Nationwide, the prevalence of autism, a disease characterized by impairments in social interaction and communication, has increased roughly ten-fold over the past 40 years. This dissertation makes four contributions to the literature on autism. First, it provides an estimate of the combined contribution of changing diagnostic criteria and diagnostic substitution to the epidemic. Diagnostic change accounts for close to one-third of the observed increase in autism caseload. Second, it includes the first study of changes in risk factors across successive birth cohorts. The risk associated with two of the most contentious factors—maternal and paternal age—change over time. Potential biological and demographic reasons for this are explored.

The final portion of the dissertation moves from the individual to the community level. Autism clusters in California are identified for the first time. Communities with extremely high relative risks, in excess of 20, are identified. In these communities, 1 in 15 children have an autism diagnosis. Differences in spatial distributions of autism and autism-MR cases are described. It is also shown that clusters differ based on residence at birth and residence at diagnosis, and that purely diagnostic clusters exist. Communities that are wealthier, have a proportionately larger Caucasian population, and have higher educational attainment rates tend to have higher rates of autism. In contrast, rates of autism-MR comorbidity within communities did not differ systematically along any of these lines. It appears that the social environments that give rise to autism with and without comorbidity are quite different. This evinces that at least part of the epidemic is social. Geographic diagnostic expansion is also identified. In the final analytic section, mechanisms that might account for diagnostic expansion beyond the boundaries of birth clusters are examined. Autism organizations play a role in spreading and/or concentrating diagnoses in certain communities. This dissertation outlines the contours of this changing epidemic, but also highlights the need for further research to understand its complex dynamics.