is a brain development disorder characterized by impaired social
interaction and communication, and by restricted and repetitive
behavior. These signs all begin before a child is three years
old. The autism spectrum disorders (ASD) also include related
conditions with milder signs and symptoms.
has a strong genetic basis, although the genetics of autism
are complex and it is unclear whether ASD is explained more
by multigene interactions or by rare mutations. In rare cases,
autism is strongly associated with agents that cause birth defects.
Other proposed causes, such as childhood vaccines, are controversial...
The prevalence of ASD is about 6 per 1,000 people, with about
four times as many boys as girls. The number of people known
to have autism has increased dramatically since the 1980s, partly
due to changes in diagnostic practice; the question of whether
actual prevalence has increased is unresolved.