As recently as 10 years ago, the American Psychiatric Association estimated the prevalence of autism to be two to 20 in every 10,000 children.1 But by 2004, a number of studies using criteria for a diagnosis of pervasive developmental disorders, also known as autism spectrum disorders (ASD), from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) noted prevalence rates of two to six per 1,000 children. Some studies from Scandinavia and Europe have cited prevalence as high as 12 per 1,000 children; prevalence in the United States has been estimated to be two to seven per 1,000 children.2

These sharply increased numbers have sparked concerns about what factors might underlie this increase in ASD diagnoses. One must be careful to evaluate these numbers with the full understanding that the definition of the autistic spectrum has changed dramatically over recent decades. In 1980, DSM-III included infantile autism and spelled out six criteria, all of which had to be met for a diagnosis.3 Then in 1994, DSM-IV established the category of pervasive spectrum disorders, which includes autistic disorder and four other disorders: Asperger’s, Rett’s, childhood disintegrative disorder, and pervasive developmental disorder not otherwise specified. Diagnosis of autistic disorder requires just six of 16 criteria (see Table 1).

What is unclear is whether the increasing numbers of children diagnosed with ASD reflect a true increase in prevalence or whether the apparent increase can be explained by changes in the diagnostic criteria. Indeed, an article published in the January 2009 issue of the Journal of Epidemiology claims that in the state of California, the increase in ASD diagnoses cannot merely be attributed to the change in diagnostic criteria.4 Authors Hertz-Picciotto and Delwiche posit a role for environmental factors as well.

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Our goal is to clarify this complicated and confusing diagnosis that manifests from infancy through adolescence with varying levels of dysfunction.