A critical period, in biology, psychology and many other fields, is a limited time in which some event can occur. Examples of this include the development of vision, birdsong and language. Studies involving depriving animals of stimuli and cases of child abuse have been used to argue that without exposure to environmental 'triggers', the onset of traits or abilities such as these is affected or prevented.
The critical period hypothesis (CPH) in linguistics and language acquisition refers to the extent to which the ability to acquire language is biologically linked to age. The hypothesis claims that there is an ideal 'window' of time to acquire language in a linguistically rich environment, after which this is no longer possible due to changes in the brain. The hypothesis has been discussed in the context of both first (FLA) and second language acquisition (SLA), and is particularly controversial in the latter. In FLA, it seeks to explain the apparent absence of language in individuals whose childhood exposure was very limited, and in SLA it is often invoked to explain variation in adults' performance in learning a second language.
The critical period hypothesis is associated with Wilder Penfield, whose 1959 work with Lamar Roberts, Speech and Brain Mechanisms, explored the neuroscience of language, concluding that it was dominant in the left hemisphere of the brain. The work focussed on how individuals with brain damage evidenced atypical linguistic performance, rather than examining neurotypical cases of 'normal' language acquisition, and the authors' conclusions were also based on the prevailing tabula rasa view that children were born without any real innate language ability. Their recommendations for language schooling recommended starting early in order to avoid fixed effects. The hypothesis was developed by Eric Lenneberg in his 1967 Biological Foundations of Language, which set the end of the critical period for native language acquisition at 12. The hypothesis has been fiercely debated since then, and has continued to inform popular assumptions about the presumed (in)ability of adults to fluently learn a second language.
- Penfield & Roberts (1959: 252; Dechert 1995: 67-94).