Critical Evidence: A Test of the Critical-Period Hypothesis for Second-Language Acquisition

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PSYCHOLOGICAL SCIENCE

Research Article
CRITICAL EVIDENCE: A Test of the Critical-Period Hypothesis for Second-Language Acquisition Kenji Hakuta,1 Ellen Bialystok,2 and Edward Wiley1 1

Stanford University and 2York University, Toronto, Ontario, Canada

Abstract—The critical-period hypothesis for second-language acquisition was tested on data from the 1990 U.S. Census using responses from 2.3 million immigrants with Spanish or Chinese language backgrounds. The analyses tested a key prediction of the hypothesis, namely, that the line regressing second-language attainment on age of immigration would be markedly different on either side of the criticalage point. Predictions tested were that there would be a difference in slope, a difference in the mean while controlling for slope, or both. The results showed large linear effects for level of education and for age of immigration, but a negligible amount of additional variance was accounted for when the parameters for difference in slope and difference in means were estimated. Thus, the pattern of decline in second-language acquisition failed to produce the discontinuity that is an essential hallmark of a critical period. The idea that there is a biologically based critical period for secondlanguage acquisition that prevents older learners from achieving nativelike competence has appeal to both theorists and social policymakers (Bailey, Bruer, Symons, & Lichtman, 2001). The critical-period hypothesis was originally proposed in the neurolinguistic literature by Penfield and Roberts (1959) and vigorously followed up by Lenneberg (1967), who speculated that maturational aspects of the brain that limited recovery from brain traumas and disorders would extend to second-language acquisition. Subsequent research using behavioral evidence appeared to confirm this hypothesis (Johnson, 1992; Johnson & Newport, 1989; Oyama, 1976; Patkowski, 1980, 1994). The measure of language proficiency in these studies varied (ratings of oral speech, grammaticality judgment tasks), but the typical result was that proficiency scores declined with increases in age of initial exposure to the second language. The claim that there is an age-related decline in the success with which individuals master a second language is not controversial. The diminished average achievement of older learners is supported by personal anecdote and documented by empirical evidence (Flege, YeniKomshian, & Liu, 1999; Stevens, 1999). What is controversial, though, is whether this pattern meets the conditions for concluding that a critical period constrains learning in a way predicted by the theory. A critical period minimally entails two characteristics: (a) a high level of preparedness for learning within a specified developmental period to ensure the domain is mastered by the species and (b) a lack of preparedness outside this period (Bornstein, 1989; Colombo, 1982). The consequence of these conditions is that the relation between learning and age is different inside and outside the critical period. Proponents of a critical-period explanation have attempted to place the description of second-language learning within these parameters. Johnson

Address correspondence to Kenji Hakuta, CERAS Building, Stanford University, Stanford, CA 94305; e-mail: hakuta@stanford.edu. VOL. 14, NO. 1, JANUARY 2003

and Newport (1989, 1991) have argued, for example, that there is a strong age-related decline in proficiency for languages learned prior to puberty (defined as 15 years old) and random variation in achievement among individuals who are exposed to a second language later in life. Such developmental discontinuity at an identifiable maturational time would constitute support for the two conditions of a critical period. The data, however, are controversial because of the difficulty in separating out the effects of age of initial exposure, duration of exposure, and social and linguistic backgrounds of the participants (see the analysis and...
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