Critical Period Hypothesis

The critical period hypothesis holds that a person can achieve native-like fluency in a language only if they acquire it before a certain age, after which the brain's capacity for language learning declines. It was first proposed by the Montreal neurologist Wilder Penfield and his co-author Lamar Roberts in their 1959 book Speech and Brain Mechanisms, and was popularized in 1967 by Eric Lenneberg's Biological Foundations of Language. The hypothesis remains the subject of a long-running debate in linguistics and second language acquisition over how tightly the ability to learn language is tied to developmental stages of the brain, with direct bearing on when and how second-language instruction is best introduced.