Genetic influences on specific versus non-specific language impairment in 4-year-old twins

The genetic and environmental etiology of speech and broader language skills was examined in terms of their concurrent relationships in young children; their longitudinal association with reading; and the role they play in defining the ‘heritable phenotype’ for specific language impairment (SLI). The work was based on a large sample of 4 1/2-year-old twins, who were assessed at home on a broad range of speech and language measures as part of the Twins Early Development Study. We found that genetic factors strongly influence variation in young children's speech in typical development as well as in SLI, and that these genetic factors also account for much of the relationship between early speech and later reading. In contrast, shared environmental factors play a more dominant role for broader language skills, and in relating these skills to later reading; isolated impairments in language as opposed to speech appear to have largely environmental origins.