Epidemiology is the science of public health. Information from epidemiologic studies can help address questions such as the frequency of occurrence of a disease, change in disease patterns over time, and the distribution of disease over different places and groups of people. Epidemiologic research can also provide information on risk factors and protective factors for disease.

In a chronic condition such as autism, important public policy decisions about the allocation of resources and provision of supports are often based on the results of epidemiologic research. Although public surveys of intellectual disability (mental retardation) began in the early nineteenth century, autistic disorder and Asperger’s disorder were first described in the 1940s, and the first population survey of autism was not undertaken until the 1960s. In many ways, the epidemiology of autism is still a new and emerging field of study (Hertz-Picciotto et al., 2006; Newschaffer et al., 2007).

The present chapter will present a review and discussion of recent epidemiologic research on autism. The focus will be on studies that address the rate of occurrence of autism spectrum disorders, changes in this rate over time, and specific risk factors. Epidemiologic research on vaccines and autism will also be discussed.

Definitions and methods

A brief review of some epidemiologic terms and methodology can facilitate understanding of the studies to be discussed. Generally speaking, the studies reviewed in this chapter are of two types: descriptive and analytic.

Descriptive studies yield information on the prevalence or incidence of a disease or disorder – of critical importance for planning services and estimating their public cost. In epidemiology, prevalence is the proportion of a population with a disease or disorder. It is defined as the number of existing cases of a disorder at a point in time, divided by the total population of concern. Incidence is the rate at which new cases of a disease or disorder arise. It is defined as the number of new cases of the disorder appearing over a defined period of time divided by the population at risk (Mausner and Kramer, 1985). Incidence data are most applicable in conditions where there is a clear onset of the disease, e.g. influenza. An epidemiologic survey might, for example, determine the incidence of influenza in Montana during a 3-month epidemic.