Introduction

Autism is a developmental disorder defined by the presence of a triad of communication, social, and stereotypical behavioral characteristics with onset before 3 years of age. Previous estimates of the incidence of autism were 2–5 cases per 10000 individuals (for review see Wing, 1993). Recent studies show a higher incidence of autism (approximately 1 in 1000), while the three to four times predominance of the disorder in males has remained constant (Bryson, 1996). Autism was first described by Kanner in 1943 in his landmark paper describing a group of children who showed language abnormalities, impairment in social interactions, and restricted interests and preoccupations. One year later, Asperger (1944) described a similar group of children. The term Asperger's syndrome is now used to describe high-functioning individuals with autistic features but relatively normal communication and cognitive skills (Gillberg, 1989; Volkmar et al., 1996).

Underlying the spectrum of autistic behaviors are undoubtedly multiple etiologies, only a small fraction of which have been so far identified. The reliance upon this behavioral definition is a consequence of a failure to identify biological markers for the majority of individuals with autistic behavior and is a source of difficulty in the design and reproducibility of functional imaging studies. The inexact nature of the diagnosis of autism and other pervasive developmental disorders is also the source of numerous practical problems for the families of autistic children in identifying appropriate medical, behavioral, and educational interventions necessary to promote optimal development of their children.