Introduction

Epidemiological surveys of autism started in the mid 1960s in England (Lotter, 1966, 1967) and have since then been conducted in many countries. Most epidemiological surveys have focused on a categorical—diagnostic approach to autism that has relied over time on different sets of diagnostic criteria; however, all surveys used a definition of autism which comprised severe impairments in communication and language, social interactions, and play and behavior. This chapter is therefore concerned with autism defined as a severe developmental disorder and not with more subtle autistic features or symptoms that occur as part of other, more specific, developmental disorders, as unusual personality traits, or as components of the lesser variant of autism thought to index genetic liability to autism in relatives. With the exception of recent studies, other pervasive developmental disorders (PDDs) falling short of diagnostic criteria for autistic disorder — pervasive developmental disorder not otherwise specified (PDD-NOS), Asperger's syndrome — were generally not included in the case definition used in earlier surveys although several epidemiological investigations yielded useful information on the rates of these particular types of PDD. These data are summarized separately. The aims of this chapter are to provide an up-to-date review of the methodological features and substantive results of published epidemiological surveys. This chapter updates our previous reviews (Fombonne, 1998, 1999, 2003a) with the inclusion of new studies made available since then.