What atypical populations can reveal about language development: The contrast between deafness and Williams syndrome

Two distinct lines of investigation are presented: the study of linguistic competence in the written language of deaf children and adults, and the study of linguistic development in children and adolescents with Williams syndrome (WS). Qualitative data focusing on spoken and written Italian and coming from cross-sectional and longitudinal studies conducted over the last 10 years are briefly reviewed and discussed. Italian people who are deaf demonstrate selective difficulties with aspects of grammatical morphology that play a syntactic rather than a semantic function. Italian people with WS display a particular asymmetric fragmentation within linguistic abilities: a profile of strength in phonological abilities but serious deficits in semantic and morphosyntactic aspects of language. The case of these two very different populations can offer us important clues for investigating which aspects of language and specifically of grammar are influenced by modality of perception.