Development.

The course of language development is exceedingly complex, characterized by massive variability across children and by multiple bursts and plateaus within individual children. This complexity is well illustrated in other chapters in this reader and in an extensive review from which this chapter is excerpted (Bates, Thal, Finlay and Clancy, in press). Learning plays an extremely important role throughout the development of language, beginning even in utero when children start to pick up language-specific preferences. Across development, language can be ‘tuned ’ in various directions depending on the nature of the input. This is not the original view of language development. Language milestones were previously believed to timed by a ‘biological clock ’ that also governed motor milestones like crawling and walking (Lenneberg, 1967). However, this notion of lockstep development (based on average onset times across different samples of children) implies a set of correlations that should hold up within individual children as well. Doubts about such a lockstep process were raised when Bates, Benigni, Brethertin, Camaioni and Volterra (1979) looked for such correlations in their longitudinal study of language and communication in a sample of healthy, normal children aged 9- 13 months. The