Critical Period Hypothesis

The critical period hypothesis states that there is a specific and limited time for language acquisition. More specifically, this hypothesis states that the first few years of a child's life is the critical time in which an individual can acquire language if presented with adequate stimuli. Proponents of the hypothesis argue that if a child does not receive the appropriate stimuli during this “critical period,” then the individual will never achieve full command of language. In other words, once a child passes a certain age without language acquisition, it is not possible to learn language at a later age.

The scientific focus on a critical time for language acquisition began in the late 1950s when neurologist Wilder Penfield discussed language acquisition from a physiological perspective. Penfield pointed to the superiority for learning language demonstrated by young children. He argued that children learn language easily before the age of 9; however, after the age of 9, learning language becomes difficult. Penfield claimed that the reason for this change in the ability to learn language was due to the plasticity of the human brain. The brain of the child is plastic, whereas the adult brain is rigid.

In 1967, linguist Eric Lenneberg further advanced the idea of a critical period for learning language in his classic work, Biological Foundations of Language. Like Penfield, Lenneberg asserted that the acquisition of language, like other biological functions, was successful only when it was stimulated at the right time and in a linguistically stimulating environment. Drawing on evidence from studies of brain growth and from clinical studies of deafness, mental retardation, and brain damage, Lenneberg claimed that there are age constraints on language acquisition caused by brain maturation. He maintained that the critical period for language learning occurs between the ages of 2 and puberty, with the crucial period occurring between 4 and 5 years of age. Lenneberg argued that before the age of 2 the brain has not developed the capacities it needs for learning language. He maintained that after puberty the brain's later-alization shuts down the brain's ability to acquire language. Therefore, if an individual did not learn language within the critical period, the individual would never be able to acquire language in any normal sense.

More recently, in 1994, psychologist and cognitive scientist Steven Pinker similarly claimed a critical time period for language acquisition by stating that language is instinctual. Language, Pinker asserted, is a biological adaptation rather than a cultural creation. He asserted that the brain contains innate means of creating an endless number of grammatical sentences from a limited vocabulary. Pinker held that the acquisition of normal language would occur for children up to 6 years of age when properly stimulated. He asserted that, after 6 years of age, the possibility of normal language acquisition declines and is rarely successful after puberty.

What evidence exists to support the critical period hypothesis? A basic limitation of the critical period hypothesis is that testing this theory, using traditional scientific methodology, is unethical. Scientists cannot intentionally isolate a child from the rest of the world for several years and then assess the effects of such isolation on language acquisition. Therefore, scientists have documented evidence of the critical period hypothesis mainly from abused and feral children who grow up deprived of exposure to language in childhood and who, consequently, do not acquire language normally. The most famous example used to demonstrate evidence of this hypothesis is the case of Genie, a pseudonym for a girl discovered in 1974, at the age of 13, strapped to a potty chair and wearing diapers. Genie had litde linguistic ability and, over several years of rehabilitation, was unable to acquire language completely, although researchers involved in Genie's rehabilitation disagreed on the [Page 246]degree to which she acquired the normal use of language.

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