Autism is a brain development disorder characterized by impairments in social interaction and communication, and restricted and repetitive behavior, all exhibited before a child is three years old. These characteristics distinguish autism from milder autism spectrum disorders (ASD).

Heritability contributes a large fraction of the risk of a child's developing the disorder, although the genetics of autism are complex, and it is generally unclear which genes are responsible. In rare cases, autism is strongly associated with agents that cause birth defects. Other proposed causes, such as the exposure of children to vaccines, are controversial and the vaccine hypotheses are unsupported by convincing scientific evidence. Most recent reviews estimate a prevalence of one to two cases per 1,000 people for autism, and about six per 1,000 for ASD, with ASD averaging a 4.3:1 male-to-female ratio. The number of people known to have autism has increased dramatically since the 1980s, at least partly due to changes in diagnostic practice; the question of whether prevalence has increased is unresolved.

Autism affects many parts of the brain; how this occurs is poorly understood. Parents usually notice signs in the first year or two of their child's life. Early intervention may help children gain self-care and social skills, although few of these interventions are supported by scientific studies. There is no cure. With severe autism, independent living is unlikely; with milder autism, there are some success stories for adults, and an autistic culture has developed, with some seeking a cure and others believing that autism is a condition rather than a disorder.

Diagnosis

Diagnosis is based on behavior, not cause or mechanism. Autism is defined in the DSM-IV-TR as exhibiting at least six symptoms total, including at least two symptoms of qualitative impairment in social interaction, at least one symptom of qualitative impairment in communication, and at least one symptom of restricted and repetitive behavior. Sample symptoms include lack of social or emotional reciprocity, stereotyped and repetitive use of language or idiosyncratic language, and persistent preoccupation with parts of objects. Onset must be prior to age three years, with delays or abnormal functioning in either social interaction, language as used in social communication, or symbolic or imaginative play. The disturbance must not be better accounted for by Rett syndrome or childhood disintegrative disorder. ICD-10 uses essentially the same definition.

Several diagnostic instruments are available. Two are commonly used in autism research: the Autism Diagnostic Interview-Revised (ADI-R) is a semistructured parent interview, and the Autism Diagnostic Observation Schedule (ADOS) uses observation and interaction with the child. The Childhood Autism Rating Scale (CARS) is used widely in clinical environments to assess severity of autism based on observation of children.

A pediatrician commonly performs a preliminary investigation by taking developmental history and physically examining the child. If warranted, diagnosis and evaluations are conducted with help from ASD specialists, observing and assessing cognitive, communication, family, and other factors using standardized tools, and taking into account any associated medical conditions. A differential diagnosis for ASD at this stage might also consider mental retardation, hearing impairment, and a specific language impairment such as Landau-Kleffner syndrome. In the UK the National Autism Plan for Children recommends at most 30 weeks from first concern to completed diagnosis and assessment, though few cases are handled that quickly in practice. ASD can sometimes be diagnosed by age 14 months, but a 2006 U.S. study found the average age of first evaluation by a qualified professional was 48 months and of formal ASD diagnosis was 61 months, reflecting an average 13-month delay, all far above recommendations.

Underdiagnosis and overdiagnosis are problems in marginal cases, and much of the recent increase in the number of reported ASD cases is likely due to changes in diagnostic practices. The increasing popularity of drug treatment options and the expansion of benefits has given providers incentives to diagnose ASD, resulting in some overdiagnosis of children with uncertain symptoms. Conversely, the cost of screening and diagnosis and the challenge of obtaining payment can inhibit or delay diagnosis. It is particularly hard to diagnose autism among the visually impaired, partly because some of its diagnostic criteria depend on vision, and partly because autistic symptoms overlap with those of common blindness syndromes.

The symptoms of autism and ASD begin early in childhood but are occasionally missed. Adults may seek retrospective diagnoses to help them or their friends and family understand themselves, to help their employers make adjustments, or in some locations to claim disability living allowances or other benefits.

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