Family history data on 99 autistic and 36 Down’s syndrome probands are reported. They confirmed a raised familial loading for both autism and more broadly defined pervasive developmental disorders in siblings (2.9% and 2.9%, respectively, vs 0% in the Down’s group) and also evidence for the familial aggregation of a lesser variant of autism, comprising more subtle communication/social impairments or stereotypic behaviours, but not mental retardation alone. Between 12.4 and 20.4% of the autism siblings and 1.6% and 3.2% of the Down’s siblings exhibited this lesser variant, depending on the stringency of its definition. Amongst autistic probands with speech, various features of their disorder (increased number of autistic symptoms; reduced verbal and performance ability) as well as a history of obstetric complications, indexed an elevation in familial loading. No such association was seen in the probands without speech, even though familial loading for the lesser variant in this subgroup, was significantly higher than in the Down’s controls. The findings suggest that the autism phenotype extends beyond autism as traditionally diagnosed; that aetiology involves several genes; that autism is genetically heterogeneous; and that obstetric abnormalities in autistic subjects may derive from abnormality in the foetus. Bolton, P., MacDonald, H., Pickles, A., Rios, P., Goode, S., Crowson, M., Bailey, A. & Rutter, M. (1994). A case-control family history study of autism. Journal of Child Psychology and Psychiatry, and Allied Disciplines, 35(5), 977-900.
Downs Syndrome exhibits powerful suggestions of neoteny with some studies calling attention to the correlations. I’ve hypotheized and discussed in detail the influence of neoteny on autism. Downs is a chromosonal condition. Autism is far more subtle. Do any of the my hypothetical causes of autism also apply to Downs? For example, are there higher rates of Downs among Minnesota Somalis?
