Familial heterogeneity in infantile autism.

The incidence of autism and cognitive disability was assessed in the biological siblings of 29 autistic probands subdivided on the basis of IQ. A significant clustering of autism and nonspecific intellectual retardation was found in the siblings of severely retarded autistic probands which was not present in the siblings of our higher-functioning autistic sample. These findings suggest that there may be etiological differences in autism, depending on the degree of associated mental retardation.