Critical period hypothesis
The critical period hypothesis (CPH) refers to a long-standing debate in linguistics and language acquisition over 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, which is very often observed to fall short of nativelike attainment. Various ages have been proposed for the supposed end of the CPH; those that point to pre-adolescent ages such as 12 have been vulnerable to alternative theories which invoke psychological or social factors applying as children move into adolescence.
History
The critical period hypothesis is associated with Wilder Penfield and Lamar Roberts, whose 1959 work Speech and Brain Mechanisms proposed biologically-based limits on the brain that were linked to age. The hypothesis was extended 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 informed popular assumptions about the presumed (in)ability of adults to fluently learn a second language.
In SLA, a weaker version of the CPH emerged in the 1970s. This refers to a sensitive period in which nativelike performance is unlikely but not ruled out.[1] The strongest evidence for the CPH is in the study of accent, where most older learners seem not to reach a native-like level. This leads some researchers to apply the CPH only to second language phonology rather than all aspects of language; indeed, a CPH was not seriously considered for syntax until the 1990s, in research that remains a minority view.[2] However, under certain conditions, native-like accent has been observed, suggesting that accent in SLA is affected by multiple factors, such as identity and motivation, rather than a biological constraint.[3]
First language acquisition
Children without language
The CPH as applied to first language acquisition proposes that a child deprived of exposure to natural language would fail to acquire it if exposure commenced only after the end of the critical period. Because testing such a theory would be unethical, in that it would involve isolating a child from the rest of the world for several years, researchers have gathered evidence of the CPH from a few victims of child abuse. The most famous example is the case of Genie (a pseudonym), who was deprived of language until the age of 13. Over the following years of rehabilitation, improvement in her ability to communicate was noted, but during this time she did not develop the language ability common to other children. However, this case has been criticised as a firm example of the critical period in action, and data has not been gathered from Genie since the 1970s.
Although there are several cases on record of deaf children being deprived of sign language, this could also count as abuse. One case in which no abuse took place is that of Chelsea, whose deafness was left undiagnosed until the age of 31. Once hearing aids had apparently restored her hearing to near-normal levels, she seemed to develop a large vocabulary while her phonology and syntax remained at a very low level.[4] The implications of this have been disputed, given the apparently unlikely circumstances of Chelsea's diagnosis.[5]