These results provide evidence for the heterogeneity of the RRBI domain of the ADI-R in terms of both clinical presentation and other correlates. In addition, the IS factor seems to be under familial (presumably genetic) control, while RSMB appears to simply reflect variation in developmental level.

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These findings on the familial aggregation of the lesser variant suggest that the genes for PDD also confer susceptibility to the lesser variant and that PDD may be a genetically heterogeneous disorder.

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It has been proposed that females at risk for autism are protected in some way, so that only those with the greatest genetic liability are affected. Consequently, affected male siblings of females with autism should be more impaired than affected male siblings of male probands. One hundred and ninety-four (194) families with a single child with autism (simplex, SPX) and 154 families with more than one child with autism (multiplex, MPX) were examined on measures of severity, including non-verbal IQ. Among SPX families, girls had lower IQ than boys, but no such differences were seen among MPX families. Similarly, the affected brothers of girls with autism were no different from affected brothers of male probands. These data suggest that MPX and SPX families differ with respect to the relationship between gender and IQ.

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