Critical period hypothesis

The hypothesis that biological development makes language learning especially constrained during a limited age range, after which native-like acquisition becomes less likely.

Language acquisition: The process by which people learn to understand and use a language. The hypothesis proposes an age-linked constraint on this broader process.
Genie (feral child): An American girl discovered in 1970 after severe social isolation and deprivation during childhood. Her limited language progress after discovery became a prominent, ethically fraught case in debates about late acquisition.
Usage-based language acquisition: An account in which linguistic knowledge emerges from repeated use, general learning abilities, and social interaction. It explains learning through experience and general cognition rather than a narrow maturational window.
Age of acquisition: The age at which a person begins learning a language or a particular linguistic feature. Studies use it to estimate age effects, but exposure histories can complicate the measure.
Neuroplasticity: The nervous system's capacity to change its structure or function in response to experience. Age-related changes in plasticity are proposed as one possible basis for the learning constraint.
Isabelle (feral child): A child found in Ohio in 1938 after spending much of early childhood isolated with her deaf mother. Her rapid language development after intervention contrasts with claims that early deprivation leaves an irreversible deficit.
Universal grammar: The proposal that humans possess an innate structural capacity that constrains possible grammars. It is often paired with age-sensitive accounts of how innate language capacities develop.
Ultimate attainment: The highest level of proficiency a learner reaches after sustained language learning. The hypothesis predicts age-related differences in eventual proficiency, not merely learning speed.
Sensitive period: A developmental interval when experience has unusually strong effects, without making later learning impossible. This less absolute account distinguishes heightened early readiness from a strict cutoff.
Chelsea (deaf woman): An American woman whose profound deafness went undiagnosed until adulthood and who then learned spoken language. Her history tests whether late first-language access can yield fluent grammar and speech.

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