Autism is classified as a neurodevelopmental disorder which manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior. Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers. While there is disagreement about the magnitude, nature, and mechanisms for such environmental factors, researchers have found seven genes prevalent among individuals diagnosed as autistic. Some estimate that autism occurs in as many as one American child in 166, however the National Institute of Mental Health gives a more conservative estimate of one in 1000. For families that already have one autistic child, the odds of a second autistic child may be as high as one in twenty. Although autism is about 3 to 4 times more common in boys, girls with the disorder tend to have more severe symptoms and greater cognitive impairment. Diagnosis is based on a list of psychiatric criteria, and a series of standardized clinical tests may also be used.

Autism may not be physiologically obvious. A complete physical and neurological evaluation will typically be part of diagnosing autism. Some now speculate that autism is not a single condition but a group of several distinct conditions that manifest in similar ways.

By definition, autism must manifest delays in "social interaction, language as used in social communication, or symbolic or imaginative play," with "onset prior to age 3 years", according to the Diagnostic and Statistical Manual of Mental Disorders. The ICD-10 also requires symptoms to be "manifest before the age of three years." There have been large increases in the reported incidence of autism, for reasons that are heavily debated by researchers in psychology and related fields within the scientific community.