Abstract
On the basis of evidence from twin and family studies, both autism and developmental language disorder—also referred to as specific language impairment (SLI)—are believed to be genetically mediated. The genetic mechanisms involved, however, are not clear. For example, the prevalence of autism is currently estimated at 1/1,000 (Fombonne 1999), whereas the recurrence risk in families is 6%–8% (Santangelo and Folstein 1999). On the basis of combined data from three population-based twin studies, the concordance rate for MZ twins is estimated to be ∼65%, contrasted with a concordance rate of 0% in DZ twins (Bailey et al. 1995). Presumably, the DZ rate of 0% is a type II error, since the combined samples contain .90, and there is no convincing evidence for perinatal or other environmental risk factors. Genetic modeling based on these data has been consistent with a model of oligogenic inheritance with epistasis (Pickles et al. 1995).
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