by Aj Webb

Created on: May 31, 2010

The basic assumptions of the Critical Period Hypothesis states that during a certain period of time the learner must be in a linguistically rich environment in order to achieve complete linguistic competence, and after this small window of time, or after a certain age, linguistic competence, in particular grammatical systems, is no longer achievable.

For first language acquisition this ‘critical period’ is the first few years for the Childs life, and for second language acquisition, the learner will never achieve native fluency of a language outside of this critical period.  There have been many age related studies for Second Language Acquisition, Oyama (1976) states that all second language learners have an identifiable foreign accent, although some possess perfect linguistic competence.  According to Robertson (2002) there are many other factors more important than age that affects a second language learner, such as motivation, input and output, commitment, time and even anxiety.

Second language acquisition is not subject to ‘critical periods’ perse, but research does show a decline in the ability to fully achieve linguistic competence as the learner’s age increases. (Bialystok and Hakuta, 1994)

According to the Critical Period Hypothesis (CPH), there's a biologically determined 'window of opportunity', ie. a critical period, during which the human brain has the capacity to acquire language (Lenneberg 1967; Penfield & Roberts 1959). Once the lateralisation of language functions has been completed (ie. the specialisation of language by either brain hemisphere; for right-handers, language functions are primarily controlled by the left hemisphere), and the brain structure is no longer plastic, the CPH predicts that the biological basis for language learning is irreversibly altered, and that, in effect, the end product can never reach the level of completion which could have been obtained, had the learning taken place within the critical period. There's some disagreement as to when the exact closure of this period occurs, but Lenneberg (1967) considers the onset of puberty the critical cut-off point for the loss of neural plasticity.

A milder version of the original hypothesis, commonly referred to as the Sensitive Period Hypothesis, suggests there's a time span during which the processing capacity of language input is optimised, but that it's still possible, although more difficult, to acquire language after its closure.

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