The concept of a critical period in language acquisition is a central tenet of many contemporary theories of language development; its first prominent proponent was linguist and neurologist Eric Lenneberg in the late 1960s. Lenneberg conceived of it as a biological structural limitation on the possibility of learning a first language (L1) outside a narrow window of developmental opportunity. Critical periods in learning are well known from studies of animal learning; for example, in some sparrow species exposure during a specific window of time is required for them to develop normal songs. In applying the concept to humans, the idea is that without exposure to language input during a specific period, language either will not be learned at all or will be learned imperfectly.

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