Reply #117: According to the CDC, the cause of autism is not known.

[Jim__]

From the CDC's website:

What causes autism?

We have learned a lot about the symptoms of ASDs and have improved efforts to track the disorders, but we still don’t know a lot about the causes of ASDs. Scientists think that both genes and the environment play a role, and there might be many causes that lead to ASDs.

Family studies have been most helpful in understanding how genes contribute to autism. Studies have shown that among identical twins, if one child has autism, then the other will be affected about 75% of the time. In non-identical twins, if one child has autism, then the other has it about 3% of the time. Also, parents who have a child with an ASD have a 2%–8% chance of having a second child who is also affected.<5>,<6>

For most people with ASDs, the cause is not known. But ASDs tend to occur more often than expected among people who have certain other medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU). Some harmful drugs taken during pregnancy also have been linked with a higher risk of autism, specifically, the prescription drug thalidomide.

CDC’s Centers for Autism and Developmental Disabilities Surveillance and Epidemiology (CADDRE) are working together on a large, population-based study to better understand the possible risk factors for and causes of autism. Called the Study to Explore Early Development (SEED), this project will help answer the many questions needed to find the causes of autism and—if possible—come up with strategies to prevent this complex disorder.