Language development provides one example that can illustrate the concept of a critical/sensitive period. Although language development is a process that psychologists have long debated, there is agreement that a strong biological basis for language acquisition exists. In 1967, Eric Lenneberg first proposed the notion of a critical period for language acquisition. He suggested that the period between infancy and puberty (the beginning of adolescence) was a critical period for language acquisition. This critical period was thought to end at puberty because of important maturational changes in the brain that occur at this time. Language must be acquired during the critical period if it is to be acquired at all. Alternatively, if the period from infancy to puberty is viewed as a sensitive period, rather than a critical period, language will be learned most easily during this period. After the sensitive period, language can be learned, but with greater difficulty and less efficiency.
What evidence is there supporting this concept of a critical period for language acquisition? How could a researcher ever test this idea? Information about this issue comes from different sources. These sources include a few unfortunate and extreme cases of childhood deprivation—children who were deprived of typical social experiences and stimulation.
Perhaps the most well-known of these was the case of Genie, who was described in a series of publications in the 1970s. Genie was essentially kept in isolation by a maltreating parent, with no exposure to language and normal social experiences for the time between toddlerhood and early adolescence. When she was discovered, she had some understanding of language but did not speak. After nearly one year of intensive training and instruction, she had a vocabulary of about 200 words and was speaking in two-word sentences. Six years later, she had made much progress, but she was still much less advanced in her language than other people her age who had normal experiences growing up.
Since Genie was able to acquire language following the onset of adolescence, the notion that language learning is impossible after the critical period cannot be supported. Rather, language learning can occur after the onset of adolescence but may be incomplete. The period between infancy and adolescence, therefore, may be a sensitive period for language learning—language can be acquired more easily during this time—rather than an absolutely critical period.
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