No one knows what causes autism. The number of children diagnosed with the disorder has increased significantly over the past decade or so, but experts are not sure whether that reflects an improvement in diagnostic awareness or a true increase in prevalence.
Recent evidence suggests that the disorder may be caused by random genetic mutations, as it is associated with advanced maternal and/or paternal age at conception. That would account for the great variability of impairment and neural systems involved.
There is also evidence that the disorder may be caused by failure of embryonic brain cells to undergo normal patterns of migration during early development, affecting later brain structure and wiring of nerve-cell circuits that control social, language, movement, and other abilities.
A sex imbalance in the number of affected children (four times more males than females) suggests the disorder may also be related to fetal exposure to abnormally high levels of testosterone in utero; many of the traits of autism are said to reflect male cognitive and behavioral preferences, such as orientation to detail rather than the big picture, affinity for things rather than social experience, facility for math and numbers, and even linguistic impairment; autistic children can accumulate a large vocabulary without being able to sustain a conversation.
A belief that autism is caused by standard childhood immunization with mercury-containing vaccines persists despite many studies discrediting the link and retraction of the original paper linking autism to immunization.