Critical Period Hypothesis

The Critical Period Hypothesis (Lenneberg 1967) claims that there is a biologically determined window — roughly from birth to puberty — during which language acquisition occurs naturally and effortlessly. After this window closes, native-like attainment becomes extremely difficult or impossible due to maturational changes in the brain (particularly lateralization of language functions).

Strong vs Weak Versions

Strong CPH: There is a hard cutoff (around puberty). After it, full native-like acquisition of any language feature is impossible. This version is largely rejected — too many counterexamples exist of post-puberty learners achieving very high proficiency.

Weak CPH (Sensitive Period): There is a gradual decline in learning capacity rather than an abrupt cutoff. Different language domains may have different sensitive periods:

• Phonology — Strongest evidence for age effects. Accent-free L2 pronunciation after puberty is rare (though not impossible). Long (1990) argued the phonological window may close as early as age 6.
• Morphosyntax — Moderate age effects. L2 grammar acquisition becomes harder with age but remains achievable to high levels.
• Vocabulary and pragmatics — Weakest age effects. Adults often learn vocabulary faster than children in the short term.

Key Evidence

• Genie case (Curtiss 1977): A child isolated until age 13 never fully acquired L1 syntax, suggesting a critical period for first language.
• Johnson & Newport (1989): Chinese and Korean immigrants to the US showed a strong negative correlation between age of arrival and grammatical accuracy — but only for arrivals before age 15.
• Birdsong (2006): Found that some late L2 learners do achieve native-like performance on grammatical judgment tasks, challenging a hard cutoff.
• Hartshorne et al. (2018): Massive dataset (670,000 participants) suggested grammatical learning ability declines sharply after about age 17, later than the traditional puberty cutoff.

Why It Matters for Teaching

The CPH is often invoked to justify "earlier is better" language policies, but the reality is nuanced:

• Older learners have cognitive advantages (metalinguistic awareness, learning strategies, literacy transfer) that younger learners lack
• The apparent superiority of young learners is largely due to amount of exposure — children in immersion get thousands more hours of input
• For pronunciation, starting early does help — but explicit phonetic training and extended L2 residence can partially compensate
• Fossilization of certain features in adult learners may reflect sensitive period effects, but also communicative sufficiency and reduced motivation to improve

Connection to Other Theories

The CPH aligns with Nativist Theory and the concept of a Language Acquisition Device — if there is innate language-specific hardware, it makes sense that it might have a biological timeline. It challenges purely environmental or cognitive accounts that predict no fundamental age constraints. The phenomenon of Fossilization in adult learners may partly be explained by sensitive period effects, though other factors (L1 transfer, input quality, motivation) also contribute.