ABSTRACT
Autism and the other pervasive developmental disorders (PDD), including childhood disintegrative disorder, Rett’s disorder, Asperger’s disorder, and PDD-not otherwise speci ed, comprise a set of disorders which share a “triad” of impairments in socialization and communication along with repetitive and ritualistic behaviors (American Psychiatric Association, 2000). Etiologically, autism is a disorder with a strong genetic basis (Bailey et al., 1995) but research has yet to identify a single gene or even a set of genes that account for the vast majority of the identi ed cases
although progress is being made (Abrahams and Geschwind, 2008). Indeed, Happe and Ronald (2008) have noted that the triad of impairments characteristic of autism fractionate when examined in large-scale twin populations and are associated with nonoverlapping genes, suggesting that it may be fruitless to assume a single cause is responsible for the disorder. To complicate the matter further, there is strong heterogeneity not only across diagnostic classes in PDD but within autism itself. For example, Miles et al. (2005) have suggested two groupings: essential versus complex autism, with the latter categorized by evidence of signi cant dysmorphology or microcephaly. Such individuals were also more likely to have an identi able syndrome. Yet the notion that autism is a syndrome with many different “organic” etiologies is not new (Coleman, 1976).
