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The critical period hypothesis holds that first language acquisition must occur before cerebral lateralization is complete, at about the age of puberty. One prediction of this hypothesis is that second language acquisition will be relatively fast, successful, and qualitatively similar to first language only if it occurs before the age of puberty. This prediction was tested by studying longitudinally the naturalistic acquisition of Dutch by English speakers of different ages. The subjects were tested 3 times during their first year in Holland, with an extensive test battery designed to assess several aspects of their second language ability. It was found that the subjects in the age groups 12-15 and adult made the fastest progress during the first few months of learning Dutch and that at the end of the first year the 8-10 and 12-15-year-olds had achieved the best control of Dutch. The 3-5-year-olds scored lowest on all the tests employed …