Persons with an autism spectrum disorder (ASD)* have impairments in social, communicative, and behavior
development that typically are present before age 3 years and that often are accompanied by abnormalities in
cognitive functioning, learning, attention, and sensory processing (1). Autism was thought previously to be
rare, but during the 1990s, the number of persons reported to be receiving services for ASDs increased
substantially (1--7). This increase has elevated public concern regarding prevalence of conditions in the
autism spectrum and underscores the need for systematic public health monitoring. The complex nature of
these behaviorally defined disorders, together with the current lack of genetic or biologic markers for early
and consistent identification, make epidemiologic investigation challenging