Autism is a brain development disorder that shows symptoms before a child is three years old and has a steady course with no remission. Its characteristic signs are impairments in social interaction and communication, as well as restricted and repetitive behavior. These characteristics distinguish autism from milder pervasive developmental disorders (PDD) or autism spectrum disorders (ASD), and reflect Leo Kanner's first descriptions of autism, which emphasized "autistic aloneness" and "insistence on sameness".
Heritability contributes about 90% of the risk of a child developing autism, but the genetics of autism are complex and typically it is unclear which genes are responsible. In rare cases, autism is strongly associated with agents that cause birth defects. Many other causes have been proposed, such as exposure of children to vaccines; these proposals are controversial and the vaccine hypotheses have no convincing scientific evidence.
Autism affects many parts of the brain, but how this occurs is poorly understood. Parents usually notice warning signs in the first year or two of their child's life. Rapid evaluation by a specialist is important as, although there is no cure, early intervention may aid attempts to help children gain self-care and social skills. However, only a very few of these interventions are supported by scientific studies. With severe autism independent living is unlikely, but with milder autism there are some success stories as adults, and an autistic culture has developed, with some seeking a cure and others believing that autism is simply another way of being.
Most recent reviews estimate a prevalence of 1–2 per 1,000 people for autism and close to 6 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. Some of this increase is undoubtedly due to changes in diagnostic practice; it is not known whether prevalence has increased as well.