The critical period hypothesis has been questioned by many researchers in recent years and is presently quite controversial.

... Typically, when pressed, people asserting the superiority of child learners resort to some variant of the "critical period hypothesis." The argument is that children are superior to adults in learning second languages because their brains are more flexible (Lenneberg, 1967; Penfield & Roberts, 1959). They can learn languages easily because their cortex is more plastic than that of older learners. (The corollary hypothesis is the "frozen brain hypothesis," applied to adult learners.) The critical period hypothesis has been questioned by many researchers in recent years and is presently quite controversial (Geneses, 1981; Harley, 1989; Newport, 1990). The evidence for the biological basis of the critical period has been challenged and the argument made that differences in the rate of second language acquisition may reflect psychological and social factors, rather then biological ones that favor child learners. For example, children may be more motivated than adults to learn the second language.

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