Critical Period Hypothesis

A theoretical framework proposing that language acquisition occurs most effectively within a biologically determined time window, typically extending from early infancy through puberty. During this period, the brain exhibits heightened neuroplasticity and specialized mechanisms for processing linguistic input, enabling children to acquire native-like proficiency in any language through mere exposure. After this critical period closes, language learning becomes increasingly effortful and rarely results in complete native-speaker competence, particularly in areas such as pronunciation and intuitive grammar.

The hypothesis manifests in various linguistic phenomena, including the superior phonological abilities of early bilinguals, the persistent foreign accents in late second-language learners, and the incomplete language development observed in cases of linguistic deprivation. Modern neurolinguistic research has refined this concept into multiple sensitive periods for different language components—phonology closing earliest, followed by morphosyntax, while vocabulary acquisition remains relatively open throughout life—suggesting a gradient rather than absolute cutoff in language learning capacity.