Abstract-me crirical-period hypothesisfor second-language acquisition was rested on doto from the 1990 U.S. Census using responses from 2.3 million immigrants with Spanish or Chinese language backgrounds. The analyses rested 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 ofrhe 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 effectsfor level ofeducation and for age of immigration, but a negligible amount of additional variance was accounted for when the parameters for diffeerence in slope and difference in means were estimated. Thus, the pattern of decline in second-language acquisitionfailed to produce the discontinuity that is an essential hallmark o f 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 lo both theorists and social policymakers (Bailey, Brner, Symons, & Lichtman, 2001). The critical-period hypothesis was originally proposed in the neurolinguistic litemture by Penfield and Robem (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 & NewpoIf 1989; 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 @) 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. hponents of a critical-period explanation have attempted to place the description of second-language learning within these pammeters. Johnso
