Critical Period

The Critical Period Hypothesis (CPH) claims that there is a maturationally constrained window during which language can be acquired with native-like outcomes; outside this window, acquisition is slower, less complete, and rarely reaches native attainment. The claim originated in first-language research and was extended, more controversially, to second-language acquisition. Current consensus has shifted from a hard "critical" period to a softer "sensitive" period of gradient decline.

Lenneberg's Original Hypothesis

Eric Lenneberg's Biological Foundations of Language (1967) proposed the CPH for L1 acquisition. Drawing on Penfield and Roberts, Lenneberg argued that language acquisition was tied to brain maturation, specifically to lateralisation of language to the left hemisphere, which he believed completed around puberty. Before puberty, the brain retained the plasticity needed to acquire language naturalistically; after puberty, lateralisation was complete and language acquisition became laborious and incomplete. Evidence came from feral children, late-deafened and late-signing individuals, and aphasia recovery patterns showing better outcomes in children than adults.

Lenneberg's neurological argument has not aged well. Subsequent imaging research has shown that lateralisation is largely complete much earlier than puberty, undermining his proposed mechanism. The behavioural pattern he described, however, has held up: extreme cases of language deprivation past childhood (Genie, the Romanian orphans) confirm that L1 acquisition is severely compromised when delayed beyond a certain age.

Extension to L2

Whether a critical period applies to L2 acquisition is the more contested question. Johnson and Newport (1989) tested 46 Korean and Chinese immigrants to the United States on grammaticality judgments in English. Within the group whose age of arrival was 3 to 15, they found a strong negative correlation between age of arrival and ultimate attainment; for the 17 to 39 group, they found no significant correlation but uniformly lower scores. This pattern, age effects within a window followed by a flat decline, became the canonical empirical signature of the CPH for L2.

DeKeyser (2000) replicated the design with 57 Hungarian-speaking adults in the United States and reached a stronger conclusion: among adult arrivals, only those with high verbal analytical ability scored within the child-arrival range. He argued for a Fundamental Difference Hypothesis: children acquire language through implicit, domain-specific mechanisms; adults must rely on explicit, problem-solving cognition, and only those with high analytical aptitude can compensate.

Birdsong and others have catalogued counter-evidence: adult learners who, by every measure, achieve native-like attainment in L2. The existence of even a few such cases, sceptics argue, refutes a hard critical period; CPH defenders respond that the cases are rare exceptions explainable by individual differences in aptitude, motivation, or input quality.

Hartshorne, Tenenbaum & Pinker (2018)

The largest-scale empirical study to date used a viral grammar quiz to collect data from 669,498 native and non-native English speakers. Hartshorne, Tenenbaum and Pinker (2018) modelled the trajectory of grammar-learning ability by disentangling three confounded variables: current age, age of first exposure, and years of experience.

Their conclusion: grammar-learning ability is preserved nearly intact until about 17.4 years of age, then declines steadily. This is much later than Lenneberg's puberty cutoff. The paper argues that the appearance of a critical period at puberty in earlier studies was an artefact of insufficient data and uncontrolled variables; with a large enough dataset, the offset is clearly later. Whether to call this a "critical" period or a sensitive period of late-adolescent decline remains a terminological choice. Slik and colleagues (2022) reanalysed the dataset and argued that the data are equally consistent with a steady decline from early childhood, without any sharply defined offset.

Critical vs. Sensitive Period

The terminology has shifted. A critical period in biology proper, like imprinting in geese, is a sharp window outside which the relevant capacity is lost entirely. Most contemporary SLA researchers prefer sensitive period: a softer construct describing graded decline rather than a cliff edge. Native-like attainment becomes progressively less likely with later age of onset, but not impossible.

Different domains may have different sensitive periods. Phonology appears most age-sensitive, with native-like accent rarely achieved past early adolescence. Morphosyntax shows pronounced age effects but with more individual variation. Lexicon and pragmatics show the smallest age effects; vocabulary growth continues into adulthood with no maturational ceiling.

Implications for ELT

The CPH has been used to argue both for very early English instruction (start before the window closes) and against the assumption that earlier is always better. The empirical record is mixed. In immersion settings, earlier starters do tend to achieve more native-like outcomes, especially in pronunciation. In typical foreign-language classrooms with limited weekly hours, however, earlier starters do not consistently outperform older starters; older learners can compensate with cognitive maturity, study skills, and explicit grammar knowledge.

Practical takeaways: pronunciation work warrants serious attention in young learners; adult learners should not be told their case is hopeless, since attainable competence is high even if native-like phonology is unlikely; and the goal of approximating an idealised native speaker is itself questionable, given the global use of English as a lingua franca where intelligibility, not native-likeness, is what matters.

References

• Birdsong, D. (Ed.) (1999). Second language acquisition and the critical period hypothesis. Lawrence Erlbaum.
• DeKeyser, R. M. (2000). The robustness of critical period effects in second language acquisition. Studies in Second Language Acquisition, 22(4), 499-533.
• Hartshorne, J. K., Tenenbaum, J. B. & Pinker, S. (2018). A critical period for second language acquisition: Evidence from 2/3 million English speakers. Cognition, 177, 263-277.
• Johnson, J. S. & Newport, E. L. (1989). Critical period effects in second language learning: The influence of maturational state on the acquisition of English as a second language. Cognitive Psychology, 21(1), 60-99.
• Lenneberg, E. H. (1967). Biological foundations of language. John Wiley & Sons.