Critical Period Hypothesis

Critical period hypothesis is the claim that there is a biologically bounded developmental window during which language acquisition can proceed with full fluency and native-like competence, after which the capacity for complete acquisition declines irreversibly. The window is generally held to extend from birth through puberty, though its exact boundaries and rigidity remain contested.

The hypothesis derives from multiple lines of evidence: the contrast between the effortless acquisition of first languages in childhood and the typically incomplete acquisition of second languages in adulthood; the case studies of feral children and deaf individuals exposed to sign language after childhood; and neurobiological evidence for declining neural plasticity with age. Chomsky and Eric Lenneberg were the most influential proponents of the biological framing.

The alternative view — sensitive period rather than critical period — holds that plasticity declines gradually and is modulated by factors other than biological maturation alone: quantity and quality of input, social motivation, affective investment, and the presence of a first language that may either facilitate or interfere with additional language acquisition. The evidence supports a softer interpretation: later learners rarely achieve native-like phonology, but can achieve native-like syntax and semantics given sufficient input and motivation.