ABSTRACT
The central topic of this volume is language competence across populations. The population of interest here is children with specific language impairment (SLI), a condition conventionally defined as one in which children seem to have the necessary developmental precursors to support language acquisition but nevertheless show language delays. This condition is sometimes described as “unexpected and unexplained variation” in language acquisition. This chapter focuses on the nature of variation within the language system, the ways in which affected children do and do not vary from unaffected children, the ways in which children with SLI are similar to and different from children with Williams syndrome (WMS), and the interpretation of a possible grammatical marker as indicative of either a general language delay or a selective delay in addition to the general delay. At the most general levels, the chapter takes up the issues of variation within elements of language, developmental variation as children move toward the adult linguistic system, variation in onset mechanisms versus asynchronous elements within the linguistic system, and variation across different clinical populations of children with language impairments.
