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.[1] In rare cases, autism is strongly associated with agents that cause birth defects.[2] Other proposed causes, such as the exposure of children to vaccines, are controversial and the vaccine hypotheses are unsupported by convincing scientific evidence.[3] 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.[4]

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.[5] With severe autism, independent living is unlikely; with milder autism, there are some success stories for adults,[6] and an autistic culture has developed, with some seeking a cure and others believing that autism is a condition rather than a disorder.[7]