J., & Moreno, S. (2013). Learning, neural plasticity and sensitive periods: implications for language acquisition, music training and transfer across the lifespan. Frontiers in systems neuroscience, 7, 90.

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Critical Period in Brain Development: Definition, Importance

Toketemu has been multimedia storyteller for the last four years. Her expertise focuses primarily on mental wellness and women’s health topics. 

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• When Does the Critical Period Begin and End?
• The Critical Period Hypothesis—What It States
• What Happens to the Brain in the Critical Period?
• What Kind of Events Impact the Brain During the Critical Period?
• How Do Adverse Events Impact the Brain?
• What's the Difference Between a Critical Period and a Sensitive Period?
• What Happens to the Brain When the Critical Period Ends?

The critical period in brain development is an immensely significant and specific time frame during which the brain is especially receptive to environmental stimuli and undergoes a series of rapid changes. 

These changes have lifelong effects as essential neural connections and pathways are established, playing a vital role in cognitive, emotional, and social development. 

This article will explore the timeline, impacting events, and subsequent consequences of the critical period on brain development. It also explores the distinction between critical periods and sensitive periods and what happens to the brain once the critical period ends.

When Does the Critical Period Begin and End? 

The starting point of the critical period is at conception. The brain starts to form and develop from the moment you are conceived. During pregnancy, a baby's brain is already beginning to shape itself for the world outside. The brain is gearing up and getting ready to absorb a massive amount of information.

The Early Years of a Child's Life

Once the baby is born, the brain kicks into high gear. The early years of a child's life, from birth to around the age of five, are generally considered the core of the critical period. The brain is incredibly absorbent during these years, taking in information rapidly. Everything from language to motor skills to social cues is being learned and processed extensively.

Different aspects of learning and development have different critical periods. For instance, the critical period for language acquisition extends into early adolescence. This means that while the brain is still very good at learning languages during early childhood, it continues to be relatively efficient at it until the teenage years.

The brain is incredibly absorbent during these years, taking in information rapidly. Everything from language to motor skills to social cues is being learned and processed extensively.

Vision Develops During This Period

On the other hand, for certain sensory abilities like vision, the critical period might end much earlier. This means that the brain is most receptive to developing visual abilities in the first few years of life, and after that, it becomes significantly harder to change or improve these abilities.

The Critical Period Hypothesis—What It States 

The brain has a certain time window when it's exceptionally good at learning new things, especially languages. This window of time is what is referred to as the "critical period."

Younger People Learn Languages Faster Than Older People

Eric Lenneberg, a neuropsychologist, introduced the Critical Period Hypothesis. He was very interested in how people learn languages . Through his observations and research, Lenneberg noticed that younger people were much more adept at learning languages than older people. This observation led him to the idea that there is a specific period during which the brain is highly efficient and capable of absorbing languages.

As You Age, It Becomes More Difficult to Absorb New Information

If the critical period is a wide open window in the early years of life, allowing the brain to take in an abundance of information quickly and efficiently, as time progresses, this window begins to close gradually. As it closes, the brain becomes less capable of easily absorbing languages.

This doesn't mean that learning becomes impossible as you age; it merely indicates that the ease and efficiency with which the brain learns start to decline.

What Happens to the Brain in the Critical Period? 

During the critical period, the brain experiences explosive growth. Let's take a look at some of the changes that happen in the brain during the critical period.

Neurons Form Connections

In the early stages, neurons in the brain start to form connections. These connections are called synapses.

Synapses are bridges that help different parts of the brain communicate with each other. In the critical period, the brain is building these bridges at an incredible pace.

Neuroplasticity Strengthens Brain Connections

As a baby interacts with the world, certain connections strengthen while others weaken. For instance, if a baby hears a lot of music, the parts of the brain associated with sounds and music will become stronger. This process of strengthening certain connections is known as brain plasticity because the brain molds itself like plastic.

Attachment to Primary Caregivers

An essential aspect of the critical period is the development of attachment to caregivers. During the early months and years, babies and toddlers form strong bonds with the people caring for them .

These attachments are critical for emotional development. When a caregiver responds to a baby's needs with warmth and care, the baby learns to form secure attachments . This lays the foundation for healthy relationships later in life.

What Happens When Children Are Not Given Attention?

What if a child is not given the attention and care they need during the critical period? This is a significant concern. Without proper attention and stimulation, the brain doesn't develop as effectively. The bridges or connections that should be built might not form properly. This can lead to various issues, including difficulty forming relationships, emotional problems, and learning difficulties.

When a child is given proper attention, stimulation, and care during the critical period, their brain thrives. The connections form rapidly and robustly. This sets the stage for better learning, emotional regulation, and relationship-building throughout life.

What Kind of Events Impact the Brain During the Critical Period? 

When a child is exposed to a rich, stimulating environment where they can play, explore, and learn, it tremendously impacts the brain. Engaging in interactive learning, being read to, and having supportive relationships with caregivers can significantly contribute to a well-developed brain.

Events such as abuse, neglect, head trauma , or extreme stress—collectively known as adverse childhood experiences (ACEs)—can be detrimental to brain development. These adverse events can impede the formation of neural connections and lead to behavioral, emotional, and cognitive difficulties later in life. "Unfortunately, disruptions to normal brain development due to environmental influences such as poverty, neglect, or exposure to toxins can cause lasting damage. This is why it is so important for children to receive adequate nutrition, stimulation, and parental care during these first few years of life; without it, they may suffer developmental delays and other issues that could potentially be avoided with proper attention,"  Harold Hong, MD , a board-certified psychiatrist says.

How Do Adverse Events Impact the Brain? 

When a child is neglected or abused, stress can impact how their brain develops. The parts of the brain involved in emotions and handling stress might not develop properly. This can make it hard for the person to manage their emotions later in life.

The hippocampus, involved in learning and memory, and the amygdala, which plays a role in emotion processing, are especially vulnerable. 

Similarly, if a child does not have enough food to eat or a safe place to live, the chronic stress of these conditions can impact brain development. The brain might focus on survival instead of other important areas of development, like learning and building relationships.

Even accidents that cause head injuries can impact the brain during the critical period. If a child experiences head trauma, it can affect the brain's development depending on the injury's severity and location.

What's the Difference Between a Critical Period and a Sensitive Period?

It is imperative to distinguish between critical periods and sensitive periods.

• Critical periods are specific windows of time during development when the brain is exceptionally receptive to certain types of learning and experiences. Once this period is over, acquiring those skills or attributes becomes significantly more challenging.
• Sensitive periods are phases in which the brain is more responsive to certain experiences. It's easier to learn or be influenced by specific experiences during sensitive periods, but unlike critical periods, missing this timeframe doesn't make it impossible to acquire those skills or traits later.

For example, while there is a critical period for acquiring native-like pronunciation and grammar, there is also a sensitive period for language learning. Children are more adept at learning new languages when they are young, but even if someone misses this window, they can still learn languages later in life.

One way to visualize the difference is to think of critical periods as a tightly defined window of time with a clear beginning and end, during which certain development must occur. In contrast, sensitive periods are more like a gradual slope, where learning at the beginning is optimal, but the ability doesn't disappear entirely over time.

What Happens to the Brain When the Critical Period Ends? 

It's essential to recognize that the end of the critical period does not mean the end of learning or brain development. Instead, it signifies a shift in how the brain learns and adapts. 

During the critical period, the brain is highly plastic, meaning it can change and form new connections rapidly. As this period ends, the brain doesn't lose this plasticity entirely, but the rate at which it can make new connections slows down. 

According to Hong, although some of these connections can still be altered by experiences later in life, such as learning a new language or practicing a skill, it is much harder to make significant changes after the critical period has ended. This highlights just how important it is for parents to provide proper care and nurture during those first few years.

The Brain Becomes More Specialized Via Adult Plasticity

The brain also becomes more specialized in the skills and information it has acquired as this period ends. During the critical period, the brain forms numerous connections, and as it ends, it starts to use these connections more efficiently for specialized tasks.

Even though the critical period ends, the brain still possesses a degree of plasticity and continues to learn throughout life. This is called adult plasticity.

Adult plasticity is not as robust during the critical period, but it allows for the continuous adaptation and learning necessary for us to navigate the ever-changing demands of life.

The conventional view is that critical periods close relatively tightly. However, research has started to challenge this rigid view. It's more accurate to say that the doors of critical periods close but do not necessarily lock.

While the brain's plasticity decreases after these periods, learning and adaptation can still take place, albeit with more effort and over a longer time. This phenomenon of 'metaplasticity'—the brain's ability to change its plasticity levels—remains an exciting area of ongoing research,  Dr. Ryan Sultan , a neuroscientist, child psychiatrist, and professor of psychiatry at Columbia University, says. 

What This Means For You

The critical period represents an invaluable window during which the foundations for cognitive, emotional, and social abilities are established. The environment, experiences, and attachments formed during this period have far-reaching consequences on a person's life.  Understanding the nuances of the critical period is essential for educators, parents, and policymakers to create nurturing environments that support healthy brain development. Providing support and early interventions for children exposed to adverse experiences is vital for ensuring their potential is not hindered by the circumstances of their early life.

Siahaan F. The critical period hypothesis of sla eric lenneberg’s . Journal of Applied Linguistics . 2022;2(1):40-45.

Nelson CA, Gabard-Durnam LJ. Early adversity and critical periods: neurodevelopmental consequences of violating the expectable environment. Trends in Neurosciences. 2020;43(3):133-143.

Colombo J, Gustafson KM, Carlson SE. Critical and sensitive periods in development and nutrition. Ann Nutr Metab . 2019;75(Suppl. 1):34-42.

Patton MH, Blundon JA, Zakharenko SS. Rejuvenation of plasticity in the brain: opening the critical period. Current Opinion in Neurobiology . 2019;54:83-89.

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Critical Period Hypothesis (CPH)

Tom Scovel writes, “The CPH [critical period hypothesis] is conceivably the most contentious issue in SLA because there is disagreement over its exact age span; people disagree strenuously over which facets of language are affected; there are competing explanations for its existence; and, to top it off, many people don’t believe it exists at all” (113). Proposed by Wilder Penfield and Lamar Roberts in 1959, the Critical Period Hypothesis (CPH) argues that there is a specific period of time in which people can learn a language without traces of the L1 (a so-called “foreign” accent or even L1 syntactical features) manifesting in L2 production (Scovel 48). If a learner’s goal is to sound “native,” there may be age-related limitations or “maturational constraints” as Kenneth Hyltenstam and Niclas Abrahamsson call them, on how “native” they can sound. Reducing the impression left by the L1 is certainly possible after puberty, but eliminating that impression entirely may not be possible.

Kenji Hakuta et al. explains that the relationship between age and L1 interference in L2 production is really not up for debate:

“The diminished average achievement of older learners is supported by personal anecdote and documented by empirical evidence….What is controversial, though, is whether this pattern meets the conditions for concluding that a critical period constrains learning in a way predicted by the theory” (31).

Some learners manage to overcome the “constraints” that Scovel believes are “probably accounted for by neurological factors that are genetically specified in our species” (114), but these learners are exceptional rather than the rule. It may be biology; it may be due to something else. The debate will continue, but evidence seems to indicate that the older learners become, the more difficult complete acquisition can be.

“David Birdsong, Looking Inside and Beyond the Critical Period Hypothesis.”  YouTube,  uploaded by IWL Channel, 09 May 2016, https://www.youtube.com/watch?v=9Bo0C4dj7Mw.

Application

Instructors should consider taking the CPH into account when assessing their students’ oral communication in the target language. When “maturational constraints” are a potential concern, it seems more fair for instructors to weight comprehension more heavily than nativeness. A thorough understanding of the CPH can also help instructors to counteract adult learners’ “self-handicapping” by helping the learners understand that, in spite of constraints due to aging, they are still capable of acquiring many–if not most–aspects of the target language.

Bibliography

Hakuta, Kenji, et al. “Critical Evidence: A Test of the Critical-Period Hypothesis for Second-Language Acquisition.”  Psychological Science , vol. 14, no. 1, 2003, pp. 31–38.  JSTOR , www.jstor.org/stable/40063748.

Hyltenstam, Kenneth, and Niclas Abrahamsson. “Comments on Stefka H. Marinova-Todd, D. Bradford Marshall, and Catherine E. Snow’s ‘Three Misconceptions about Age and L2 Learning’: Age and L2 Learning: The Hazards of Matching Practical ‘Implications’ with Theoretical ‘Facts.’”  TESOL Quarterly , vol. 35, no. 1, 2001, pp. 151–170.  JSTOR , www.jstor.org/stable/3587863.

Nemer, Randa. “Critical Period Hypothesis.”  Prezi,  04 Dec. 2013, https://prezi.com/zzuch40ibrlq/critical-period-hypothesis-sla/#.

Scovel, Tom.  Learning New Languages . Heinle & Heinle, 2001.

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The Critical Period Hypothesis: A coat of many colours

• David Singleton

Research on age-related effects in L2 development often invokes the idea of a critical period – the postulation of which is customarily referred to as the Critical Period Hypothesis. This paper argues that to speak in terms of the Critical Period Hypothesis is misleading, since there is a vast amount of variation in the way in which the critical period for language acquisition is understood – affecting all the parameters deemed to be theoretically significant and indeed also relating to the ways in which the purported critical period is interpreted in terms of its implications for L2 instruction. The paper concludes that the very fact that there are such diverse and competing versions of the Critical Period Hypothesis of itself undermines its plausibility.

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Original research article, critical period in second language acquisition: the age-attainment geometry.

• Teachers College, Columbia University, New York City, NY, United States

One of the most fascinating, consequential, and far-reaching debates that have occurred in second language acquisition research concerns the Critical Period Hypothesis [ 1 ]. Although the hypothesis is generally accepted for first language acquisition, it has been hotly debated on theoretical, methodological, and practical grounds for second language acquisition, fueling studies reporting contradictory findings and setting off competing explanations. The central questions are: Are the observed age effects in ultimate attainment confined to a bounded period, and if they are, are they biologically determined or maturationally constrained? In this article, we take a sui generis , interdisciplinary approach that leverages our understanding of second language acquisition and of physics laws of energy conservation and angular momentum conservation, mathematically deriving the age-attainment geometry. The theoretical lens, termed Energy Conservation Theory for Second Language Acquisition, provides a macroscopic perspective on the second language learning trajectory across the human lifespan.

Introduction

The Critical Period Hypothesis (CPH), as proposed by [ 1 ], that nativelike proficiency is only attainable within a finite period, extending from early infancy to puberty, has generally been accepted in language development research, but more so for first language acquisition (L1A) than for second language acquisition (L2A).

In the context of L2A, there are two parallel facts that appear to compound the difficulty of establishing the validity of CPH. One is that there is a stark difference in the level of ultimate attainment between child and adult learners. “Children eventually reach a more native-like level of proficiency than learners who start learning a second language as adults” ([ 2 ], p . 360). But this fact exists alongside another fact, namely, that there are vast differences in ultimate attainment among older learners. [ 3 ] observed:

Although few adults, if any, are completely successful, and many fail miserably, there are many who achieve very high levels of proficiency, given enough time, input, and effort, and given the right attitude, motivation, and learning environment. ( p . 13).

The dual facets of inter-learner differential success are at the nexus of second language acquisition research. As [ 4 ] once noted:

One of the enduring and fascinating problems confronting researchers of second language acquisition is whether adults can ever acquire native-like competence in a second language, or whether this is an accomplishment reserved for children who start learning at a relatively early age. As a secondary issue, there is the question of whether those rare cases of native-like success reported amongst adult learners are indeed what they seem, and if they are, how it is that such people can be successful when the vast majority are palpably not. ( p . 219).

The primary question Kellerman raised here is, in essence, a critical period (CP) question, concerning differential attainment between child and adult learners, and his secondary question relates to differential attainment among adult learners.

As of this writing, neither question has been settled. Instead, the two phenomena are often seen conflated in debates, including taking evidence for one as counter-evidence for the other (see, e.g., [ 5 ]). By and large, it would seem that the debate has come down to a matter of interpretation; the same facts are interpreted differently as evidence for or against CPH (see, e.g., [ 5 – 7 ]). This state of affairs, tinted with ideological differences over the role of nature and/or nature in language development, continues to put a tangible understanding of either phenomenon out of reach, let alone a coherent understanding of both phenomena. In order to break out of the rut of ‘he said, she said,” we need to engage in systems thinking.

Our research sought to juxtapose child and adult learners, as some researchers have, conceptually, attempted (see, e.g., [ 8 – 11 ]). Specifically, we built on and extended an interdisciplinary model of L2A, Energy Conservation Theory for L2A (ECT-L2A) [ 12 , 13 ], originally developed to account for differential attainment among adult learners, to child learners. In so doing, we sought to gain a coherent understanding of the dual facets of inter-learner differential success in L2A, in addition to mathematically obtaining the geometry of the age-attainment function, a core concern of the CPH/L2A debate.

In what follows, we first provide a quick overview of the CPH research in L2A. We then introduce ECT-L2A. Next, we extend ECT-L2A to the age issue, mathematically deriving the age-attainment function. After that, we discuss the resultant geometry and the fundamental nature of CPH/L2A, and, more broadly, L2 attainment across the human lifespan. We conclude by suggesting a number of avenues for furthering the research on CPH within the framework of ECT-L2A.

However, before we proceed, it is necessary to note two “boundary conditions” we have set for our work. First, the linguistic domain in which we theorize inter-learner differential attainment concerns only the grammatical/computational aspects of language, or what [ 14 , 15 ] calls basic language cognition, which concerns aspects of language where native speakers show little variance. As [ 2 ] has aptly pointed out, much of the confusion in the CPH-L2A debate is attributable to a lack of agreement on the scope of linguistic areas affected by CP. Second, we are only concerned with naturalistic acquisition (i.e., acquisition happens in an input-rich or immersion environment), not instructed learning (i.e., an input-poor environment). These two assumptions are often absent in CPH/L2A research, leading to the different circumstances under which researchers interpret the CP notion and empirical results (for discussion, see [ 7 ]).

The critical period hypothesis in L2A

To date, two questions have dominated the research and debate on CPH/L2A: What counts as evidence of a critical period? What accounts for the age-attainment difference between younger learners and older learners? More than 4 decades of research on CPH/L2A- from [ 16 ] to [ 17 ] to [ 18 ]—have, in the main, found an inverse correlation between the age of acquisition (AoA) and the level of grammatical attainment (see also [ 19 ], for a meta-analysis); “the age of acquisition is strongly negatively correlated with ultimate second language proficiency for grammar as well as for pronunciation” ([ 20 ], p . 88).

However, views are almost orthogonal over whether the observed inverse correlation can count as evidence of CPH or the observed difference is attributable to brain maturation (see, e.g., [ 5 , 7 , 21 – 35 ]).

For some researchers, true evidence or falsification of CPH for L2A must be tied to whether or not late learners can attain a native-like level of proficiency (e.g., [ 36 ]). Others contend that the nativelikeness threshold, in spite of it being “the most central aspect of the CPH” ([ 2 ], p . 362), is problematic, arguing that monolingual-like native attainment is simply impossible for L2 learners [ 37 , 38 ]. Echoing this view, [ 39 ] offered:

[Sequential] bilinguals are not “two monolinguals in one” in any social, psycholinguistic, or cognitive neurofunctional sense. From this perspective, it is of questionable methodological value to quantify bilinguals’ linguistic attainment as a proportion of monolinguals’ attainment, with those bilinguals reaching 100% levels of attainment considered nativelike. ( p . 121).

In the meantime, empirical research into adult learners have consistently produced evidence of selective nativelike attainment, that is, nativelikeness is attained vis-à-vis some aspects of the target language but not others. These studies employed a variety of methodologies, including cross-sectional studies and longitudinal case studies (see, e.g., [ 40 – 56 ]). Some researchers (e.g., [ 55 , 57 ]) take the selective nativelikeness as falsifying evidence of CPH/L2A; other researchers disagree (see, e.g., [ 36 ]).

Leaving aside the vexed issue of nativelikeness, 1 Birdsong [ 58 ], among others, postulated that CPH/L2A must ultimately pass geometric tests: if studies comparing younger learners and older learners yield the geometry of a “stretched Z” for the age-attainment function, that would prove the validity of CPH/L2A, or falsify it, if otherwise. The stretched Z or inverted S [ 20 ] references a bounded period in which the organism exhibits heightened neural plasticity and sensitivity to linguistic stimuli from the environment. This period has certain temporal and geometric features. Temporally, it extends from early infancy to puberty, coinciding with the time during which the brain undergoes maturation [ 1 , 36 , 59 – 62 ]. Geometrically, this period should exhibit two points of inflection or discontinuities, viz, “an abrupt onset or increase of sensitivity, a plateau of peak sensitivity, followed by a gradual offset or decline, with subsequent flattening of the degree of sensitivity” ([ 58 ], p . 111).

By the temporal and geometric hallmarks, few studies seem to have confirmed CPH/L2A, not even those that have allegedly found stark evidence. A case in point is the [ 17 ] study, which reported what appears to be clear-cut evidence of CPH/L2A: r = −.87, p <.01 for the early age of arrival (AoA) group and r = −.16, p >.05 for the late AoA group. As Johnson and Newport described it, “test performance was linearly related to [AoA] up to puberty; after puberty, performance was low but highly variable and unrelated to [AoA],” which supports “the conclusion that a critical period for language acquisition extends its effects to second language acquisition” ( p . 60). However, this claim has been contested.

Focusing on the geometry of the results, [ 58 ] pointed out that the random distribution of test scores within the late AoA group “does not license the conclusion that “through adulthood the function is low and flat” or the corresponding interpretation that “the shape of the function thus supports the claim that the effects of age of acquisition are effects of the maturational state of the learner” ([ 17 ], p . 79)” ( p . 117). Birdsong argued that if CPH holds for L2A, the performance scores of the late AoA group should be distributed horizontally in addition to showing marginal correlation with age. Accordingly, the random distribution of scores could only be taken as indicative of “a lack of systematic relationship between the performance and the AoA and not of a “levelling off of ultimate performance among those exposed to the language after puberty” ([ 17 ], p . 79)” ([ 58 ], p . 118).

Interpreting the same study, other researchers such as [ 20 ] did not set their sights as much on the random distribution of the performance scores among the late learners as on the discontinuity between the early AoA and late AoA groups, arguing that the qualitative difference is sufficient evidence of CPH/L2A.

If geometric satisfaction is one flash point in CPH/L2A research, explaining random distribution of performance scores or, essentially, differential attainment among late learners counts as another. Analyses of late learners’ ultimate attainment (e.g., [ 10 , 22 , 26 , 43 , 63 – 67 ]) have yielded a host of cognitive, socio-psychological, or experiential factors that can be associated with inter-learner differential attainment among late learners. The question, then, is whether or not these non-age factors confound, or even interact with, the age or maturational effect (see discussion in [ 2 , 68 – 72 ]. As Newport [ 7 ] aptly asked, “why cannot other variables interact with age effects?” ( p . 929).

These are undoubtedly complex questions for which sophisticated solutions are needed—beyond the methodological repairs many have thought are solely needed in advancing CPH/L2A research (see, e.g., [ 19 , 67 ]). In the remainder of this article, we take a different tack to the age issue, adopting a theoretical, hybrid approach, ECT-L2A [ 12 , 13 ], to mathematically derive the age-attainment function.

Energy-Conservation Theory for L2A

ECT-L2A is a theoretical model originally developed to account for the divergent states of ultimate attainment in adult L2A [ 12 , 13 ]. Drawing on the physics laws of energy conservation and angular momentum conservation, it theorizes the dynamic transformation and conservation of internal energies (i.e., from the learner) and external energies (i.e., from the environment) in rendering the learner’s ultimate attainment. This model, thus, takes into account nature and nurture factors, and specifically, uses five parameters - the linguistic environment or input, learner motivation, learner aptitude, distance between the L1 and the target language (TL) and the developing learner—and their interaction to account for levels of L2 ultimate attainment.

ECT-L2A draws a number of parallels between mechanical energies and human learning energies: kinetic energy for motivation and aptitude energy, potential energy for environmental energy, 2 and centrifugal energy for L1-TL deviation energy (for discussion, see [ 12 ]). These energies each perform a unique yet dynamic role. As the learner progresses in the developmental process, the energies shift in their dominance, while the total energy remains constant.

Mathematically, ECT-L2A reads as follows:

where ζ r denotes the learner’s motivational energy, r the learner’s position in the learning process relative to the TL, η the distance between L1 and TL, and ρ the input of TL. According to Eq. 1 , the total learning energy, ∈ , comes from the sum of motivation energy ζ r , aptitude (a constant) Λ , deviation energy η 2 r 2 , and environmental energy - ρ r .

The energy types included in Eq. 1 are embodiments of nature and nurture contributions. The potential energy or TL traction, - ρ r , represents the external or environmental energy, while the kinetic or motivational energy, ζ r , along with aptitude, Λ , and the centrifugal or deviation energy η 2 r 2 represent the internal energies.

Under the overarching condition of the total energy being the same or conserved throughout the learning process, ϵ = constant , each type of energy performs a different role, with one converting to another over time as the position of the learner changes in the developmental process.

For mathematical and conceptual convenience, (1) is rewritten into (2) which contains the effective potential energy, U eff (r) .

where U e f f r = η 2 r 2 − ρ r . In other words, the effective potential energy is the sum of deviation energy and the potential energy (see further breakdown in the next section).

The L2A energy system as depicted here is true of every learner, meaning that the total energy is constant for a single learner. But the total energy varies from learner to learner. Accordingly, different learners may reach different levels of ultimate attainment (i.e., closer or more distant from the TL), r 0 . This is illustrated in Figure 1 , where r 0 and r 0 ′ represent the ultimate attainments for learners with different amounts of total energy, ϵ >0 or ϵ <0.

FIGURE 1 . Inter-learner differential ultimate attainment as a function of different amounts of total energy: ϵ >0; ϵ < 0; ϵ = ϵ min [ 12 , 13 ].

Key to understanding Figure 1 is that it is the individual’s total energy that determines their level of attainment. Of the three scenarios on display here, ECT-L2A is only concerned with the case of ϵ ≥0, which represents the unbound process (r 0 , ∞), ignoring the bounded processes of ϵ < 0; ε = ϵmin.

The central thesis of ECT-L2A, as expressed in Eq. 1 , is that the moment a learner begins to receive substantive exposure to the TL, s/he enters a ‘gravitational’ field or a developmental ecosystem in which s/he is initially driven by kinetic or motivational energy, increasingly subject to the traction of the potential or environmental energy, but eventually stonewalled by the deviation energy or centrifugal barrier, resulting in an asymptotic endstate. This trajectory is further elaborated below.

The developmental trajectory depicted and forecast by ECT-L2A

The L2A trajectory begins with the learner at the outset of the learning process or at infinity (r = ∞). Initially, their progression toward the central source, i.e., the TL, is driven almost entirely by their motivation energy and aptitude, as expressed in Eq. 3 .

As learning proceeds, but with r still large (i.e., the learner still distant from the target) and the deviation energy much weaker than the environmental energy, η 2 r 2 ≪ ρ r (due to the second power of r ), the motivation energy rises as a result of its “interaction” with the environmental energy− ρ r , in which case the environmental energy transfers to the motivation energy. Mathematically, this is expressed in Eq. 4 .

As learning further progresses, the environmental energy - ρ r becomes dominant before yielding to the deviation energy η 2 r 2 . Eventually, the deviation energy overrides the environmental energy, as expressed in Eq. 1 , repeated below as Eq. 5 for ease of reference.

The deviation energy is so powerful that it draws the learner away from the target and their learning reaches an asymptote, where their motivation energy becomes minimal, ζ ( r 0 ) = 0, as expressed in Eq. 6 .

At this point, all other energies submit to the deviation energy, including the initial motivation energy ζ (∞) and some of the potential or environmental energy. Consequently, further exposure to TL input would not be of substantive help, meaning that it would not move the learner markedly closer to the target.

Figure 2 gives a geometric expression of the L1-TL deviation η, which is akin to the angular momentum of an object moving in a central force field [ 73 – 75 ]. The deviation from the TL, signifying the distance between the L1 and the TL, varies with different L1-TL pairings. For example, the distance index, according to the Automated Similarity Judgment Program Database [ 76 ], is 90.25 for Italian and English but 100.33 for Italian and Chinese.

FIGURE 2 . Geometric description of the deviation parameter η .

Figure 3 illustrates differential ultimate attainment (indicated by r 0 ) as a function of the deviation parameter η. As η increases, the level of attainment is lower or the attainment is further away from the target ( r = 0).

FIGURE 3 . Effective potentials U eff with different values of η [ 12 , 13 ].

For adult L2A, ECT-L2A predicts, inter alia , that high attainment is possible but full attainment is not. In other words, near-nativelike attainment is possible, but complete-nativelike attainment is not. ECT-L2A also predicts that while motivation and aptitude are part and parcel of the total energy of a given L2 learner, their role is largely confined to the earlier stage of development. Most of all, ECT-L2A predicts that the L1-L2 deviation is what keeps L2 attainment at asymptote.

For L2 younger learners, ECT-L2A also makes a number of predictions to which we now turn.

ECT-L2A vis-à-vis younger learners

As highlighted above, the deviation energy is what leads L2 attainment to an asymptote. It follows that as long as η (i.e., the L1-TL distance) is non-zero, the learner’s ultimate attainment, r 0 , will always eventuate in an asymptote. As shown in Figure 3 , the larger the deviation r 0 , the more distant the ultimate attainment r 0 is from the TL. Put differently, a larger η portends that learning would reach an asymptote earlier or that the ultimate attainment would be less native-like. But how does that work for child L2A?

On the ECT-L2A account, it is the low η value that determines child learners’ superior attainment. In child L2 learners, the deviation is low, because of the incipient or underdeveloped L1. However, as the L1 develops, the η value grows until it becomes a constant, presumably happening around puberty 3 , hence coinciding with the offset of the critical period [ 1 ]. As shown in Figures 1 , 3 , the smaller the deviation, η, the closer r 0 (i.e., the ultimate level of attainment) is to the TL or the higher the ultimate attainment.

From Eq. 6 the ultimate attainment of any L2 learner, irrespective of age, can be mathematically derived:

where ε = ϵ – Λ (i.e., total energy minus aptitude). r 0 here again denotes ultimate attainment. The upper panel in Figure 4 displays the geometry of ultimate attainment as a function of deviation, η.

FIGURE 4 . Double non-linearity of r 0 η [ (A) : first non-linearity] and η t [ (B) : second non-linearity] at early AoA.

For a given child learner, η is a constant, but different child learners can have a different η value, depending on their AoA . Herein lies a crucial difference from adult learning where η is a constant for all learners because of their uniform late AoA or age of acquisition and because their L1 has solidified. Adult learning starts at a time when the deviation between their L1 and the TL has become fixed, so to speak, as a result of having mastered their L1 (see the lower panel of Figure 4 ).

Further, for child L2 learners, η is simultaneously a function of their AoA, a proxy for time ( t ), and can therefore be expressed as η(t). This deviation function of time varies in the range of 0 ≤ η t ≤ η max . Accordingly; Eq. 7 can be mathematically rewritten into (8):

Assuming that as t grows or as AoA increases, η increases slowly and smoothly from 0 to η max until it solidifies into a constant, which marks the onset of adult learning, η( t ) can mathematically be expressed as (9).

where a is a constant. The geometry of the deviation function of time is illustrated in the lower panel of Figure 4 .

Figure 4 displays a double non-linearity characterizing L2 acquisition by young learners, with (A) showing the first order of non-linearity of r 0 η , that is, ultimate attainment as a function of deviation or the L1-TL distance (computed via Eq. 7 ), and with (B) displaying the second order of non-linearity, η (t), that is, η changing with t , age of acquisition (computed through Eq. 9 ).

Figure 5 illustrates ultimate attainment as a function of AoA, r 0 (t), and its derivative against t , d r 0 d t , which naturally yields three distinct periods: a critical period, t critical ; a post-critical period, t p-critical ; and an adult learning period, t adult . Within the critical period, t critical , r 0 ≅ 0 , meaning there is no real difference in attainment as age of acquisition increases. But within the post-critical period, t p-critical , r 0 changes dramatically, with d r 0 d t peaking and waning until it drops to the level approximating that of the adult period. Within the adult period, t adult , r 0 remains a constant, as attainment levels off.

FIGURE 5 . Ultimate attainment (the blue line) as a function of age of acquisition ( t ) and its derivatives giving three distinct periods (the orange line).

ECT-L2A, therefore, identifies three learning periods. First, there is a critical period, t critical , within which attainment is nativelike, r 0 ≅ 0. Notice that the blue line in Figure 5 is the lowest during the critical period, signifying that the attainment converges on the target, but it is the highest during the adult period, meaning that the attainment diverges greatly from the target. The offset of the critical period is smooth rather than abrupt, with the impact of deviation, η , slowly emerging at its offset. During this period, the L1 is surfacing, yet with negligible deviation from the TL and weak in strength.

Key to understanding this account of the critical period is the double non-linearity: first, ultimate attainment as a function of L1-TL deviation ( r 0 ( η ), see (A) in Figure 4 ); and second, L1-TL deviation as a function of AoA ( η ( t ); see (B) in Figure 4 ). Crucially, this double non-linearity extends a critical “point” into a critical “period” .

Second, there is a post-critical period, t p-critical , 0 < r 0 ≤ r 0 ( η max ), within which, with advancing AoA, the L1-L2 deviation grows larger and stronger, resulting in ultimate attainment that is increasingly lower (i.e., increasingly non-nativelike). The change rate of r 0 , its first derivative to time, d r 0 d t , is dramatic, waxing and waning. As such, the post-critical period is more complex and nuanced than the critical period. During the post-critical period, as the learner’s L1 becomes increasingly robust and developed, the deviation becomes larger, resulting in a level of attainment increasingly away from the target (i.e., increasingly non-nativelike).

Third, there is an adult learning period, t adult , η = η max ≅ constant, where, despite the continuously advancing AoA, the deviation reaches its maximum and remains a constant, as benchmarked in indexes of crosslinguistic distance (see, e.g., the Automated Similarity Judgment Program Database [ 76 ]). As a result, L2 ultimate attainment turns asymptotic (for discussion, see [ 12 , 13 ]).

The three periods mathematically produced by ECT-L2A coincide with the stretched “Z” slope that some researchers have argued (e.g., [ 17 , 58 , 59 ]) constitutes the most unambiguous evidence for CPH/L2A, and by extension, for a maturationally-based account of the generic success or lack thereof (i.e., nativelike or non-nativelike L2 proficiency) in early versus late starters. For better illustration of the stretched “Z,” we can convert Figure 5 into Figure 6 , using Eq. 10 .

where a t t stands for level of attainment. According to Eq. 10 , the smaller the r 0 is, the higher the attainment is.

FIGURE 6 . Level of attainment as a function of AoA.

In sum, ECT-L2A mathematically establishes the critical period geometry. That said, the geometry, as seen in Figure 6 , exhibits anything but abrupt inflections; the phase transitions are gradual and smooth. The adult period, for example, does not exhibit a complete “flattening” but markedly lower attainment with continuous decline (cf. [ 7 , 23 , 28 ]). 4

Explaining CPH/L2A

As is clear from the above, on the ECT-L2A account of the critical period, η (i.e., L1-TL deviation) is considered an inter-learner variable and, at once, a proxy for age of acquisition, t . More profoundly, however, ECT-L2A associates η with neural plasticity or sensitivity (cf [ 77 ]). The relationship between plasticity, p ( t ), and deviation function, η (t) , is expressed as (11):

Thus, the relationship between plasticity and the deviation function is one of inverse correlation. During the critical period, η = η min (i.e., minimal L1-TL deviation) and p = p max (i.e., maximal plasticity); conversely, during the adult learning period, η = η max (i.e., maximal L1-TL deviation) and p = p min (i.e., minimal plasticity). In short, an increased deviation, η (t) , corresponds to a decrease of plasticity, p (t) , and vice versa , as illustrated in Figure 7 .

FIGURE 7 . Plasticity as a function of age of acquisition.

Illustrated in Figure 7 is that neural plasticity, first proposed by [ 78 ] as the underlying cause of CP, is at its highest during the critical period and, as [ 79 ] put it, it “endures within the confines of its onset and offset” ( p . 182). But it begins to decline and drops to a low level during the post-critical period, and remains low through the adult learning period. 5 It would, therefore, seem reasonable to call the first period “critical” and the second period “sensitive.” It is worth mentioning in passing that the post-critical or sensitive period has thus far received scant empirical attention in CPH/L2A research.

Temporally, following the [ 59 ] conjecture, the critical period should last through early childhood from birth to age six, and the sensitive period should offset around puberty (see also [ 2 , 20 , 36 , 67 , 71 ]). Crucially, both periods are circumscribed, exhibiting discontinuities, with the critical period exhibiting maximal sensitivity, the sensitive period declining, though, for the most part, still far greater, sensitivity than the adult learning period. This view of a changing underlying mechanism across the three periods of AoA and attainment resonates with the Language as a Complex Adaptive System perspective (see, e.g., [ 80 ]). [ 81 ], for example, noted that “the processing mechanisms that underlie [language development] … are fundamentally non-linear. This means that development itself will frequently have phase-like characteristics, that there may be periods of extreme sensitivity to input (‘critical periods’)” ( p . 431).

ECT-L2A as a unifying model

ECT-L2A, by virtue of identifying the L1-TL deviation, η, as a lynchpin for age effects, provides an explanation for the differential ultimate attainment of early versus late starters. Essentially, in early AoA, η is a temporal and neuro-functional proxy tied respectively to a developing L1 and to a changing age and changing neuroplasticity. In contrast, in late AoA, η is a constant, due to the L1 being fully developed and the brain fully mature. This takes care of the first facet of inter-learner differential attainment. What about the second facet, viz., the inter-learner differential attainment among late learners?

ECT-L2A (as expressed in Eq. 1 ) is a model of an ecosystem where there is an interplay between learner-internal and environmental energies. In line with the general finding from L2 research that individual difference variables are largely responsible for inter-learner differential attainment of nativelike proficiency in adult learners (see, e.g., [ 27 , 35 , 77 , 82 , 83 ]), ECT-L2A specifically ties motivation and aptitude to kinetic energy, only to provide a more nuanced picture of the changing magnitude of individual difference variables.

Figure 8 illustrates the twin facets of inter-learner differential attainment. First, attainment varies as a function of AoA. Second, attainment varies within and across the three learning periods as a function of individual learners with different amounts of total energy, ϵ 1 < ϵ 2 < ϵ 3 . As shown, individual differences play out the least among learners of AoA falling within the critical period but the most within the adult learning period, consistent with the general findings from L2 research (see, e.g., [ 2 , 3 , 43 , 63 , 65 , 67 , 84 , 85 ]). During the post-critical or sensitive period, individual differences are initially non-apparent but become more pronounced with increasing AoA. 6

FIGURE 8 . Level of attainment for total energies ϵ 1 < ϵ 2 < ϵ 3 .

ECT-L2A thus offers a coherent explanation for variable attainment in late learners. First and foremost, it posits that individual learners’ total energy or “carrying capacity” [ 86 ] is different, which leads to different levels of attainment. Second, although the internal (motivation and aptitude) and external (environment) energies interact over time, ultimately it is the deviation energy η 2 r 2 that dominates and stalls the learner at asymptote (see Eq. 6 ). This account provides a much more nuanced perspective on the role of individual differences than has been given in the current L2A literature.

Extant empirical studies investigating individual difference variables through correlation analysis have mostly projected a static view of the role (some of) the variables play in L2A. In contrast, ECT-L2A gives a dynamic view and, more importantly, an interactive view. In the end, the individual difference variables are part of a larger ecosystem within which they do not act alone, but rather interact with other energies (i.e., potential energy and deviation energy), waxing and waning as a result of energy conservation.

In this article, we engaged with a central concern in the ongoing heated debate on CPH/L2A, that is, the geometry of age differences. Within the framework of ECT-L2A, an interdisciplinary model of L2 attainment, we mathematically derived the age-attainment function and established the presence of a critical period in L2A. Importantly, this period is part of a developmental trajectory that comprises three learning periods: a critical period, a post-critical or sensitive period, and an adult period.

ECT-L2A has thus far demonstrated a stunning internal consistency in that it mathematically identifies younger learners’ superior performance to adult learners’ as well as the differential attainment among adult learners.

ECT-L2A, while in broad agreement with an entrenchment-transfer account from L2A research that essentializes the role of the L1 in L2 attainment (see, e.g., [ 5 , 11 , 87 – 90 ]), provides a dynamic account of that role and its varying contributions to the different age-related learning periods. Furthermore, ECT-L2A offers an interactive account whereby the L1, as part of the deviation energy, interacts with other types of learner-internal and learner-external energies. Above all, ECT-L2A, by virtue of summoning internal and external energies, gives a coherent explanation for the twin facets of inter-learner differential success—as respectively manifested between younger and older learners and among older learners.

Validation of ECT-L2A is, however, required. Many questions warrant investigation. On this note, Johnson and Newort’s view [ 17 ], in particular, that the goal of any L2A theory should be to account for three sets of facts—a) gradual decline of performance, b) the age at which a decline in performance is detected, and c) the nature of adult performance—resonates with us. Although ECT-L2A shines a light on all three, further work is clearly needed. More specific to the focus of the present article, three sets of questions can be asked in relation to the three learning periods ECT-L2A has identified.

In the spirit of promoting collective intelligence, we present a subset of these questions below in the hope that they will spark interest among researchers across disciplines and inspire close-up investigations leveraging a variety of methodologies.

First, for the critical period:

1. When does the decline of learning begin?

2. How does it relate to the status of L1?

3. What is plasticity like in this period?

4. What does plasticity entail?

5. How is it related to a developing L1 and a developing L2?

Answers to these questions can, at least in part, be found in the various literatures across disciplines. But approaching these questions in relation to one another—as opposed to discretely—would likely yield a more systematic, holistic and coherent understanding. Or perhaps, in search of answers to any of these questions, one may realize that the existing understanding is way too shallow or inadequate. For instance, [ 18 ] cited “a lack of interference from a well-learned first language” as one of the possible causes of the age-attainment function in younger versus older learners. But what has not yet been established is the nature of the younger learners’ L1. What does “well-learned” mean? Is it established or is it still developing? At minimum, it cannot be a unitary phenomenon, given the age span of young learners.

Second, for the post-critical or sensitive period, ECT-L2A mathematically identifies two sub-periods. Thus, questions such as the following should be examined:

6. What prompts the initial dramatic decline of attainment?

7. How does each of the sub-periods relate to the status of L1?

8. How does the decline relate to changing plasticity?

9. How does it relate to grammatical performance?

Third, for the adult learning period, questions such as the following warrant close engagement:

10. How do learners with the same L1 background differ from each other in their L2 ultimate attainment?

11. How do learners with different L1 backgrounds differ from one another in their L2 ultimate attainment?

12. How is the trajectory of each type of energy, endogenous or exogenous, related to the level of attainment?

Investigating these questions, among others, will lead us to a better understanding not only of the critical period but also of L2 learning over the arc of human life.

The theoretical and practical importance of gaining a robust and comprehensive understanding of how age affects the L2 learning outcome calls for systematic investigations. To that end, ECT-L2A has offered a systems thinking perspective and framework.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Acknowledgments

We greatly appreciate the insightful and perceptive comments made by the reviewers on an earlier version of this article, and take sole responsibility for any error or omission.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

1 Despite the centrality of “nativelikeness” to the Critical Period Hypothesis [ 1 ], studies in L2A have increasingly moved away from the use of the term in favor of “the level of ultimate attainment” [ 2 ].

2 The potential energy in ECT-L2A is akin to gravitational potential energy. As such it defines the central source field, serving as the primary energy that dynamically converts to other types of energy: kinetic energy and centrifugal energy. Similarly, the potential energy of L2A defines the field of learning. It stands for TL environment or input, serving as primary energy, dynamically converting to motivational and L1-TL deviation energies. An essential premise of ECT-L2A is the existence of potential energy. This premise is consistent with that underpinning L2A studies on CPH and ultimate attainment.

3 That is when the L1 becomes entrenched.

4 Looking back on the [ 17 ] study, [ 7 ], taking account of developments in the intervening 3 decades in understanding changes in the brain during adulthood, updated the earlier assertation about the stability of age effects in adulthood, noting that “it is more accurate to hypothesize that L2 proficiency SHOULD continue to decline during adulthood” and that “a critical or sensitive period for language acquisition is not absolute or sudden” ( p . 929, emphasis in original). She further argued that “[t]he lack of flattening of age function at adulthood in many studies does not mean that learning is not constrained by biologically based maturational changes” (ibid).

5 The plasticity never completely disappears, but rather becomes asymptotic.

6 Age and attainment function appears to follow a power law in that age effects are greatest during the critical period, less so during the post-critical or sensitive period, and weakest during the adult learning period (see Figure 8 ). Similarly, Figure 7 exhibits a power law relationship between age and plasticity: Plasticity is at its peak during the critical period, declines during the post-critical or sensitive period, and plateaus in the adult learning period.

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Keywords: ultimate attainment, critical period, second language acquisition, physics laws, energy conservation, angular momentum conservation, inter-learner differential attainment

Citation: Han Z and Bao G (2023) Critical period in second language acquisition: The age-attainment geometry. Front. Phys. 11:1142584. doi: 10.3389/fphy.2023.1142584

Received: 11 January 2023; Accepted: 02 March 2023; Published: 20 March 2023.

Reviewed by:

Copyright © 2023 Han and Bao. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: ZhaoHong Han, [email protected]

This article is part of the Research Topic

Social Physics and the Dynamics of Second Language Acquisition

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Research Article

The Critical Period Hypothesis in Second Language Acquisition: A Statistical Critique and a Reanalysis

* E-mail: [email protected]

Affiliation Department of Multilingualism, University of Fribourg, Fribourg, Switzerland

• Jan Vanhove

• Published: July 25, 2013
• https://doi.org/10.1371/journal.pone.0069172
• Reader Comments

17 Jul 2014: The PLOS ONE Staff (2014) Correction: The Critical Period Hypothesis in Second Language Acquisition: A Statistical Critique and a Reanalysis. PLOS ONE 9(7): e102922. https://doi.org/10.1371/journal.pone.0102922 View correction

In second language acquisition research, the critical period hypothesis ( cph ) holds that the function between learners' age and their susceptibility to second language input is non-linear. This paper revisits the indistinctness found in the literature with regard to this hypothesis's scope and predictions. Even when its scope is clearly delineated and its predictions are spelt out, however, empirical studies–with few exceptions–use analytical (statistical) tools that are irrelevant with respect to the predictions made. This paper discusses statistical fallacies common in cph research and illustrates an alternative analytical method (piecewise regression) by means of a reanalysis of two datasets from a 2010 paper purporting to have found cross-linguistic evidence in favour of the cph . This reanalysis reveals that the specific age patterns predicted by the cph are not cross-linguistically robust. Applying the principle of parsimony, it is concluded that age patterns in second language acquisition are not governed by a critical period. To conclude, this paper highlights the role of confirmation bias in the scientific enterprise and appeals to second language acquisition researchers to reanalyse their old datasets using the methods discussed in this paper. The data and R commands that were used for the reanalysis are provided as supplementary materials.

Citation: Vanhove J (2013) The Critical Period Hypothesis in Second Language Acquisition: A Statistical Critique and a Reanalysis. PLoS ONE 8(7): e69172. https://doi.org/10.1371/journal.pone.0069172

Editor: Stephanie Ann White, UCLA, United States of America

Received: May 7, 2013; Accepted: June 7, 2013; Published: July 25, 2013

Copyright: © 2013 Jan Vanhove. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: No current external funding sources for this study.

Competing interests: The author has declared that no competing interests exist.

Introduction

In the long term and in immersion contexts, second-language (L2) learners starting acquisition early in life – and staying exposed to input and thus learning over several years or decades – undisputedly tend to outperform later learners. Apart from being misinterpreted as an argument in favour of early foreign language instruction, which takes place in wholly different circumstances, this general age effect is also sometimes taken as evidence for a so-called ‘critical period’ ( cp ) for second-language acquisition ( sla ). Derived from biology, the cp concept was famously introduced into the field of language acquisition by Penfield and Roberts in 1959 [1] and was refined by Lenneberg eight years later [2] . Lenneberg argued that language acquisition needed to take place between age two and puberty – a period which he believed to coincide with the lateralisation process of the brain. (More recent neurological research suggests that different time frames exist for the lateralisation process of different language functions. Most, however, close before puberty [3] .) However, Lenneberg mostly drew on findings pertaining to first language development in deaf children, feral children or children with serious cognitive impairments in order to back up his claims. For him, the critical period concept was concerned with the implicit “automatic acquisition” [2, p. 176] in immersion contexts and does not preclude the possibility of learning a foreign language after puberty, albeit with much conscious effort and typically less success.

sla research adopted the critical period hypothesis ( cph ) and applied it to second and foreign language learning, resulting in a host of studies. In its most general version, the cph for sla states that the ‘susceptibility’ or ‘sensitivity’ to language input varies as a function of age, with adult L2 learners being less susceptible to input than child L2 learners. Importantly, the age–susceptibility function is hypothesised to be non-linear. Moving beyond this general version, we find that the cph is conceptualised in a multitude of ways [4] . This state of affairs requires scholars to make explicit their theoretical stance and assumptions [5] , but has the obvious downside that critical findings risk being mitigated as posing a problem to only one aspect of one particular conceptualisation of the cph , whereas other conceptualisations remain unscathed. This overall vagueness concerns two areas in particular, viz. the delineation of the cph 's scope and the formulation of testable predictions. Delineating the scope and formulating falsifiable predictions are, needless to say, fundamental stages in the scientific evaluation of any hypothesis or theory, but the lack of scholarly consensus on these points seems to be particularly pronounced in the case of the cph . This article therefore first presents a brief overview of differing views on these two stages. Then, once the scope of their cph version has been duly identified and empirical data have been collected using solid methods, it is essential that researchers analyse the data patterns soundly in order to assess the predictions made and that they draw justifiable conclusions from the results. As I will argue in great detail, however, the statistical analysis of data patterns as well as their interpretation in cph research – and this includes both critical and supportive studies and overviews – leaves a great deal to be desired. Reanalysing data from a recent cph -supportive study, I illustrate some common statistical fallacies in cph research and demonstrate how one particular cph prediction can be evaluated.

Delineating the scope of the critical period hypothesis

First, the age span for a putative critical period for language acquisition has been delimited in different ways in the literature [4] . Lenneberg's critical period stretched from two years of age to puberty (which he posits at about 14 years of age) [2] , whereas other scholars have drawn the cutoff point at 12, 15, 16 or 18 years of age [6] . Unlike Lenneberg, most researchers today do not define a starting age for the critical period for language learning. Some, however, consider the possibility of the critical period (or a critical period for a specific language area, e.g. phonology) ending much earlier than puberty (e.g. age 9 years [1] , or as early as 12 months in the case of phonology [7] ).

Second, some vagueness remains as to the setting that is relevant to the cph . Does the critical period constrain implicit learning processes only, i.e. only the untutored language acquisition in immersion contexts or does it also apply to (at least partly) instructed learning? Most researchers agree on the former [8] , but much research has included subjects who have had at least some instruction in the L2.

Third, there is no consensus on what the scope of the cp is as far as the areas of language that are concerned. Most researchers agree that a cp is most likely to constrain the acquisition of pronunciation and grammar and, consequently, these are the areas primarily looked into in studies on the cph [9] . Some researchers have also tried to define distinguishable cp s for the different language areas of phonetics, morphology and syntax and even for lexis (see [10] for an overview).

Fourth and last, research into the cph has focused on ‘ultimate attainment’ ( ua ) or the ‘final’ state of L2 proficiency rather than on the rate of learning. From research into the rate of acquisition (e.g. [11] – [13] ), it has become clear that the cph cannot hold for the rate variable. In fact, it has been observed that adult learners proceed faster than child learners at the beginning stages of L2 acquisition. Though theoretical reasons for excluding the rate can be posited (the initial faster rate of learning in adults may be the result of more conscious cognitive strategies rather than to less conscious implicit learning, for instance), rate of learning might from a different perspective also be considered an indicator of ‘susceptibility’ or ‘sensitivity’ to language input. Nevertheless, contemporary sla scholars generally seem to concur that ua and not rate of learning is the dependent variable of primary interest in cph research. These and further scope delineation problems relevant to cph research are discussed in more detail by, among others, Birdsong [9] , DeKeyser and Larson-Hall [14] , Long [10] and Muñoz and Singleton [6] .

Formulating testable hypotheses

Once the relevant cph 's scope has satisfactorily been identified, clear and testable predictions need to be drawn from it. At this stage, the lack of consensus on what the consequences or the actual observable outcome of a cp would have to look like becomes evident. As touched upon earlier, cph research is interested in the end state or ‘ultimate attainment’ ( ua ) in L2 acquisition because this “determines the upper limits of L2 attainment” [9, p. 10]. The range of possible ultimate attainment states thus helps researchers to explore the potential maximum outcome of L2 proficiency before and after the putative critical period.

One strong prediction made by some cph exponents holds that post- cp learners cannot reach native-like L2 competences. Identifying a single native-like post- cp L2 learner would then suffice to falsify all cph s making this prediction. Assessing this prediction is difficult, however, since it is not clear what exactly constitutes sufficient nativelikeness, as illustrated by the discussion on the actual nativelikeness of highly accomplished L2 speakers [15] , [16] . Indeed, there exists a real danger that, in a quest to vindicate the cph , scholars set the bar for L2 learners to match monolinguals increasingly higher – up to Swiftian extremes. Furthermore, the usefulness of comparing the linguistic performance in mono- and bilinguals has been called into question [6] , [17] , [18] . Put simply, the linguistic repertoires of mono- and bilinguals differ by definition and differences in the behavioural outcome will necessarily be found, if only one digs deep enough.

A second strong prediction made by cph proponents is that the function linking age of acquisition and ultimate attainment will not be linear throughout the whole lifespan. Before discussing how this function would have to look like in order for it to constitute cph -consistent evidence, I point out that the ultimate attainment variable can essentially be considered a cumulative measure dependent on the actual variable of interest in cph research, i.e. susceptibility to language input, as well as on such other factors like duration and intensity of learning (within and outside a putative cp ) and possibly a number of other influencing factors. To elaborate, the behavioural outcome, i.e. ultimate attainment, can be assumed to be integrative to the susceptibility function, as Newport [19] correctly points out. Other things being equal, ultimate attainment will therefore decrease as susceptibility decreases. However, decreasing ultimate attainment levels in and by themselves represent no compelling evidence in favour of a cph . The form of the integrative curve must therefore be predicted clearly from the susceptibility function. Additionally, the age of acquisition–ultimate attainment function can take just about any form when other things are not equal, e.g. duration of learning (Does learning last up until time of testing or only for a more or less constant number of years or is it dependent on age itself?) or intensity of learning (Do learners always learn at their maximum susceptibility level or does this intensity vary as a function of age, duration, present attainment and motivation?). The integral of the susceptibility function could therefore be of virtually unlimited complexity and its parameters could be adjusted to fit any age of acquisition–ultimate attainment pattern. It seems therefore astonishing that the distinction between level of sensitivity to language input and level of ultimate attainment is rarely made in the literature. Implicitly or explicitly [20] , the two are more or less equated and the same mathematical functions are expected to describe the two variables if observed across a range of starting ages of acquisition.

But even when the susceptibility and ultimate attainment variables are equated, there remains controversy as to what function linking age of onset of acquisition and ultimate attainment would actually constitute evidence for a critical period. Most scholars agree that not any kind of age effect constitutes such evidence. More specifically, the age of acquisition–ultimate attainment function would need to be different before and after the end of the cp [9] . According to Birdsong [9] , three basic possible patterns proposed in the literature meet this condition. These patterns are presented in Figure 1 . The first pattern describes a steep decline of the age of onset of acquisition ( aoa )–ultimate attainment ( ua ) function up to the end of the cp and a practically non-existent age effect thereafter. Pattern 2 is an “unconventional, although often implicitly invoked” [9, p. 17] notion of the cp function which contains a period of peak attainment (or performance at ceiling), i.e. performance does not vary as a function of age, which is often referred to as a ‘window of opportunity’. This time span is followed by an unbounded decline in ua depending on aoa . Pattern 3 includes characteristics of patterns 1 and 2. At the beginning of the aoa range, performance is at ceiling. The next segment is a downward slope in the age function which ends when performance reaches its floor. Birdsong points out that all of these patterns have been reported in the literature. On closer inspection, however, he concludes that the most convincing function describing these age effects is a simple linear one. Hakuta et al. [21] sketch further theoretically possible predictions of the cph in which the mean performance drops drastically and/or the slope of the aoa – ua proficiency function changes at a certain point.

• PPT PowerPoint slide
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The graphs are based on based on Figure 2 in [9] .

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Although several patterns have been proposed in the literature, it bears pointing out that the most common explicit prediction corresponds to Birdsong's first pattern, as exemplified by the following crystal-clear statement by DeKeyser, one of the foremost cph proponents:

[A] strong negative correlation between age of acquisition and ultimate attainment throughout the lifespan (or even from birth through middle age), the only age effect documented in many earlier studies, is not evidence for a critical period…[T]he critical period concept implies a break in the AoA–proficiency function, i.e., an age (somewhat variable from individual to individual, of course, and therefore an age range in the aggregate) after which the decline of success rate in one or more areas of language is much less pronounced and/or clearly due to different reasons. [22, p. 445].

DeKeyser and before him among others Johnson and Newport [23] thus conceptualise only one possible pattern which would speak in favour of a critical period: a clear negative age effect before the end of the critical period and a much weaker (if any) negative correlation between age and ultimate attainment after it. This ‘flattened slope’ prediction has the virtue of being much more tangible than the ‘potential nativelikeness’ prediction: Testing it does not necessarily require comparing the L2-learners to a native control group and thus effectively comparing apples and oranges. Rather, L2-learners with different aoa s can be compared amongst themselves without the need to categorise them by means of a native-speaker yardstick, the validity of which is inevitably going to be controversial [15] . In what follows, I will concern myself solely with the ‘flattened slope’ prediction, arguing that, despite its clarity of formulation, cph research has generally used analytical methods that are irrelevant for the purposes of actually testing it.

Inferring non-linearities in critical period research: An overview

Group mean or proportion comparisons.

[T]he main differences can be found between the native group and all other groups – including the earliest learner group – and between the adolescence group and all other groups. However, neither the difference between the two childhood groups nor the one between the two adulthood groups reached significance, which indicates that the major changes in eventual perceived nativelikeness of L2 learners can be associated with adolescence. [15, p. 270].

Similar group comparisons aimed at investigating the effect of aoa on ua have been carried out by both cph advocates and sceptics (among whom Bialystok and Miller [25, pp. 136–139], Birdsong and Molis [26, p. 240], Flege [27, pp. 120–121], Flege et al. [28, pp. 85–86], Johnson [29, p. 229], Johnson and Newport [23, p. 78], McDonald [30, pp. 408–410] and Patowski [31, pp. 456–458]). To be clear, not all of these authors drew direct conclusions about the aoa – ua function on the basis of these groups comparisons, but their group comparisons have been cited as indicative of a cph -consistent non-continuous age effect, as exemplified by the following quote by DeKeyser [22] :

Where group comparisons are made, younger learners always do significantly better than the older learners. The behavioral evidence, then, suggests a non-continuous age effect with a “bend” in the AoA–proficiency function somewhere between ages 12 and 16. [22, p. 448].

The first problem with group comparisons like these and drawing inferences on the basis thereof is that they require that a continuous variable, aoa , be split up into discrete bins. More often than not, the boundaries between these bins are drawn in an arbitrary fashion, but what is more troublesome is the loss of information and statistical power that such discretisation entails (see [32] for the extreme case of dichotomisation). If we want to find out more about the relationship between aoa and ua , why throw away most of the aoa information and effectively reduce the ua data to group means and the variance in those groups?

Comparison of correlation coefficients.

Correlation-based inferences about slope discontinuities have similarly explicitly been made by cph advocates and skeptics alike, e.g. Bialystok and Miller [25, pp. 136 and 140], DeKeyser and colleagues [22] , [44] and Flege et al. [45, pp. 166 and 169]. Others did not explicitly infer the presence or absence of slope differences from the subset correlations they computed (among others Birdsong and Molis [26] , DeKeyser [8] , Flege et al. [28] and Johnson [29] ), but their studies nevertheless featured in overviews discussing discontinuities [14] , [22] . Indeed, the most recent overview draws a strong conclusion about the validity of the cph 's ‘flattened slope’ prediction on the basis of these subset correlations:

In those studies where the two groups are described separately, the correlation is much higher for the younger than for the older group, except in Birdsong and Molis (2001) [ =  [26] , JV], where there was a ceiling effect for the younger group. This global picture from more than a dozen studies provides support for the non-continuity of the decline in the AoA–proficiency function, which all researchers agree is a hallmark of a critical period phenomenon. [22, p. 448].

In Johnson and Newport's specific case [23] , their correlation-based inference that ua levels off after puberty happened to be largely correct: the gjt scores are more or less randomly distributed around a near-horizontal trend line [26] . Ultimately, however, it rests on the fallacy of confusing correlation coefficients with slopes, which seriously calls into question conclusions such as DeKeyser's (cf. the quote above).

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Lower correlation coefficients in older aoa groups may therefore be largely due to differences in ua variance, which have been reported in several studies [23] , [26] , [28] , [29] (see [46] for additional references). Greater variability in ua with increasing age is likely due to factors other than age proper [47] , such as the concomitant greater variability in exposure to literacy, degree of education, motivation and opportunity for language use, and by itself represents evidence neither in favour of nor against the cph .

Regression approaches.

Having demonstrated that neither group mean or proportion comparisons nor correlation coefficient comparisons can directly address the ‘flattened slope’ prediction, I now turn to the studies in which regression models were computed with aoa as a predictor variable and ua as the outcome variable. Once again, this category of studies is not mutually exclusive with the two categories discussed above.

In a large-scale study using self-reports and approximate aoa s derived from a sample of the 1990 U.S. Census, Stevens found that the probability with which immigrants from various countries stated that they spoke English ‘very well’ decreased curvilinearly as a function of aoa [48] . She noted that this development is similar to the pattern found by Johnson and Newport [23] but that it contains no indication of an “abruptly defined ‘critical’ or sensitive period in L2 learning” [48, p. 569]. However, she modelled the self-ratings using an ordinal logistic regression model in which the aoa variable was logarithmically transformed. Technically, this is perfectly fine, but one should be careful not to read too much into the non-linear curves found. In logistic models, the outcome variable itself is modelled linearly as a function of the predictor variables and is expressed in log-odds. In order to compute the corresponding probabilities, these log-odds are transformed using the logistic function. Consequently, even if the model is specified linearly, the predicted probabilities will not lie on a perfectly straight line when plotted as a function of any one continuous predictor variable. Similarly, when the predictor variable is first logarithmically transformed and then used to linearly predict an outcome variable, the function linking the predicted outcome variables and the untransformed predictor variable is necessarily non-linear. Thus, non-linearities follow naturally from Stevens's model specifications. Moreover, cph -consistent discontinuities in the aoa – ua function cannot be found using her model specifications as they did not contain any parameters allowing for this.

Using data similar to Stevens's, Bialystok and Hakuta found that the link between the self-rated English competences of Chinese- and Spanish-speaking immigrants and their aoa could be described by a straight line [49] . In contrast to Stevens, Bialystok and Hakuta used a regression-based method allowing for changes in the function's slope, viz. locally weighted scatterplot smoothing ( lowess ). Informally, lowess is a non-parametrical method that relies on an algorithm that fits the dependent variable for small parts of the range of the independent variable whilst guaranteeing that the overall curve does not contain sudden jumps (for technical details, see [50] ). Hakuta et al. used an even larger sample from the same 1990 U.S. Census data on Chinese- and Spanish-speaking immigrants (2.3 million observations) [21] . Fitting lowess curves, no discontinuities in the aoa – ua slope could be detected. Moreover, the authors found that piecewise linear regression models, i.e. regression models containing a parameter that allows a sudden drop in the curve or a change of its slope, did not provide a better fit to the data than did an ordinary regression model without such a parameter.

To sum up, I have argued at length that regression approaches are superior to group mean and correlation coefficient comparisons for the purposes of testing the ‘flattened slope’ prediction. Acknowledging the reservations vis-à-vis self-estimated ua s, we still find that while the relationship between aoa and ua is not necessarily perfectly linear in the studies discussed, the data do not lend unequivocal support to this prediction. In the following section, I will reanalyse data from a recent empirical paper on the cph by DeKeyser et al. [44] . The first goal of this reanalysis is to further illustrate some of the statistical fallacies encountered in cph studies. Second, by making the computer code available I hope to demonstrate how the relevant regression models, viz. piecewise regression models, can be fitted and how the aoa representing the optimal breakpoint can be identified. Lastly, the findings of this reanalysis will contribute to our understanding of how aoa affects ua as measured using a gjt .

Summary of DeKeyser et al. (2010)

I chose to reanalyse a recent empirical paper on the cph by DeKeyser et al. [44] (henceforth DK et al.). This paper lends itself well to a reanalysis since it exhibits two highly commendable qualities: the authors spell out their hypotheses lucidly and provide detailed numerical and graphical data descriptions. Moreover, the paper's lead author is very clear on what constitutes a necessary condition for accepting the cph : a non-linearity in the age of onset of acquisition ( aoa )–ultimate attainment ( ua ) function, with ua declining less strongly as a function of aoa in older, post- cp arrivals compared to younger arrivals [14] , [22] . Lastly, it claims to have found cross-linguistic evidence from two parallel studies backing the cph and should therefore be an unsuspected source to cph proponents.

The authors set out to test the following hypotheses:

• Hypothesis 1: For both the L2 English and the L2 Hebrew group, the slope of the age of arrival–ultimate attainment function will not be linear throughout the lifespan, but will instead show a marked flattening between adolescence and adulthood.
• Hypothesis 2: The relationship between aptitude and ultimate attainment will differ markedly for the young and older arrivals, with significance only for the latter. (DK et al., p. 417)

Both hypotheses were purportedly confirmed, which in the authors' view provides evidence in favour of cph . The problem with this conclusion, however, is that it is based on a comparison of correlation coefficients. As I have argued above, correlation coefficients are not to be confused with regression coefficients and cannot be used to directly address research hypotheses concerning slopes, such as Hypothesis 1. In what follows, I will reanalyse the relationship between DK et al.'s aoa and gjt data in order to address Hypothesis 1. Additionally, I will lay bare a problem with the way in which Hypothesis 2 was addressed. The extracted data and the computer code used for the reanalysis are provided as supplementary materials, allowing anyone interested to scrutinise and easily reproduce my whole analysis and carry out their own computations (see ‘supporting information’).

Data extraction

In order to verify whether we did in fact extract the data points to a satisfactory degree of accuracy, I computed summary statistics for the extracted aoa and gjt data and checked these against the descriptive statistics provided by DK et al. (pp. 421 and 427). These summary statistics for the extracted data are presented in Table 1 . In addition, I computed the correlation coefficients for the aoa – gjt relationship for the whole aoa range and for aoa -defined subgroups and checked these coefficients against those reported by DK et al. (pp. 423 and 428). The correlation coefficients computed using the extracted data are presented in Table 2 . Both checks strongly suggest the extracted data to be virtually identical to the original data, and Dr DeKeyser confirmed this to be the case in response to an earlier draft of the present paper (personal communication, 6 May 2013).

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Results and Discussion

Modelling the link between age of onset of acquisition and ultimate attainment.

I first replotted the aoa and gjt data we extracted from DK et al.'s scatterplots and added non-parametric scatterplot smoothers in order to investigate whether any changes in slope in the aoa – gjt function could be revealed, as per Hypothesis 1. Figures 3 and 4 show this not to be the case. Indeed, simple linear regression models that model gjt as a function of aoa provide decent fits for both the North America and the Israel data, explaining 65% and 63% of the variance in gjt scores, respectively. The parameters of these models are given in Table 3 .

The trend line is a non-parametric scatterplot smoother. The scatterplot itself is a near-perfect replication of DK et al.'s Fig. 1.

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The trend line is a non-parametric scatterplot smoother. The scatterplot itself is a near-perfect replication of DK et al.'s Fig. 5.

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To ensure that both segments are joined at the breakpoint, the predictor variable is first centred at the breakpoint value, i.e. the breakpoint value is subtracted from the original predictor variable values. For a blow-by-blow account of how such models can be fitted in r , I refer to an example analysis by Baayen [55, pp. 214–222].

Solid: regression with breakpoint at aoa 18 (dashed lines represent its 95% confidence interval); dot-dash: regression without breakpoint.

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Solid: regression with breakpoint at aoa 18 (dashed lines represent its 95% confidence interval); dot-dash (hardly visible due to near-complete overlap): regression without breakpoint.

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Solid: regression with breakpoint at aoa 16 (dashed lines represent its 95% confidence interval); dot-dash: regression without breakpoint.

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Solid: regression with breakpoint at aoa 6 (dashed lines represent its 95% confidence interval); dot-dash (hardly visible due to near-complete overlap): regression without breakpoint.

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In sum, a regression model that allows for changes in the slope of the the aoa – gjt function to account for putative critical period effects provides a somewhat better fit to the North American data than does an everyday simple regression model. The improvement in model fit is marginal, however, and including a breakpoint does not result in any detectable improvement of model fit to the Israel data whatsoever. Breakpoint models therefore fail to provide solid cross-linguistic support in favour of critical period effects: across both data sets, gjt can satisfactorily be modelled as a linear function of aoa .

On partialling out ‘age at testing’

As I have argued above, correlation coefficients cannot be used to test hypotheses about slopes. When the correct procedure is carried out on DK et al.'s data, no cross-linguistically robust evidence for changes in the aoa – gjt function was found. In addition to comparing the zero-order correlations between aoa and gjt , however, DK et al. computed partial correlations in which the variance in aoa associated with the participants' age at testing ( aat ; a potentially confounding variable) was filtered out. They found that these partial correlations between aoa and gjt , which are given in Table 9 , differed between age groups in that they are stronger for younger than for older participants. This, DK et al. argue, constitutes additional evidence in favour of the cph . At this point, I can no longer provide my own analysis of DK et al.'s data seeing as the pertinent data points were not plotted. Nevertheless, the detailed descriptions by DK et al. strongly suggest that the use of these partial correlations is highly problematic. Most importantly, and to reiterate, correlations (whether zero-order or partial ones) are actually of no use when testing hypotheses concerning slopes. Still, one may wonder why the partial correlations differ across age groups. My surmise is that these differences are at least partly the by-product of an imbalance in the sampling procedure.

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The upshot of this brief discussion is that the partial correlation differences reported by DK et al. are at least partly the result of an imbalance in the sampling procedure: aoa and aat were simply less intimately tied for the young arrivals in the North America study than for the older arrivals with L2 English or for all of the L2 Hebrew participants. In an ideal world, we would like to fix aat or ascertain that it at most only weakly correlates with aoa . This, however, would result in a strong correlation between aoa and another potential confound variable, length of residence in the L2 environment, bringing us back to square one. Allowing for only moderate correlations between aoa and aat might improve our predicament somewhat, but even in that case, we should tread lightly when making inferences on the basis of statistical control procedures [61] .

On estimating the role of aptitude

Having shown that Hypothesis 1 could not be confirmed, I now turn to Hypothesis 2, which predicts a differential role of aptitude for ua in sla in different aoa groups. More specifically, it states that the correlation between aptitude and gjt performance will be significant only for older arrivals. The correlation coefficients of the relationship between aptitude and gjt are presented in Table 10 .

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The problem with both the wording of Hypothesis 2 and the way in which it is addressed is the following: it is assumed that a variable has a reliably different effect in different groups when the effect reaches significance in one group but not in the other. This logic is fairly widespread within several scientific disciplines (see e.g. [62] for a discussion). Nonetheless, it is demonstrably fallacious [63] . Here we will illustrate the fallacy for the specific case of comparing two correlation coefficients.

Apart from not being replicated in the North America study, does this difference actually show anything? I contend that it does not: what is of interest are not so much the correlation coefficients, but rather the interactions between aoa and aptitude in models predicting gjt . These interactions could be investigated by fitting a multiple regression model in which the postulated cp breakpoint governs the slope of both aoa and aptitude. If such a model provided a substantially better fit to the data than a model without a breakpoint for the aptitude slope and if the aptitude slope changes in the expected direction (i.e. a steeper slope for post- cp than for younger arrivals) for different L1–L2 pairings, only then would this particular prediction of the cph be borne out.

Using data extracted from a paper reporting on two recent studies that purport to provide evidence in favour of the cph and that, according to its authors, represent a major improvement over earlier studies (DK et al., p. 417), it was found that neither of its two hypotheses were actually confirmed when using the proper statistical tools. As a matter of fact, the gjt scores continue to decline at essentially the same rate even beyond the end of the putative critical period. According to the paper's lead author, such a finding represents a serious problem to his conceptualisation of the cph [14] ). Moreover, although modelling a breakpoint representing the end of a cp at aoa 16 may improve the statistical model slightly in study on learners of English in North America, the study on learners of Hebrew in Israel fails to confirm this finding. In fact, even if we were to accept the optimal breakpoint computed for the Israel study, it lies at aoa 6 and is associated with a different geometrical pattern.

Diverging age trends in parallel studies with participants with different L2s have similarly been reported by Birdsong and Molis [26] and are at odds with an L2-independent cph . One parsimonious explanation of such conflicting age trends may be that the overall, cross-linguistic age trend is in fact linear, but that fluctuations in the data (due to factors unaccounted for or randomness) may sometimes give rise to a ‘stretched L’-shaped pattern ( Figure 1, left panel ) and sometimes to a ‘stretched 7’-shaped pattern ( Figure 1 , middle panel; see also [66] for a similar comment).

Importantly, the criticism that DeKeyser and Larsson-Hall levy against two studies reporting findings similar to the present [48] , [49] , viz. that the data consisted of self-ratings of questionable validity [14] , does not apply to the present data set. In addition, DK et al. did not exclude any outliers from their analyses, so I assume that DeKeyser and Larsson-Hall's criticism [14] of Birdsong and Molis's study [26] , i.e. that the findings were due to the influence of outliers, is not applicable to the present data either. For good measure, however, I refitted the regression models with and without breakpoints after excluding one potentially problematic data point per model. The following data points had absolute standardised residuals larger than 2.5 in the original models without breakpoints as well as in those with breakpoints: the participant with aoa 17 and a gjt score of 125 in the North America study and the participant with aoa 12 and a gjt score of 117 in the Israel study. The resultant models were virtually identical to the original models (see Script S1 ). Furthermore, the aoa variable was sufficiently fine-grained and the aoa – gjt curve was not ‘presmoothed’ by the prior aggregation of gjt across parts of the aoa range (see [51] for such a criticism of another study). Lastly, seven of the nine “problems with supposed counter-evidence” to the cph discussed by Long [5] do not apply either, viz. (1) “[c]onfusion of rate and ultimate attainment”, (2) “[i]nappropriate choice of subjects”, (3) “[m]easurement of AO”, (4) “[l]eading instructions to raters”, (6) “[u]se of markedly non-native samples making near-native samples more likely to sound native to raters”, (7) “[u]nreliable or invalid measures”, and (8) “[i]nappropriate L1–L2 pairings”. Problem No. 5 (“Assessments based on limited samples and/or “language-like” behavior”) may be apropos given that only gjt data were used, leaving open the theoretical possibility that other measures might have yielded a different outcome. Finally, problem No. 9 (“Faulty interpretation of statistical patterns”) is, of course, precisely what I have turned the spotlights on.

Conclusions

The critical period hypothesis remains a hotly contested issue in the psycholinguistics of second-language acquisition. Discussions about the impact of empirical findings on the tenability of the cph generally revolve around the reliability of the data gathered (e.g. [5] , [14] , [22] , [52] , [67] , [68] ) and such methodological critiques are of course highly desirable. Furthermore, the debate often centres on the question of exactly what version of the cph is being vindicated or debunked. These versions differ mainly in terms of its scope, specifically with regard to the relevant age span, setting and language area, and the testable predictions they make. But even when the cph 's scope is clearly demarcated and its main prediction is spelt out lucidly, the issue remains to what extent the empirical findings can actually be marshalled in support of the relevant cph version. As I have shown in this paper, empirical data have often been taken to support cph versions predicting that the relationship between age of acquisition and ultimate attainment is not strictly linear, even though the statistical tools most commonly used (notably group mean and correlation coefficient comparisons) were, crudely put, irrelevant to this prediction. Methods that are arguably valid, e.g. piecewise regression and scatterplot smoothing, have been used in some studies [21] , [26] , [49] , but these studies have been criticised on other grounds. To my knowledge, such methods have never been used by scholars who explicitly subscribe to the cph .

I suspect that what may be going on is a form of ‘confirmation bias’ [69] , a cognitive bias at play in diverse branches of human knowledge seeking: Findings judged to be consistent with one's own hypothesis are hardly questioned, whereas findings inconsistent with one's own hypothesis are scrutinised much more strongly and criticised on all sorts of points [70] – [73] . My reanalysis of DK et al.'s recent paper may be a case in point. cph exponents used correlation coefficients to address their prediction about the slope of a function, as had been done in a host of earlier studies. Finding a result that squared with their expectations, they did not question the technical validity of their results, or at least they did not report this. (In fact, my reanalysis is actually a case in point in two respects: for an earlier draft of this paper, I had computed the optimal position of the breakpoints incorrectly, resulting in an insignificant improvement of model fit for the North American data rather than a borderline significant one. Finding a result that squared with my expectations, I did not question the technical validity of my results – until this error was kindly pointed out to me by Martijn Wieling (University of Tübingen).) That said, I am keen to point out that the statistical analyses in this particular paper, though suboptimal, are, as far as I could gather, reported correctly, i.e. the confirmation bias does not seem to have resulted in the blatant misreportings found elsewhere (see [74] for empirical evidence and discussion). An additional point to these authors' credit is that, apart from explicitly identifying their cph version's scope and making crystal-clear predictions, they present data descriptions that actually permit quantitative reassessments and have a history of doing so (e.g. the appendix in [8] ). This leads me to believe that they analysed their data all in good conscience and to hope that they, too, will conclude that their own data do not, in fact, support their hypothesis.

I end this paper on an upbeat note. Even though I have argued that the analytical tools employed in cph research generally leave much to be desired, the original data are, so I hope, still available. This provides researchers, cph supporters and sceptics alike, with an exciting opportunity to reanalyse their data sets using the tools outlined in the present paper and publish their findings at minimal cost of time and resources (for instance, as a comment to this paper). I would therefore encourage scholars to engage their old data sets and to communicate their analyses openly, e.g. by voluntarily publishing their data and computer code alongside their articles or comments. Ideally, cph supporters and sceptics would join forces to agree on a protocol for a high-powered study in order to provide a truly convincing answer to a core issue in sla .

Supporting Information

Dataset s1..

aoa and gjt data extracted from DeKeyser et al.'s North America study.

https://doi.org/10.1371/journal.pone.0069172.s001

Dataset S2.

aoa and gjt data extracted from DeKeyser et al.'s Israel study.

https://doi.org/10.1371/journal.pone.0069172.s002

Script with annotated R code used for the reanalysis. All add-on packages used can be installed from within R.

https://doi.org/10.1371/journal.pone.0069172.s003

Acknowledgments

I would like to thank Irmtraud Kaiser (University of Fribourg) for helping me to get an overview of the literature on the critical period hypothesis in second language acquisition. Thanks are also due to Martijn Wieling (currently University of Tübingen) for pointing out an error in the R code accompanying an earlier draft of this paper.

Author Contributions

Analyzed the data: JV. Wrote the paper: JV.

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The Critical Period Hypothesis and Language Acquisition

Concept map.

Eric Lenneberg's contributions to linguistics revolve around the biological foundations of language and the critical period hypothesis. His work, alongside Noam Chomsky, challenged behaviorist views, emphasizing an innate capacity for language learning linked to early childhood. Lenneberg's theories on brain development and language acquisition have influenced educational and therapeutic practices in language learning.

Eric Lenneberg

Early life and education.

Lenneberg was born in Germany and later obtained degrees from the University of Chicago and Harvard University

Biological Foundations of Language

Critical Period Hypothesis

Lenneberg proposed that there is an optimal time window in early childhood for acquiring language

Cerebral Lateralization

Lenneberg hypothesized that the brain's hemispheres are equally involved in language acquisition during the critical period

Collaboration with Noam Chomsky

Lenneberg and Chomsky worked together to refute the behaviorist view of language learning and advocate for an innate, neurobiological basis for language acquisition

Implications of the Critical Period Hypothesis

Bilingualism.

Lenneberg's theory is supported by the proficiency of children exposed to multiple languages before puberty in bilingual environments

Language Rehabilitation

Lenneberg's research has guided therapeutic methods for language rehabilitation after brain injuries by acknowledging the time-sensitive nature of language acquisition

Challenges for Adults

Adults often face difficulties in achieving native-like proficiency in a second language, especially in areas such as pronunciation and syntax, supporting Lenneberg's hypothesis

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Eric Lenneberg's birthplace and date

Born in Düsseldorf, Germany on September 19, 1921

Lenneberg's migration to the US and reason

Emigrated to the US in 1938 to escape Nazism

Lenneberg's academic path and degrees

Bachelor of Philosophy from University of Chicago in 1942; PhD from Harvard in 1956

In 1967, ______'s influential book, '______ ______ of Language,' discussed the optimal time frame for language learning in early childhood.

Eric Lenneberg Biological Foundations

The ______ ______ hypothesis, introduced by Lenneberg, suggests that language acquisition should ideally occur before the onset of ______.

critical period puberty

Lenneberg's critical period concept

Critical period is a developmental stage when the brain is optimally primed for language acquisition.

Role of cerebral lateralization post-critical period

After the critical period, language functions become more lateralized to the left hemisphere.

Impact of reduced neuroplasticity on language learning

As neuroplasticity decreases with age, acquiring new languages becomes more challenging.

In studying how we pick up language, ______ worked with ______, to challenge the idea that language is learned only through environmental influence.

Lenneberg Noam Chomsky

Critical Period Hypothesis in Language Education

Suggests early language exposure is crucial for proficiency; impacts teaching methods.

Genie's Case and Language Acquisition

Shows difficulty in learning language post-critical period; evidence for Lenneberg's theory.

Adult Language Learning Challenges

Adults struggle with native-like pronunciation and syntax; supports critical period concept.

Lenneberg's studies emphasized the importance of a ______ for language acquisition and have shaped educational and therapeutic language practices.

critical period

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Eric Lenneberg: A Trailblazer in Linguistic Theory

Eric Heinz Lenneberg was a prominent figure in the realm of linguistics, celebrated for his groundbreaking work on language development and acquisition. Born on September 19, 1921, in Düsseldorf, Germany, Lenneberg escaped the rise of Nazism by emigrating to the United States in 1938. His academic journey led him to the University of Chicago, where he obtained a Bachelor of Philosophy in 1942, and subsequently to Harvard University, where he earned his PhD in Psychology and Social Relations in 1956. Lenneberg's research was characterized by a deep interest in the biological foundations of language, which formed the basis of his scholarly contributions.

The Biological Foundations of Language and the Critical Period Hypothesis

Eric Lenneberg's seminal work, "Biological Foundations of Language," published in 1967, was a transformative text in the study of language acquisition. In this book, he introduced the critical period hypothesis, which asserts that there is an optimal time window in early childhood for acquiring language. Lenneberg proposed that this critical period lasts until around the onset of puberty, after which the ability to learn a language to native-level fluency significantly decreases. His hypothesis emphasized the role of biological factors in language learning, suggesting that the capacity for language is an innate feature of human biology.

The Relationship Between Language Acquisition and Brain Development

Lenneberg's theory delved into the neurological aspects of language learning, with a particular focus on the concept of cerebral lateralization. He hypothesized that during the critical period, the brain's hemispheres are equally involved in language acquisition. As the critical period ends, there is a pronounced shift towards lateralization, with the left hemisphere generally becoming more dominant for language functions. This change is associated with a reduction in neuroplasticity, which makes learning a language more difficult as one gets older.

Lenneberg and Chomsky: Refuting Behaviorist Views on Language Learning

In his research on language acquisition, Lenneberg collaborated with Noam Chomsky, another influential figure in linguistics. Together, they refuted the behaviorist view that language learning is solely a product of environmental stimuli and reinforcement. Lenneberg and Chomsky advocated for an innate, neurobiological basis for language learning that operates independently of one's cultural or genetic background. This perspective highlighted the natural and universal characteristics of language acquisition, proposing that all humans have an intrinsic capacity for language.

Practical Consequences of the Critical Period Hypothesis

The critical period hypothesis has significant implications for language education and therapy. For example, children exposed to multiple languages before puberty in bilingual environments often become proficient in both, which supports Lenneberg's theory. In contrast, the case of 'Genie,' a girl deprived of linguistic stimulation until the age of 13, underscores the difficulties of language acquisition outside the critical period. Similarly, adults often face challenges in achieving native-like proficiency in a second language, especially in areas such as pronunciation and syntax, lending further credence to Lenneberg's hypothesis.

The Lasting Impact of Lenneberg's Contributions

The work of Eric Lenneberg has had a lasting impact on the fields of linguistics, neurolinguistics, and language teaching. His insights into the biological underpinnings of language acquisition have informed the development of educational strategies that leverage the critical period and have guided therapeutic methods for language rehabilitation after brain injuries. By acknowledging the innate ability for language and the time-sensitive nature of its acquisition, Lenneberg's research has provided a comprehensive framework for understanding the mechanisms of language learning and processing in the human brain.

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Critical Periods in Science and the Science of Critical Periods: Canine Behavior in America

Brad bolman.

1 Institute on the Formation of Knowledge, University of Chicago

This article offers a canine history of the “critical period” concept, situating its emergence within a growing, interdisciplinary network of canine behavior studies that connected eugenically minded American veterinarians, behavioral geneticists, and dog lovers with large institutional benefactors. These studies established both logistical and conceptual foundations for large‐scale science with dogs while establishing a lingering interdependence between American dog science and eugenics. The article emphasizes the importance of dogs as subjects of ethological study, particularly in the United States, where some of the earliest organized efforts to analyze canine behavior began. Further, the article argues that the “critical period” is important not only for its lasting prominence in multiple fields of scientific inquiry, but also as a historiographical tool, one that invites reflection on the tendency of historians to emphasize a particular narrative structure of scientific advancement.

For over seventy years, scientists have been fond of “critical periods.” Linguist Eric H. Lenneberg's influential Biological Foundations of Language introduced the “critical period” for language acquisition in 1967, the idea that languages are most easily learned during particular developmental phases. 1 The United States “Head Start” program, established by President Lyndon B. Johnson in 1965, provided early childhood education to low‐income families and emerged during a period of psychological focus on “critical periods” in children's development. 2 For adults, on the other hand, therapists have explored whether there are “critical periods” for intervention into psychosis, 3 and, more recently, neuroscientists have used the “critical period” to describe phases of neuronal plasticity. 4 As psychologist John Colombo argues, the elasticity of the “critical period” concept allows “the inclusion of many phenomena from a wide range of disciplines.” 5

Variants on the term exist, but “critical period” remains a dominant way of understanding developmental change across fields. Following the citational trail of contemporary uses, however, returns us to an unexpected place: mid‐century studies of canine social behavior, a project led by behavioral geneticist and comparative psychologist John Paul Scott at the Jackson Memorial Laboratory in Bar Harbor, Maine. Scott's influential overview of the concept, “Critical Periods in Behavioral Development,” which appeared in Science in 1962, has framed discussion, and everyone from Lenneberg to the students of neuronal plasticity link their work, however elliptically, to Scott's. 6

This article, in turn, offers a canine history of the “critical period” concept, situating its emergence within a growing, interdisciplinary network of canine behavior studies that connected eugenically minded American veterinarians, behavioral geneticists, and dog lovers with large institutional benefactors, especially the Rockefeller Foundation. Jackson was preceded by the “Dog Farm” of anatomist Charles Rupert Stockard, who turned to dogs in order to test hypotheses about the endocrine system and human “racial degeneration.” Although historians of biology have typically regarded Stockard as an embarrassing oddity, I argue that this project established both logistical and conceptual foundations for large‐scale studies of dogs, while also revealing a lingering interdependence between American dog science and eugenics. 7 Research with dogs as experimental subjects, the article shows, involved a more complicated network of stakeholders than work with other laboratory organisms; dogs were discipline‐defying, inviting analyses of genetics, psychology, and behavior as well as emotions, personality, and heredity, which pushed studies in multiple simultaneous directions and disrupted efforts at unification. The article is thus simultaneously a history of canine behavior studies and a prehistory of canine “ethology”: a story of the numerous cross‐cutting fields which attempted to understand what dogs do . 8

I trace the origin of the Jackson studies from Stockard's aborted project as well as a shared interest in dogs between the Rockefeller Foundation's Alan Gregg and Jackson director Clarence Cook Little. 9 I argue that the practical complexity of training and understanding the behavior of dogs, especially the financial costs of doing so during a period when “standard” animals became a de facto expectation for animal experimentation, encouraged researchers to focus on the early stages of life – “critical periods” – because preliminary mistakes could undermine years of careful research. In telling this story, the article emphasizes the importance of dogs as subjects of behavioral study, particularly in the United States, where some of the earliest organized efforts to analyze canid behavior began.

The article builds on existing studies of experimental animals and “model organisms,” work which has revealed how nonhuman beings become crucial elements within both the conceptual and practical infrastructure of scientific work. 10 Because dogs are expensive research subjects, large‐scale experimentation historically required wealthy institutional funders, whose own aims and proclivities shaped the forms that canine behavior studies took, and the article thus emphasizes the importance of external funders in producing canine science. Yet dogs have been comparatively undertheorized as research organisms by historians of science, and, concomitantly, canine studies have been treated as minor pieces in the development of the science of behavior. One key reason is that it was only in the late twentieth century that social behavior studies of dogs were (re)conceptualized as canine ethology : during the development of the discipline of ethology, dogs appeared too complex for the study of “instinct” that Richard W. Burkhardt argues characterized the early field. 11 Most historical writing about ethology thus focuses on the key research organisms of the field's founders: notably, birds and fish. That focus has been important, but it has left historians and philosophers with few tools to approach the contemporary prominence of studies of canine behavior, which have seen a sharp increase over the last thirty years and made dogs a behavior “scholar's best friend.” 12 The article ends by exploring two outgrowths of the Jackson work, comparative studies of the behavior of dogs and wolves by Raymond and Lorna Coppinger and Harry and Martha Frank, which reveal how Scott and Fuller's “social behavior” program was reimagined as a founding moment in the development of canine “ethology.”

Inattention to dogs has also produced gaps in our understanding of the most influential concept to come from canine behavior research: the “critical period.” I argue that the “critical period,” which Scott and Fuller rearticulated and popularized, is important not only for its lasting prominence in multiple fields of scientific inquiry, but also as a historiographical device. The Jackson project emerged at what many understood to be a “critical period” in American scientific activity and behavioral study, which allowed for pathbreaking research that could not be replicated later; the popularizers of the term also used it to understand their own lives. By historicizing both the studies and the concept, I aim to reveal tensions in its use as a framing device. Histories of science have long been tied, both implicitly and explicitly, to a critical period approach focused on particular early moments of dramatic expansion or insight for scientific projects. That approach has value, but I argue also for an attunement to knowledge production which appears less critical. The excavation of canine behavior science here offers an abbreviated example of how this might proceed.

Late in 1925, primatologist Robert Mearns Yerkes received a request for Yale's support in research on experimental breeding in dogs. “The races of dogs are just as numerous as the races of men,” explained young dog breeder Leon F. Whitney, former Executive Secretary of the American Eugenics Society. Whitney was a prominent, if at times controversial, voice in the American eugenics movement: his popular book Sex and Birth Control suggested a path to racial “improvement” through controlling the reproduction of “degenerates.” 13 As Stefan Kühl has shown, Whitney was also important in bridging America and Nazi Germany, expressing his admiration for Nazi sterilization laws and sending a copy of his The Case for Sterilization to Adolf Hitler. 14 For Whitney, canine studies offered essential evidentiary support for such projects: “I believe that so much of eugenical and genetical importance can be found out from dogs.” 15 Yerkes, another noted eugenicist, was supportive, but Whitney's proposal arrived just a few months late: that March, Charles Rupert Stockard had received funding from John D. Rockefeller's General Education Board (GEB) to undertake a similar program with nearly parallel justifications, bringing far more scientific prestige than Whitney himself.

Stockard, a leading American anatomist and embryologist, graduated from Columbia in 1906 and began teaching at Cornell Medical College in New York City. There he would rise to chair the anatomy department and remain for the rest of his career. In 1925, he met with Wickliffe Rose, head of the GEB, to inquire about support for “a breeding experiment with dogs,” requesting $12–20,000 in order to purchase a fifty‐acre farm, hire staff, construct relevant buildings, and acquire animals. 16 Stockard's focus was the endocrine system and inheritance of “glandular types.” The glands, historian Michael Pettit argues, offered a useful framework for theorizing biological determinants of individual character before DNA. 17 Such research was frequently coextensive with eugenic studies, and Stockard's was no exception: “Many of these breeds of dogs have been bred pure for several hundred years,” he noted, “and show definite glandular complexes entirely comparable to similar glandular conditions seen in peculiar human beings.” 18 His embryological work also attuned Stockard to the early stages of organismic development – what he called, like some other developmentally‐attuned biologists, “critical moments.” 19 Stockard thought breeding experiments with dogs might reveal the importance of such moments with explicit lessons for human beings.

In The Physical Basis of Personality , an adaptation of his Lane Medical Lectures at Stanford University in 1930, Stockard offered some of the first conclusions from the ongoing work, which he anticipated would take at least another decade to bear fruit. “[I]t is of great significance,” he noted, “that certain human freaks practically parallel in their growth and form these diversified canine types.” 20 Many of the modern dog breeds were evidence of glandular defects, Stockard wrote, and “Strangely enough, dog fanciers have been unconsciously selecting and preparing a splendid array of material, most ideally suited for a scientific investigation both of the inheritance of endocrine gland disturbance and of the effects of such disturbances on the growth and form of the mammalian body.” 21 In their pursuit of unique appearances and capacities, dog breeders had completed most of the leg work of producing an excellent laboratory organism for eugenic analysis.

Stockard's “Experimental Morphology Station,” which most knew as the “Cornell Dog Farm,” received over $400,000 (close to $8 million today) during its years of operation. What the GEB and later Rockefeller Foundation (RF) received from their investments, however, was less clear: the Dog Farm released no annual or monthly reports, and Stockard published minimally about the work. One reason was the unanticipated difficulty of working with dogs as experimental subjects: scattered notes reveal that the project's earliest years were spent simply determining how to breed, feed, and keep dogs in a healthy and sustainable manner. Dogs could not live entirely inside, like the hamsters Stockard was used to, but outdoor life invited parasites, which spread easily between animals. “As quickly as the animals are cleaned of hookworm they become reinfected by larvae eggs from the ground,” Stockard moaned in February 1928. 22 Constantly thinking of the dogs as human analogs, he compared the laboratory's disease challenges to those of urbanization: “It is much the same sanitary proposition which a human community faces in growing from a sparsely settled village condition into a thickly populated town.” 23 Dogs made for persuasive experimental subjects, but their management was far beyond what Stockard predicted.

Stockard was himself obsessively focused on the glands, but the dogs seemed to demand broader, behavioral consideration, so in 1932 he brought in William T. James, who worked at Cornell under experimental psychologist Howard Liddell, one of the leading American practitioners of Pavlovian conditioning. 24 James constructed new experimental apparatuses to hold dogs in place during physiological studies and found that heredity strongly influenced the presence of dominance behaviors, in turn determining how well dogs responded to laboratory conditioning. 25 Stockard, however, died unexpectedly in 1939, leaving James’ work and his own magnum opus on the dog studies unfinished. “In regard to the behavior of the different types, the work has just begun,” James wrote in a memo on the future of Stockard's farm. 26 Desperate to keep his research active, James suggested converting the farm into a more general experimental facility for those interested in dogs, or perhaps a production site for purebred laboratory animals. His pleas held little sway: the “Dog Farm” disappeared shortly thereafter, and James returned to Ithaca, with Liddell's help, where he used equipment and animals from Stockard's facilities to build a new experimental dog kennel, shaping canine research at Cornell for years to come. 27

2. Little's Dogs

Alan Gregg, head of the Rockefeller Foundation Medical Sciences Division, which funded Stockard's research in later years, had hoped the project would generate insights into the inheritance of mental ability, with implications for educational policy. Even after, he remained interested in further work on dogs, and contacted prominent scientific figures between 1942 to 1943 about the possibility of such dog studies. 28 Edwin B. Wilson, a statistician at Harvard's School of Public Health, suggested Clarence Cook Little, director of the Jackson Memorial Laboratory: Little had already started breeding dogs for his cancer studies and even judged in the Boston Dog Show. 29 Before turning to mice during his graduate studies with Harvard biologist William Castle, Little had “wanted to follow in his father's footsteps and breed dogs […] for genetic study,” and continued to publish small articles and book reviews on doggy and eugenic topics. 30 In a review of Elliott Humphrey and Lucien Warner's Working Dogs in 1935, Little noted that “dogs, properly studied, would provide a sorely needed foundation for a great deal of progress in comparative psychology.” 31 Gregg's suggestion found open ears.

The logistics were manageable, Little felt, and a laboratory could outsource the housing of dogs to local farmers in order to save money. Without explicitly volunteering Jackson, he made clear that his facilities could handle the project – Stockard's histologist, Emilia M. Vicari, was also already working there. To run the program, Little recommended Hans Grüneberg, a geneticist working with J. B. S. Haldane in England. Grüneberg and Little had corresponded over the years about mice, and Little thought the young émigré could sustain the dog program after his own eventual retirement. In 1944, Little inquired whether Grüneberg might be interested in participating in something that would be “far greater” than what “Stockard and others” had attempted, 32 a possible reference to Grüneberg's caustic criticism of Stockard's work as a “nightmare dog show” in The Eugenics Review . 33 The Jackson job, Little insisted, was “the best opportunity that exists today in mammalian genetics.”

Little further proposed that John Paul Scott, whose “traits and abilities […] would supplement those of Gruneberg successfully,” be brought in as second‐in‐command. 34 University of Chicago geneticist Sewall Wright, whom Little frequently consulted, had recommended Scott, a former student who had visited Jackson as a summer researcher in 1938 and 1939. 35 Scott jumped at the opportunity, excited to escape a frustrating position at Wabash College and to pursue a new interest: from 1938–1942, he had become acutely interested in applying biological techniques to the study of sociality, a new approach which he called “sociobiology.” 36 From this work, Scott developed the concept of the “behavior pattern,” a unit of behavior with definite function, one which paralleled Tinbergen's “ethogram” – Scott claimed he was not aware of Tinbergen until much later. 37 Jackson appeared to be the ideal place to expand on this early approach. Grüneberg, caught up in the hostilities of World War II, could neither accept nor refuse, so planning continued without him.

Dog work was expected to take place at “Hamilton Station,” a fifty‐five‐acre farm across from Jackson's main building, donated in 1940. The site had three barns, which could be retrofitted to house dogs and other facilities, as well as a small histopathology lab. Its size revealed the infrastructural challenges of work with canines: where mice could be kept within a few large rooms, dogs were thought to require large pens as well as frequent access to the outdoors – an increase of space by multiple orders of magnitude. If kenneled too tightly together, fighting and disease outbreaks tended to rise up, which had been Stockard's great challenge. In addition, the cost of continuously feeding even a dozen dogs was significant, and researchers routinely complained about how much of their budgets went to food.

When Grüneberg ultimately declined Little's offer, it was left to Scott, whose intellectual inclinations were broader than Little or Gregg recognized, to run the program. In an early memo, Scott specified interest in “differences in the physiological background of behavioral and emotional differences,” and under his guidance the Jackson study of the genetics of intelligence quickly became a study of genetics and behavior, with growing emphasis on the latter. 38

The new study was announced to the public in The New York Times in May 1945. 39 Representatives from America's humane societies promptly wrote to Little in protest, but he brushed away the concerns: their precious little dogs, locked away in apartments, were treated worse than the study's animals would be. Broadly, however, the project was met with support: in November, Gregg met with British statistician and biologist Ronald Fisher – another prominent eugenicist – who noted that “one of the most valuable things that could be done in genetics would be work with dogs in studying temperament and nervous disposition.” 40 Walker M. Dawson, who had planned a study of canine intelligence at the Department of Agriculture's Beltsville experiment station to follow up Stockard's project – later scrapped due to World War II – arrived toward the end of January 1946 to advise. 41

As construction neared completion on the “behavior laboratory” in February, Scott planned a conference on “Genetics and Social Behavior,” intending to bring together leading lights and promising young scientists to “build up enthusiasm and lay the foundations for cooperative work.” 42 A list of key topics for the conference heavily emphasized the behavioral elements of the research, but included a significant number of comparative psychologists, too, because many moved somewhat fluidly between the fields of “behavior” and “psychology.” 43 Little was preoccupied with lab management and other concerns, while Gregg was slowly edging toward retirement and avoided dictating the direction of research projects funded by Rockefeller. As both increasingly took a backseat, behavior's centrality grew. Rather than carefully interbreed dogs to identify particular visual or cognitive traits, as Stockard had done, or set out to produce a hyper‐intelligent dog, as Gregg and Little had imagined, the program of research emerging from the conference placed a heavy emphasis on using learning tasks and behavioral observation to understand the social life of dogs. For Scott and many others, the violence of World War II had demonstrated the necessity of analyzing the causes of both aggressive and cooperative behaviors.

With Grüneberg out of the picture, Scott needed his own second‐in‐command and invited John L. Fuller, an MIT‐trained physiologist who spent the war years at the University of Maine working on submarine detection and marine biology, to join the staff in the summer of 1947. With an additional grant, Scott was able to purchase an electroencephalograph machine for Fuller to carry out brain measurements, which were considered essential to understand the cognitive aspects of canine behavior. 44 Scott also invited James to visit Hamilton and to guide Fuller on the construction, assembly, and testing of apparatuses to restrain dogs in experimental settings. 45 Stockard's project thus left a technological imprint on the work at Jackson, and a lively exchange between the dog project at Jackson and work at Cornell continued: Scott visited the dog facilities which James built at Cornell for inspiration in 1945, and, in exchange, James brought dogs from Jackson to the University of Georgia for his own new colony. Jackson had begun raising five breeds of dog, selected originally by Little to represent different “personality” types: Basenji, Beagle, Cocker Spaniel, Shetland Sheep Dog, and Wire‐haired Fox Terrier. Each breed also reached a similar size at maturity, alleviating the need for testing devices of multiple dimensions, which was particularly important because apparatuses, including specially crafted test boxes, were often built by the laboratory at no small cost.

The “School for Dogs,” as Scott and Fuller sometimes called their project, progressed rapidly. Aware of the challenges that befell Stockard, Gregg urged them not to mine “more than you can smelt,” and to “run the smelting as much as possible”: publish often, in other words, rather than wait for the study's conclusion. 46 By March 1948, the lab had isolated multiple behavioral “abnormalities,” such as excessive fearfulness, and planned twenty‐four outdoor pens in order to establish a baseline for what constituted “normal” behavior in a somewhat controlled setting. 47 In August, Fuller indicated they had identified genetic differences in juvenile adjustment to group behavior and in the attitude of female dogs to weaning. 48

Because staff was limited, Scott and Fuller took on responsibility not just for overseeing the project, but also for caring for the dogs. Scott required “strict adherence” to rules of interaction with the dogs: physical punishment and loud noises were not permitted, special handling and petting were to be avoided, and rewards for participation in tests had to be uniform. 49 The resulting experimentation was a mix of traditional pet training strategies and problem‐solving and behavioral tasks – the type of conditioning work pioneered by Liddell, James, and others – carried out by experimenters (or “teachers,” as they were sometimes called). 50 As an example of the former, one of the lab's crucial early tests was leash training, which involved rewards for walking with an experimenter on a leash. This contrasted with more complicated experiments to ascertain differences in breed ability, such as a tracking test in which “fish juice was smeared on metal plates” which were placed alongside non‐treated plates to see how dogs differentiated between smells. 51

Some canine behavior tests preexisted Jackson: Pavlov, for instance, had pioneered tests of how dogs learned to avoid stimuli. But the Jackson lab carefully revised these tests and devised numerous others, such as the leash test or a test of “climbing” ability that involved having the dogs walk across a narrow, elevated plank. Results were carefully tracked using dozens of paper forms, carefully catalogued in relation to individually identifiable animals. The outcome was a sizable, quasi‐bureaucratic apparatus documenting skill and ability, literalizing the “school” metaphor. The battery of tests was advanced enough that Scott and Fuller assembled a Manual of Dog Testing Techniques , which included not only guidance for carrying out tests but also for handling dogs and constructing devices. 52 They forwarded it to universities and prominent scientists around the world, including members of the “Committee for the Study of Animal Societies Under Natural Conditions” (CSASUNC), the organization Scott was secretary of and which had emerged directly from the first Jackson conference. 53 The manual's short bibliography included only one book on husbandry: Leon Whitney's How to Breed Dogs (staff member Edna DuBuis had visited Whitney for advice on doing so in 1948). 54

The lab also opened itself to short‐term visits from experimenters eager to make use of high‐quality dogs. So‐called “standard” dogs, those with known ancestry and health histories, had become a focus of researchers in America and Europe during the end of the first half of the twentieth century, but remained difficult to raise or acquire. 55 Jackson was one of the first places to make such animals available, and James was only the first of a steady stream of experts to visit Hamilton Station in order to work with them. Those who could not travel to Bar Harbor could also purchase dogs at a premium, one of the first instances in which purpose‐bred “scientific dogs” could be bought directly. In this way, Hamilton Station instantiated many of James’ original hopes for the aftermath of Stockard's farm: a semi‐permanent site for the production of high‐quality experimental dogs as well as for visiting researchers hoping to utilize the facilities.

What kind of science was happening at Hamilton Station exactly? Scott and Fuller understood and described their research as a study of “genetics and social behavior.” They were interested, the two wrote in 1965, in “behavioral systems,” groups of related behavior patterns that possessed a common function, which situated their work in “the domain of the twin sciences of psychology and ethology.” 56 Much of their work indeed mirrored ethological research, which Scott and Fuller described as a “collector's job” of noting basic kinds of behavior, but it differed in refusing the concept of “instinct,” which was “too inexact to be of much value in modern scientific work in the description of behavior.” Canine behavior was too complex for bifurcation into “instinctive” and “learned” behaviors: patterns and behavioral systems were intricately interwoven and learned.

How, then, did behavioral systems work? In 1950, Scott and Mary‐‘Vesta Marston first publicly invoked the “critical period,” a concept which would frame the lab's work for years after, in an article describing key developmental phases in dogs’ learning. Scott's grant proposal in 1949 contained his own first explicit reference to “critical periods,” 57 but historian Gregg Mitman notes that Scott's graduate training included a strong emphasis on the interaction between organism and environment during development, which “infused” the “notions of thresholds and critical periods in developmental biology” into his research. 58 In graduate work with “monster” guinea pigs, for example, he had shown growth could be accelerated at a “particular period” by certain genes, a finding that Scott connected to Stockard's “critical moment,” the phase when “abnormal” development could be avoided. 59 Lorenz's “imprinting,” from the 1930s, was also influenced by embryological approaches such as Stockard's, the same ones that geneticist Richard B. Goldschmidt drew upon to argue for a “critical period” in the production of “phenocopies” in Scientific American in 1949. 60 Scott was familiar with much of this literature and saw a constellation of “critical period” research connecting Stockard, Lorenz, and himself.

Yet where “imprinting” referred to specific attachments formed at particular moments in the lifecycle (and implied instinct ), Scott behavioralized the “critical period,” rearticulating the term as a general way of naming rapid changes to behavioral systems during multiple phases of development. Scott's emphasis in the lab was on early experience, and preparatory studies of basic canine behavior, undertaken to ensure that as much as possible was known about each dog so results would not be marred by improper care or treatment, had revealed the existence of “certain natural periods in development during which the puppy is unusually susceptible to environmental influences.” 61 Frugal necessity thus partly inspired the lab's key concept, but so did its setup and Scott's long‐term attention to noting behavior patterns: Jackson had just enough dogs, experimenters worked closely enough with each of them, and dog behavior appeared complex‐but‐recognizable enough that periods of learning and behavioral change could be tracked and quantified in a more highly systematic way than comparable primate labs could.

Quickly, the “critical period” dominated research at Hamilton. The same year as Scott and Marston's article, summer student Barbara Arndt explored the effect of environment on the “critical period” for young beagles, 62 and Fuller and others followed with a study of conditioned avoidance in puppies which adopted the “critical period” frame. 63 Researchers carefully observed and noted the appearance of behaviors, such as nursing and crawling in the “neonatal” period or walking and play‐fighting in the “transition” period. Persistent conditioning, the tool of choice for dogs generally and part of the lab's commitment to training tests, revealed temporal demarcations between the periods: after about ten days of life, puppies were dramatically more capable of learning new skills than beforehand. In much of this work, which detached the “critical period” from embryology and connected it instead to behavior and learning across an organism's life, the concept remained a “postulate” or “hypothesis.” But Scott also promised in 1950 that the hypothesis, “which has been a contribution of the group as a whole,” would “lead to important progress” in understanding “abnormal behavior.” 64 (For Scott, that particularly meant aggression and asocial tendencies.) The early eugenic project of producing “deviant” types and Little and Gregg's vision of an “ideal” hyper‐intelligent dog had set the stage for the “critical period,” but the latter's emphasis on learned behavior shifted the focus away from inherent, genetic difference.

In 1951, Scott, Fuller, and associate Emil Fredericson published a more extensive inquiry into the concept. Scott's preliminary research had shown that periods of extensive isolation for lambs produced marked antisocial behavior and pointed to “the major importance of early experience.” Yet similarly isolated puppies quickly readjusted themselves to social life with other animals, producing the “superficial conclusion” that early experiences were not important for shaping behavior. 65 The existence of “critical periods,” which could differ across species, resolved the paradox and allowed for both results to hold simultaneously. “The changes which take place are so precisely timed and the metamorphosis in behavior which occurs just before three weeks of age is so remarkable that the dog is […] a wonderful animal for making experiments on the effects of early experience,” Scott wrote. 66 The accelerated pace of canine lives, it was argued, could more quickly reveal such developmental milestones than studies of human beings. 67

Not everyone agreed: external advisor Frank A. Beach, who long believed the lab should focus on genetics rather than social behavior, wrote Fuller to express misgivings about the thoroughness of proof regarding the “critical period.” 68 In a 1954 review article for the Psychological Bulletin , Beach and Julian Jaynes noted that it was “tempting” to suggest similarities between the proposed critical periods in dogs and parallel phases in human development, “but this would be unwarranted on the basis of present knowledge.” That did not stop an eager public from doing so. Parade magazine writers Karl Kohrs and Sid Ross shared news in September 1950 from the “world‐famous animal laboratory” of the “critical period” in which puppies needed to learn new behaviors. The findings, they explained, were suggestive for human development: “What researchers now want to know is this: ‘Is there a “critical period” in the life of human beings?’” 69 Because Parade appeared across major American newspapers, much of the country was introduced to the Hamilton Station project that September; a similar, shorter article appeared in national newspapers months earlier. 70 The critical period was, Scott noted, potentially “a long step forward in preventive mental hygiene.” 71 Although popular attention centered on its implications for human learning, dog owners were also quick to take up the concept, with three of the basic periods Scott emphasized (neonatal, socialization, and sexual maturity) becoming core phases in puppy training guidance, particularly for guide dogs. 72

1950s America also offered a supportive social context for the term: it was a “critical period in our history,” a “critical period in the Korean peace talks,” a “critical period for corn production,” or a “critical period in building up Western defenses,” depending upon the day and the newspaper. 73 In the post‐war United States, many understood themselves to have witnessed profound historical change and to be standing at the precipice of further transformations. But the “critical period” also emerged at a threshold moment for Hamilton Station: with behavioral tests completed, the lab began crossbreeding its two behavioral “extremes” – cocker spaniels and basenjis; the former understood to be most human‐oriented, the latter more “primitive” and wolf‐like – to better understand how heredity shaped behavior, and Scott and Fuller published widely (Beach thought too many were of minor importance). After its neonatal period, spent building facilities and operational capacity, Hamilton was now ready to share and socialize.

But problems were not far off. Almost immediately after a positive progress report in 1953, a distemper epidemic swept through Hamilton and killed most of the puppies at a weaning stage, halting crossbreeding for nearly a year. 74 In March 1955, as Scott and Fuller were planning for the future of the project, which would soon run out of funding, RF officers confidentially agreed the behavior studies were “not a project for which the Foundation would like to continue full support,” due to concerns both about Scott's leadership abilities and the likely cost of future work. 75 Yet many acknowledged that terminating the studies would eliminate one of the more substantial behavior programs in existence: “A similar meritorious project could not be conducted less expensively elsewhere,” noted Kansas anatomist William C. Young in 1955. 76 “It seems to me,” wrote psychologist Donald O. Hebb, “that Bar Harbor and Orange Park [the site of Yerkes’ primatology lab] ought to be the two great assets of American (and any other) psychology.” 77 With an end nonetheless in sight, Scott insisted that the project would “stand as the best performed and most impressive piece of research which has ever been done in the field of genetics and behavior.” 78 The differences of opinion were striking, and they reflected the ambiguous position of controlled behavioral studies of dogs, which interested some American researchers but convinced many others only of the necessity of further work on nonhuman primates – or human beings.

Scott and Fuller stayed at Jackson for a period, but the meat of their behavior work ended. The effects of their painstaking studies were diffuse, but no less significant for it. The lab spurred the CSASUNC, the Animal Behavior Society, and the New York Zoological Society's “Wildlife Research Station” in Jackson Hole, Wyoming, and dogs were sent from Bar Harbor to labs across America and Europe. Hamilton Station, Beach noted, had “a very considerable effect on stimulating work on animal behavior in general.” 79 This had always been Scott's hope: “I have deliberately and strongly presented the critical period hypothesis in the hope that this will catalyze work elsewhere,” he explained in 1951. “A good research hypothesis has often had a very stimulating effect in other fields of research.” 80 So it was with the “critical period.”

Few could afford to produce an experimental system comparable to Jackson's, where it took years and thousands of dollars to get close to “good” results. Breeding purebred animals was complicated and slow, and training the dogs required substantial investment and expertise. Hamilton Station had resolved many of Stockard's early problems concerning keeping and feeding experimental dogs, but preparatory work took so long that it nearly cannibalized the study once more. Subsequent research on canid behavior followed, but nearly always with smaller and shorter‐term experimental setups: many wanted to expand on Scott and Fuller's studies, but few could afford to. 81 Instead, with limited prospects for establishing full behavior laboratories, future work with canids would instead increasingly focus on applied research and comparative analysis of different canids, combining the techniques of Jackson with the methods of “natural” or outdoor observation more characteristic of European ethologists. Two of the more prominent examples reveal a remarkable degree of parallelism: they both drew heavily on Scott and Fuller's work, were both carried out by cooperative husband‐wife pairs (Scott noted late in life that his own wife should have been a listed coauthor on most of his publications), 82 and both ultimately resulted from curiosities about pet dogs. They also returned attention to the “critical period” and helped to ethologize canine behavior studies.

Raymond C. Coppinger studied animal behavior at the University of Massachusetts in Amherst, exploring experience and novelty as factors in the feeding behavior of birds; his wife, Lorna, studied their visual acuity. The two shared a semi‐rural home in central Massachusetts and, after the chance purchase of a Husky‐mix, visited veterinarian and former sled dog racer Charles Belford to ask if the dog could compete. The question, Belford explained, could ultimately be answered only by watching the dog run. After exploratory attempts, Coppinger caught the dog sledding bug: from the initial Husky, he purchased others, filled the yard with animals, won regional races, and gradually established his own strain of racing dogs. 83 When Hampshire College opened in 1969, Coppinger became one of the founding faculty and turned increasingly to studying canine behavior, drawing on ethological approaches from his early work with birds. Much of his early research remained centered on sled dogs: analyses of how they maintained body temperature, for example, or sweated. 84 Lorna, meanwhile, finished her degree and published one of the first histories of sled dog racing. In the mid‐1970s, however, they began a new project on the performance of sheep‐protecting dogs – with its own, distinct Rockefeller roots.

Winthrop Rockefeller, the grandson of John D. Rockefeller, built a 927‐acre experimental farm on Petit Jean Mountain, northwest of Little Rock, Arkansas, called “Winrock Farms” in the 1960s. After his death in 1973, the property was converted into a charitable research and training facility known as the Winrock International Livestock Research and Training Center – “a Rand Corporation […] for animal agriculture,” as one commentator put it. 85 In 1976, Winrock hosted leaders from the nation's sheep industry along with scientists, such as the Coppingers, to discuss new strategies for effective range management. Wolves and other canids had wrought havoc on herds grazing on public lands after bans on a poison known as “compound 1080” in 1972. A 1979 directive requiring nonlethal, noncapture methods on federal lands exacerbated the challenge. 86 Although a number of “modern” approaches to predator control were attempted, including chemical sheep collars and traps, few proved fully effective in slowing predation. Deploying dogs as protection, a practice common in Europe but relatively rare in the United States, emerged as an alternative. The Denver Wildlife Research Center had trained Hungarian “Komondor” dogs in 1976 for this reason, but the program was cancelled despite evidence of success. 87 William Dietel of the Rockefeller Brothers Fund, who knew the Coppingers from his time teaching at the University of Massachusetts and Amherst College, brought in Raymond and Lorna for the project. 88 After a year traveling across the United States to visit farms for their “Livestock Dog Project,” it became clear that the breeds currently in use were unsuccessful and untrusted – as likely to eat the sheep as protect them.

With funding from Winrock, the Coppingers visited Europe and acquired ten puppies from three Old World shepherding breeds: Italian Maermmano‐Abruzzeses, Yugoslavian Šarplaninacs, and Anatolian shepherd dogs. They raised the puppies near Hampshire and placed seventy of the progeny on farms and ranches around the country. 89 Contrary to some expectations, the dogs performed remarkably well in limiting wolf activity. More surprisingly, unlike herding breeds such as Border Collies, which required careful training, the guarding behaviors among European shepherd dogs appeared largely unlearned. The dogs adapted quickly to life with sheep, performing sexual and playful behaviors: as the Coppingers summarized, “The dog is treating the sheep like other dogs; the sheep are treating the dogs like other sheep.” 90 The findings accorded with what Scott and Fuller found at Jackson, but further emphasized the power of Lorenz's “imprinting”: dogs bonded to sheep during early critical periods, yet guarding behaviors reappeared in each generation in contrast to other breeds raised under similar conditions.

As their explorations of canine behavior deepened, the Coppingers turned explicitly to Scott and Fuller's Genetics and the Social Behavior of Dogs (1965), one of the first scientific books on dog behavior they could find, and Konrad Lorenz's Man Meets Dog . Visiting Lorenz in 1977, they were greeted by a surprising statement regarding some of this theories of canine origins: “I just want to say, everything I've written about dogs is wrong.” 91 Rather than the specifics, then, they adopted Lorenz's naturalistic ethological approach and his concept of imprinting, while they glommed on to the “critical period” from Jackson: the “period when dominance hierarchies are formed and dogs learn […] what species they belong to.” 92 It was the critical period that seemed to explain differences in behavior between Border Collies and Maermmano‐Abruzzeses. The pet dog industry, the Coppingers wrote, “still doesn't seem to understand its significance for traits such as temperament and learning.” 93 Still focused on determining which dogs were most “intelligent,” dog owners failed to recognize how “critical periods” shape behavior and that what dogs do is more important than what they are . The Coppingers affirmed what Scott considered the basic finding of the Jackson studies: “[F]or the most part, there are relatively few differences in […] pure intelligence, among dogs.” 94 Instead, differences in behavior had to do with motivation and emerged from early life experiences. Almost fifty years late, Scott and Fuller had finally found an eager and attentive audience. But by publishing results in journals like Ethology and describing their work as “ethology” as much as “social behavior” studies, the Coppingers also helped to legitimate the dog as an object of mainstream ethological inquiry.

A parallel ethological project was started by Harry Frank, a psychologist at the University of Michigan in Flint, in the 1970s. While still a graduate student at the University of Colorado in 1969, Frank acquired a Malamute‐Elkhound mix. After “being erroneously informed that malamutes are at least 1/8 wolf,” he began to notice similarities in his dog's behavior and that of wolves, triggering an interest in comparisons of the two canids. 95 Frank and his wife, Martha G. Frank, acquired a few wolves from a Minnesota game park and raised the animals themselves, studying their behavior from 1979 to 1981 as part of what they called the “University of Michigan canine information‐processing project.” 96 To explore how the animals learned and evaluated information, the Franks employed a series of cognitive tasks, most of which came from Scott and Fuller's Manual of Dog Testing Techniques , and drew on unpublished data that Scott shared with them directly to establish cross‐species comparisons.

Because they also added a few newly developed tests, the Franks employed four of their own Alaskan Malamute puppies for comparative results. Malamutes, unlike the breeds employed at Hamilton Station, were imagined to be relatively free of selection for specialized traits, were “lupine” in their size and general morphology, and evolved in the same environment as the timber wolf, but Frank hypothesized evolutionarily divergent cognitive capabilities. Wolves would have “the capacity for foresight, mental representation, and the beginnings of an understanding of means‐ends relations,” while domestic dogs would underperform on behavioral tasks except when the experimenter's presence mattered. 97 The Franks found those differences, but also noted caveats in normalizing the animals’ early experience. The wolves’ test performance was nearly identical to that of Scott and Fuller's basenjis, for instance, suggesting little fundamental difference between dogs and wolves. What hamstrung the wolf project most was the limited nature of its sample: only one breed of dog and a handful of test subjects. The Franks could not replicate the expansiveness of Hamilton Station, which left definitive answers for future researchers somewhere down the line, but their study exists alongside the work of the Coppingers as two of the key projects in a “second‐wave” of canine behavior studies that have today brought attention to dog ethology as a key area of academic and popular interest. 98

Scott never gave up the critical period concept, continuing to publish about it long after he left Bar Harbor. Although he at first understood the “critical period” in connection to behavioral development, in 1974, he and co‐authors argued that a “general theory” of critical periods could be used to understand organizational processes in any living system. 99 Many others, in a wide array of fields, have taken up and made use of the term. But Scott also continued to use it as a way to understand his life, work, and place within history; all the world was a critical period, starting or ending. Reflecting on his career in 1986, Scott noted that he left Jackson for Bowling Green because the latter “was undergoing a critical period” during which he could help build the psychology program. His arrival to Jackson in 1945, too, was timed for the start of a critical period: “It was getting reorganized and expanding, and just about everything was possible.” 100 If he stayed for his whole career, however, Scott worried that he would “run out of new ideas.” Why did some of his concepts catch on widely, while others struggled? “The answer seems to lie in timing […] for some of them the time was right.” 101

It is easy to dismiss the critical period as something Scott could not relinquish. But the concept also offers an approach, as he recognized, for understanding and describing the conditions for and state of scientific research. The dog behavior project at Hamilton Station emerged during a critical period of funding for biological programs in the United States and required many pieces to align perfectly: intersections of eugenics and dog breeding, Gregg's institutional connections, Little's canine fascination, the mid‐ and post‐war period of expanded interest in behavior. The major sums of money funneled into the project allowed for an expansive research program that rapidly produced journal articles, conference presentations, new academic networks, a string of students, and a new understanding of canine behavior, including the “critical period” itself. Hamilton Station emerged at the right moment and, despite work in decades since, has been difficult to replicate or surpass. The Coppingers and Franks worked in the shadow of that critical period.

In this article, I have used Scott's critical period, at multiple levels, to tell a new story about the science of canine behavior and its relation to ethology more broadly. Hopefully, that new account comes at the right time. In making this move explicitly, I also mean to draw attention to how historians of science are themselves attracted to critical periods: of scientific discovery, travel, and organizational or disciplinary growth; of conceptual “births,” work in its “infancy,” and “mature” scientific programs. Just as the scientific “critical period” moved within a constellation of parallel terms – essential moments, vital phases, important periods – so, too, have historians participated within a broader discursive formation we could think of as a critical period approach. Much history of science, as well as Science and Technology Studies, notes historian Dániel Margócsy, “tends to focus on beginnings and growth” rather than breakdowns and endings. 102 The early moments or productive middle phases in experimental communities remain key set pieces in stories of the development of science. Perhaps they should be: Scott's insistence on the concept might lead us to ask whether there truly are “critical periods” in science. But that approach can also generate an analytic perspective that overlooks subtler, subterranean processes which may support both the emergence and disappearance of scientific projects. Canine behavior studies are not the center of the story of ethology, but they are still significant to it, woven from lost and forgotten roots. Attention to failed programs and network breakdowns, to “how experiments age,” might be essential for understanding how some periods become (regarded as) so critical. 103

My approach here has been to tie the “critical” periods to those less obviously so, to use Scott's concept to understand the humans who produced it, as much as the dogs. Numerous scholars of science have focused on the Jackson project's emergence, yet few have carefully analyzed its linkages to Stockard, without whom it would have been unthinkable. The breakdown of Stockard's network freed up individuals, such as James, who transferred devices and techniques to Jackson and elsewhere. Stockard, in turn, required the sudden arrival of substantial institutional funding, but equally the American eugenics movement's ambiguous relation to the breeding of dogs. Many of the same actors, such as Whitney, floated across projects, often at the margins, sometimes accomplishing but just as often frustrating their hopes. More recent scientific studies of canine behavior, including work by the Coppingers and Franks, relied upon the prolific critical period at Hamilton Station, but they also emerged from their own particular moments, propelled by a resurgence of interest in pet and working dogs. Those studies, joining broader ethological attention to larger mammals, resituated dogs and other canids as key pieces in the work and the story of ethology, one that we are only beginning to fully understand.

B. Bolman, Ber. Wissenschaftsgesch. 2022 , 45 , 112. [ PMC free article ] [ PubMed ]

n1 Lenneberg 1967.

n2 “To some extent, this obsession arose because psychology was overwhelmed by the ethological concept of critical periods,” write Edward Zigler and Victoria Seitz 1982, on 84.

n3 Birchwood et al. 1998.

n4 Fagiolini and Hensch 2000.

n5 Colombo 1982, on 260.

n6 Scott 1962.

n7 Panofsky 2014 followed Paul in suggesting minimal eugenic connection between Jackson and earlier programs. On Whitney's eugenics in a transnational perspective, see Kühl 1994. On Whitney in America, see Lovett 2007. On American eugenics more broadly, see Kevles 1995; Cooke 1998; Godin 2007. An excellent historiographical overview is Paul 2016. On Stockard's embryology and broader irrelevance, see Clarke 1987; Pauly 2002. Pauly 1996 notes there is no significant biographical account of Stockard.

n8 In this article, I generally use “ethology” in its more common usage as a reference to the scientific study of behavior. The term has been traditionally associated with the three winners of the 1973 Nobel Prize for Physiology or Medicine: Karl von Frisch, Konrad Lorenz, and Nikolaas Tinbergen. See also Dhein 2022, this issue.

n9 On Little, see Rader 2004; Brandt 2009. Few have explored the significance of his attention to dogs or the implications of the behavior program. Exceptions include Paul 1991.

n10 For a recent overview of the literature on model organisms, see Ankeny and Leonelli 2020.

n11 In Burkhardt's thorough study of ethology, dogs receive only occasional reference. See Burkhardt 2005. On dogs as research subjects, see Todes 2001; Kirk 2010; Kirk 2014; Bolman 2018.

n12 Aria et al. 2021.

n13 Caudill 1993, on 1–2.

n14 Kühl 1995, on 85.

n15 Leon Fradley Whitney to Robert M Yerkes, 19 October 1925, p. 1, Robert Mearns Yerkes papers (MS 569), Manuscripts and Archives, Yale University Library, Box 52, Folder 1007.

n16 “Stockard, Dr. Charles R. (Extract from Interviews with Doctor Rose),” 18 March 1925, General Education Board records (FA058), Rockefeller Archive Center [henceforth GEB RAC], Box 703, Folder 7235.

n17 Pettit 2013.

n18 Charles Rupert Stockard to Wickliffe Rose, 30 March 1925, GEB RAC, Box 703, Folder 7235.

n19 Stockard 1921.

n20 Stockard 1931a, on vii.

n21 Ibid., on 223.

n22 Charles Rupert Stockard to Abraham Flexner, 23 February 1928, GEB RAC, Box 703, Folder 7235.

n23 Stockard 1931b, on 262.

n24 On Liddell, see Kirk and Ramsden 2018. On the American Pavlovian use of dogs, see Ramsden 2018.

n25 James 1939.

n26 William T. James, “[Dog Farm Memorandum],” 1939, p. 1 Rockefeller Foundation records, Rockefeller Archive Center [henceforth RF RAC], SG 1.1, Series 100, Box 81, Folder 977.

n27 William T. James, „Autobiography of William T. James,“ 1988, on 14, William T. James papers, University of Georgia Archives, Box 2, Folder 17.

n28 Paul 1991, on 273.

n29 Edwin Bidwell Wilson to Alan Gregg, 27 December 1943, RF RAC, SG 1.2, Series 200 A, Box 133, Folder 1189.

n30 Rader 2004, on 25.

n31 Little 1935. See also Rader 2004, on 253.

n32 Clarence Cook Little to Hans Grüneberg, 16 October 1944, 1, Grüneberg, Professor Hans, „Codebreakers: Makers of Modern Genetics,“ Professor Hans Grüneberg papers, Wellcome Library, online: https://wellcomecollection.org/works/jzjgce9q (accessed 2 December 2021).

n33 Grüneberg 1942.

n34 Clarence Cook Little to Alan Gregg, 17 October 1944, RF RAC, SG 1.2, Series 200 A, Box 133, Folder 1189.

n35 Scott 1989, on 408.

n36 Cora Stuhrmann traces the development of “behavioural ecology” and its intersections with the more popular variant of “sociobiology.” See Stuhrmann 2022, this issue.

n37 Scott 1989, on 406.

n38 Alan Gregg to Clarence Cook Little, 16 March 1945, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1190.

n39 [Anon.] 1945.

n40 Alan Gregg, “Diary,” 1945, on 216, RF RAC, RG 12, Officers’ Diaries, F−L, „Gregg, Alan,“ online: https://dimes.rockarch.org/objects/Xxa5PUbEgYeDCzqSppgWMP (accessed 2 December 2021).

n41 Friendly 1939; Dawson 1937.

n42 John Paul Scott to Alan Gregg, 26 December 1945, RF RAC, SG 1.2, Series 200A, Box 134, Folder 1190; John Paul Scott to Alan Gregg, 6 February 1946, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1190.

n43 John Paul Scott to Robert S. Morison, 26 April 1946, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1190.

n44 John Paul Scott to Robert S. Morison, 22 January 1947, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1191.

n45 Fuller 1985, on 98.

n46 Alan Gregg to John Paul Scott, 13 November 1946, RF RAC, SG 1.2, Series 200A, Box 134, Folder 1190.

n47 Clarence Cook Little to Alan Gregg, 22 May 1948, RF RAC, SG 1.2, Series 200A, Box 134, Folder 1191.

n48 Robert S. Morison, “RSM Interview – Thursday, August 12, 1948,” n.d., RF RAC, SG 1.2, Series 200A, Box 134, Folder 1191.

n49 Staff of the Division of Behavior Studies 1950.

n50 Fuller 1985, on 100.

n51 Scott and Fuller 1965, on 22.

n52 Sophia Gräfe shows how central a multitude of both visual and observational tools were to the discipline of ethology more broadly. See Gräfe 2022, this issue.

n53 John Paul Scott to Alan Gregg, 11 May 1950, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1192.

n54 Staff of the Division of Behavior Studies 1950, on 72.

n55 See Bolman 2021.

n56 Scott and Fuller 1965, on 60.

n57 “Future Plans for Research on Genetics and Social Behavior at the Division of Behavior Studies, Roscoe B. Jackson Memorial Laboratory: Report to the Rockefeller Foundation,” 23 December 1949, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1191.

n58 Mitman 1990, on 9.

n59 Scott 1937.

n60 Goldschmidt 1949, on 47. Scholars often treat “imprinting” and “critical period” synonymously: John Colombo, for instance, credits Lorenz with originating the “critical period” despite the latter not making particular use of it. See Colombo 1982.

n61 Scott and Marston 1950, on 25.

n62 Barbara Arndt, „Effects of environmental differences during the critical period on a litter of beagle dogs,“ Summer and Academic Year Student Reports 58, The Joan Staats Library, online: https://mouseion.jax.org/strp/58/ .

n63 Fuller et al. 1950.

n64 “Progress Report to the Rockefeller Foundation,” Research on Genetics and Social Behavior (Bar Harbor, ME: Division of Behavior Studies, Roscoe B. Jackson Memorial Laboratory, 1 July 1950), p. 2, RF RAC, SG 1.2, Series 200A, Box 134, Folder 1192.

n65 Scott et al. 1951, on 162.

n66 John Paul Scott to Robert S. Morison, 8 January 1951, p. 1, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1192.

n67 Scott et al. 1951, on 162. For more on human ethological studies, see Odenwald 2022, this issue.

n68 Frank A. Beach to John L. Fuller, 7 December 1950, Archives of the History of American Psychology, University of Akron, John L. Fuller papers, “Frank A. Beach”.

n69 Kohrs and Ross 1950.

n70 Ellis 1950

n71 Scott to Morison, 8 January 1951, p. 2, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1192.

n72 Pfaffenberger and Scott 1959; Feuerbacher and Wynne 2011, on 56.

n73 These examples were taken at random from prominent American dailies.

n74 John Paul Scott to Robert S. Morison, 3 August 1955, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1194.

n75 Robert S. Morison, “RSM Diary, New Haven, Connecticut,” 8 July 1955, p. 2, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1194.

n76 William C. Young to Robert S. Morison, 29 October 1955, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1194.

n77 Donald O. Hebb to Robert S. Morison, 26 October 1955, p. 1, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1194.

n78 Scott to Morison, 3 August 1955, p. 2, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1194.

n79 Robert S. Morison, “RSM Diary, New Haven, Connecticut,” 8 July 1955, RF RAC, SG 1.2, Series 200A, Box 134, Folder 1194.

n80 John Paul Scott to Robert S. Morison, 17 December 1951, p. 2, RF RAC, SG 1.2, Series 200 A, Box 134, Folder 1192.

n81 Many other ethologists did manage to maintain large, long‐term projects. See Milam 2022, this issue.

n82 Scott 1989.

n83 Coppinger and Coppinger 2001, on 13–14.

n84 Sands et al. 1977; Phillips et al. 1981.

n85 [Anon.] 1976.

n86 Coppinger and Coppinger 1980.

n87 Linhart et al. 1979, on 239. See also, on shepherding dogs, Haraway 2003.

n88 Coppinger and Coppinger 2016, on xv.

n89 Coppinger and Coppinger 1980, on 18.

n90 Ibid., on 24.

n91 Coppinger and Coppinger 2001, on 35.

n92 Ibid., on 106.

n93 Ibid., on 223.

n94 Scott 1986, on 12.

n95 Feuerbacher and Wynne 2011, on 62.

n96 Frank and Frank 1987, on 145.

n97 Feuerbacher and Wynne 2011, on 63.

n98 Feuerbacher and Wynne 2011.

n99 Scott et al. 1974.

n100 Scott 1986, on 24.

n101 Scott 1989, on 428.

n102 Margócsy 2017, on 308.

n103 Bolman 2018. See also Marshall 2021.