U.S. National Library of Medicine
National Institutes of Health
Autism is a developmental disorder that appears in the first 3 years of life, and affects the brain’s normal development of social and communication skills.

Causes, incidence, and risk factors
Autism is a physical condition linked to abnormal biology and chemistry in the brain. The exact causes of these abnormalities remain unknown, but this is a very active area of research. There are probably a combination of factors that lead to autism.

Genetic factors seem to be important. For example, identical twins are much more likely than fraternal twins or siblings to both have autism. Similarly, language abnormalities are more common in relatives of autistic children. Chromosomal abnormalities and other nervous system (neurological) problems are also more common in families with autism.

A number of other possible causes have been suspected, but not proven. They involve:

Diet
Digestive tract changes
Mercury poisoning
The body’s inability to properly use vitamins and minerals
Vaccine sensitivity