Critical Period Hypothesis

Supporting Theories

There are many theories that explain the critical period hypothesis. One of these is brain plasticity theory, which posits that the increase in age reduces the malleability of the brain and therefore reduces the ability of the individual to acquire language. Lenneberg believed that after lateralization—the development of specialized functions for each side of the brain—the brain loses plasticity. He claimed that lateralization of the language function is normally completed at puberty, making postadolescent language acquisition difficult. Another theory is imprinting. Imprinting can be compared to the taking of a still photograph, when a specific moment (i.e., behavior) becomes fixed in time. As opposed to instinct, imprinting theory relies on the acquisition of a particular behavior through imitation during the critical period in which learning of that behavior occurs, such as Lorenz's research with the Greylag goose.

Implications for Second-Language Learning

Both of these theories can be extended to explain the more difficult process of learning a second language. Indeed, the conclusion of the existence of a critical period for first-language acquisition has led many researchers to argue that there is also a critical period for second-language learning, since some of the same general language mechanisms that seem to “shut down” for first-language acquisition must logically affect subsequent language-learning endeavors. Lenneberg, whose research and evidence was based on first-language acquisition, generates several claims regarding second-language acquisition. He argued that most people (including adults) can learn a second language, although there is a rapid increase in what he calls “language-learning blocks” after puberty, resulting in the need for a conscious and labored effort (unlike the less laborious acquisition process of prepubescent children). Lenneberg also pointed out that foreign accents are not easily overcome after puberty.

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