Why do adult brains struggle to learn a new language?

Critical periods exist extensively across the animal kingdom. ‘Imprinting’, for example, is a phenomenon whereby newly-hatched birds must interact with their mothers within a short ‘critical period’ in order to successfully identify them as their parent [1]. A critical period for ocular development has also been identified in multiple species, including humans [2]. One of the most contested critical periods, however, is that of language acquisition. The critical period hypothesis for language acquisition states that there is a discrete window of time in an individual’s development where they are able to develop a language with full proficiency, and after this time, full fluency cannot be achieved. Most believe that this critical period ends at some point around the end of puberty, as this coincides with the completion of brain lateralisation – the process by which certain brain functions specialise to either the left or the right hemisphere [3]. It is also widely accepted that the critical period is itself a result of brain lateralisation, as this is the point where the brain loses its plasticity – an essential quality for language acquisition [4].

One of the most famous pieces of evidence in this field is the case of Genie – a child raised in complete isolation. Her father kept her in a cage for all hours of the day, preventing her from ever being spoken to, or interacted with. When rescued at the age of thirteen, she was adopted by researchers who attempted to teach her language, in hopes that she would provide them with invaluable evidence for or against the critical period hypothesis. Although Genie was able to acquire a solid vocabulary, she was never able to learn the grammatical structures required to combine words into coherent sentences, suggesting that she had already reached the end of her critical period [5]. In a similar case to Genie’s, a child called Isabelle was raised in a solitary, isolated room by a deaf and mute mother, and was therefore, not exposed to any form of language. Isabelle, however, was rescued before puberty, at the age of six. As predicted, Isabelle was eventually able to reach full linguistic proficiency [6]. These cases of children raised without language are strong evidence for the critical period hypothesis, as they demonstrate an evident change during a well-defined period that removes the ability to fully acquire language proficiency.

It is well documented that adults find it much more difficult to learn a second language than children, with most adults never reaching full fluency with regards to grammar and syntax [7]. Johnson and Newport tested Chinese and Korean children who had recently immigrated to the US, and compared their English proficiency against their age of arrival (when they began learning the language). They found that English proficiency decreases with increased age of arrival, with a significant drop during puberty. Furthermore, the same measures showed no correlation when using subjects with arrival ages above seventeen, suggesting that after the critical period is over, the ability of language acquisition plateaus and decreases no further [8]. These findings were corroborated in 2003 by a study using Spanish immigrants, with identical trends identified [9]. This evidence from second- language learners provides solid evidence for the trend in language acquisition ability decreasing with age introduced by the critical period hypothesis.

So, why does the critical period exist? Of course, language acquisition itself provides an evolutionary advantage, as language proficiency and reproductive potential are closely positively correlated [10], however, the advantage provided by the critical period is less clear. James Hurford claimed that the critical period is merely a result of the “lack of selection pressures that reinforce acquiring multiple languages” [11].  He argues that although the acquisition of a first language does produce an evolutionary advantage, the acquisition of a second does not, and as the vast majority of humans acquire their first during their first few years, the ability to acquire language later in life (e.g. after the critical period) is superfluous and has, therefore, not evolved.

So if you’re trying to learn a new language, and are struggling to differentiate your “bonjour”s from your “au revoir”s – at least you now know it’s not your fault… it’s that pesky adult brain of yours!


Chas Alexander Smith



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  2. Ma, W., Li, Y. and Tao, H. (2013). Downregulation of Cortical Inhibition Mediates Ocular Dominance Plasticity during the Critical Period. Journal of Neuroscience, 33(27), pp.11276-11280.
  3. Knecht, S., Flöel, A., Dräger, B., Breitenstein, C., Sommer, J., Henningsen, H., Ringelstein, E. and Pascual-Leone, A. (2002). Degree of language lateralization determines susceptibility to unilateral brain lesions.Nature Neuroscience, 5(7), pp.695-699.
  4. Lenneberg, E. (1967). The Biological Foundations of Language. Hospital Practice, 2(12), pp.59-67.
  5. Curtiss, S., Fromkin, V., Krashen, S., Rigler, D. and Rigler, M. (1974). The Linguistic Development of Genie. Language, 50(3), p.528.
  6. Cole, W. and Cox, R. (1968).Social foundations of education. New York: American Book Co., p.115.
  7. Meara, P., Fitzpatrick, T. and Barfield, A. (2009).Lexical processing in second language learners. 1st ed. Bristol, UK: Multilingual Matters, pp.57-74.
  8. Johnson, J. and Newport, E. (1989). Critical period effects in second language learning: The influence of maturational state on the acquisition of English as a second language.Cognitive Psychology, 21(1), pp.60-99.
  9. Hakuta, K., Bialystok, E. and Wiley, E. (2003). Critical Evidence.Psychological Science, 14(1), pp.31-38.
  10. Pinker, S. and Bloom, P. (1990). Natural language and natural selection.Behavioral and Brain Sciences, 13(4), pp.707-727.
  11. Hurford, J. (1991). The evolution of the critical period for language acquisition.Cognition, 40(3), pp.159-201.

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