Autism is a behavioral disorder characterized by impaired social interaction, impaired communication, and a restricted range of activities and interests that emerge during infancy or early childhood (American Psychiatric Association [APA], 1987,1994). Although most individuals with autism (75%) have mental retardation, manifestation of the behavioral criteria for autism described in the Diagnostic and Statistical Manual of Mental Disorders (DSM) vary in frequency, type, and severity across individuals with the diagnosis. Some individuals who do not meet the diagnostic criteria for autism exhibit behaviors that fulfill only a subset of these criteria. For example, fragile X syndrome has been associated with autism, yet reports on the prevalence of nonverbal communication deficits and stereotypic behavior in this population vary greatly. In contrast, among children with Rett syndrome the prevalence and specificity of autistic behaviors has been more consistently established. Although the etiology of autism is currently unknown, there is considerable evidence to suggest organic involvement. For instance, data from magnetic resonance imaging studies support cerebellar abnormalities in autism, particularly in the vermal regions,