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.
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