The last 2 decades have seen tremendous growth in the reported prevalence of autism spectrum disorders (ASD). They have moved from a relatively rare condition (perhaps 1 in 10 000) to a recently calculated 1 in 68 rate in the United States.1 The rapid rise has led to numerous theoretical explanations, with changes in diagnostic criteria and the possibility of diagnostic substitution standing apart from most. Other epidemiologic approaches that have considered what is different now compared with 30, 40, and 50 years ago and examined sociologic changes, public health policies, and environmental exposures have accumulated some degree of support for each of these.