The Genetics of Autism: What We Know

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Autism is a brain development disorder. It causes problems in the way a child communicates, behaves and interacts with others. Symptoms can range from mild to severe. Researchers still don't know what causes autism. However, they understand it better than they did in the past.

Experts used to think autism was a type of schizophrenia. Some thought bad parenting was the cause. There was also a time when some people believed vaccines caused autism.

As people's understanding of autism has changed, so has its name. Today, the full name for autism is autism spectrum disorder (ASD). The name reflects that there are various types of the condition. ASD includes:

A diagnosis of ASD usually occurs by age 3. Some children with ASD have mental retardation. Others have average or above average intelligence.

Is ASD a genetic disorder?

Even though scientists don’t know the cause of ASD, research now shows that genes play a role. For instance, identical twins share 100% of their genes. Some studies show that if one identical twin has ASD, the other twin has about a 60% chance of having ASD. Other research has found the percentage can be as high as 95. But because that number isn't 100%, other factors also are involved.

Researchers also are trying to find out which genes are involved in autism. So far they've identified clusters of genes as possible suspects. There also may be something in a child’s environment before or after birth that triggers these genes to cause autism. Possible triggers include viral infections and exposure to chemicals. Chemical imbalances in the body also may trigger autism.

Who is at risk for ASD?

There's no simple answer to this question. Genetic diseases often are predictable. ASD is not. You might carry genes that raise your child’s risk. That’s called susceptibility. But your child might not have ASD if nothing triggers those genes. 

Here's what the experts do know about risks for ASD:

  • Boys are almost five times more likely than girls to develop ASD.

  • Children who have brothers or sisters with ASD are at higher risk.

  • Some medicines taken during pregnancy may cause genetic changes that lead to ASD. Examples are valproic acid and thalidomide. Valproic acid (Depakote) is an anticonvulsant used to treat seizures, bipolar disorder, and to help prevent migraine headaches. Thalidomide is effective treatment for multiple myeloma and leprosy.

  • Children of older parents are at higher risk.

  • Being born with some other genetic diseases makes ASD more likely. These diseases include fragile X syndrome, tuberous sclerosis, neurofibromatosis and Rett syndrome.

What the Future Holds

Researchers around the world are trying to learn more about the genetics of ASD. They're checking into gene mutations (a change or defect in a normal gene) as well as inherited gene mutations that may increase the risk of developing ASD. They're also looking for triggers that may cause ASD, such as exposure to certain chemicals or infections. What they find might lead to a good screening test for autism. That has happened with other genetic diseases. The research also may lead to breakthroughs in earlier diagnosis, prevention and treatment.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Sep 18
THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.

  1. What Is Autism? Autism Science Foundation.

  2. Learning About Autism. National Institutes of Health.

  3. Genetic Factors in Autism Spectrum Disorders. Asperger/Autism Network.

  4. Causes. Autism Society.

  5. Autism Spectrum Disorder (ASD). Centers for Disease Control and Prevention.

  6. Genetic factors in Autism Spectrum Disorders. Asperger/Autism Network.

  7. Sandin S, Lichtenstein P, Kuja-Halkola R, et al. The Familial Risk of Autism. JAMA. 2014;311(17):1770-1777.

  8. Valproic Acid. Medline Plus, U.S. National Library of Medicine, National Institutes of Health.

  9. Thalidomide. Medline Plus, U.S. National Library of Medicine, National Institutes of Health.