In most cases, it is not possible to identify any specific event that triggers autistic disorder. About 7 in every 10 children and adolescents with autistic disorder also have mental retardation or other problems with their brain function or structure. Recent studies estimate that as many as 14 children out of 10,000 may have autism or a related condition. About 125,000 Americans are affected by these disorders, and nearly 4,000 families across the country have two or more children with autism. Three times as many boys as girls have autism.
Researchers are still unsure about what causes autism.
Several studies suggest that autistic disorder might be caused by a combination of biological factors, including exposure to a virus before birth, a problem with the immune system, or genetics. Studies of families and twins suggest a genetic basis, especially in families where more than one child has autism. Brothers and sisters of people with autism are about 50 times more likely to have the disorder than other people. It is important for scientists to find the genes that are responsible for autism because knowledge of these genes could help doctors easily diagnose the disorder in other children. Other studies have found that people with autistic disorder have problems in several parts of their brains.