Scientists aren’t certain what causes autism, but it’s likely that both genetics and environment play a role.     Researchers have identified a number of genes associated with the disorder.  Studies of people with autism have found irregularities in several regions of the brain.  Other studies suggest that people with autism have abnormal levels of serotonin or other neurotransmitters in the brain.  These abnormalities suggest that autism could result from the disruption of normal brain development early in fetal development caused by defects in genes that control brain growth and that regulate how neurons communicate with each other.  While these findings are intriguing, they are preliminary and require further study.  The theory that parental practices are responsible for autism has now been disproved.

It is suspected that there may be multiple causes for ASD due to a complex interaction of genetic and environmental factors. Family studies have shed the most light on the genetic contribution to autism. Studies of twins have shown that in identical twins there is about a 75% rate of both twins having autism, while in non-identical twins this occurs about 3% of the time. The inheritance pattern is complex and suggests that a number of genes are involved.
For the majority of people with ASDs, the cause is not known; however, ASD tends to occur more frequently than expected among individuals who have certain other medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU). Some harmful substances ingested during pregnancy also have been associated with an increased risk of autism, specifically, the prescription drug thalidomide.

Sources:  
Mauk JE, Reber M, Batshaw ML.  Autism and other pervasive developmental disorders, 4th edition. In: ML Batshaw, editor.  Children with disabilities. Baltimore: Paul H. Brookes; 1997.
Powers MD.  What is autism?  In: MD Powers, editor.  Children with autism: a parents' guide, 2nd edition.  Bethesda, MD: Woodbine House; 2000. pp. 1-44.

Date: June 27, 2006
Content source: National Center on Birth Defects and Developmental Disabilities

What role does inheritance play?
Recent studies strongly suggest that some people have a genetic predisposition to autism.  In families with one autistic child, the risk of having a second child with the disorder is approximately 5 percent, or one in 20.  This is greater than the risk for the general population.   Researchers are looking for clues about which genes contribute to this increased susceptibility.  In some cases, parents and other relatives of an autistic child show mild impairments in social and communicative skills or engage in repetitive behaviors.   Evidence also suggests that some emotional disorders, such as manic depression, occur more frequently than average in the families of people with autism.