Causes of Autism

Written by the Healthline Editorial Team | Published on July 23, 2014
Medically Reviewed by Monica Gross, M.D. on July 23, 2014

Causes of Autism

The primary cause of autism spectrum disorders (ASDs) remains a mystery. However, experts believe that they have pinpointed some factors that may increase a child’s risk for for developing autism.

Differences in the Brain

Different parts of the brain can show evidence of autism at work. Some research indicates that problems in the sections of the brain that are responsible for body movements and emotion could indicate higher autism risk.

Other research has focused on the brain’s development and function. The brains of people with autism often show differences in size and structure, compared to those of people without ASD. Also, normal differences in gene expressions between the frontal and temporal cortex are not present in the brains of people with ASD.

Neurotransmitters are brain chemicals that aid in communication between the brain and the rest of the body. These are also being studied for their role in autism development.

Genetic Factors

Researchers are looking at genetic links in families. Studies have shown that autism is more common in the siblings of an autistic child than in the general population. This suggests a genetic contribution to autism development.

Some medical conditions have been linked to an increased risk of autism. These include genetic disorders such as Fragile X syndrome and tuberous sclerosis. The link between tuberous sclerosis and ASD is unclear. But studies have shown that ASD rates are much higher among children with tuberous sclerosis.

Some ASD statistics provided by the U.S. Centers for Disease Control and Prevention (CDC) include:

  • If one identical twin has an ASD, there is a 60 to 96 percent chance the other twin will have some sort of ASD.
  • Although fraternal twins have a lower chance of both having autism, if one has an ASD the other has up to a 24 percent chance.
  • If a family already has one child with autism, the chance that any other children will have ASD is between two to eight percent.
  • About 10 percent of children with an ASD also have another genetic, neurological, or metabolic disorder.

Exposure to Drugs and Chemicals


The CDC states that the some drugs show a link to ASDs. When taken during pregnancy, thalidomide and valproic acid may pose increased risks of ASDs for the fetus. Thalidomide is used to treat patients recently diagnosed with multiple myeloma. Valporic acid is used to treat a number of conditions, including certain types of seizures and mania symptoms in people with bipolar disorder. It is also used as a preventative treatment for migraines.


Environmental toxins are also suspected for increasing risk for ASDs. A study of the San Francisco Bay Area suggested a link between autism and air polluted with mercury, cadmium, nickel, and vinyl chloride.

Common Misconceptions

Thimerosal, a mercury-based preservative used in vaccinations in the past, has also been suspected of causing ASDs. Despite many well-designed studies, there is still no research that links mercury-containing vaccines to ASDs. Thimerosal has not been used in vaccinations in the United States since 1999, so exposure is limited. In fact, the CDC published a study in 2010 on the linkage. The study stated that “prenatal and infant exposure to vaccines and immunoglobulins that contain thimerosal does not increase risk for autism spectrum disorder.”

There have been several worldwide studies to determine if the MMR vaccine can cause ASDs. This vaccine is critical in preventing measles, mumps, and rubella. A 1998 article in The Lancet suggested a possible relationship between the MMR vaccine and the development of ASDs. Six years later, The Lancet published a retraction. The journal’s editors stated that no connection exists between the MMR vaccine and autism.

The CDC and other reputable sources continue to maintain that there are no confirmed links between MMR vaccinations and ASDs.

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Show Sources

  • Wakefield, A., Murch, S., Davies, S., Waker-Smith, J., Linnell, J., Casson, D., et al. (1998). Ileal-lymphoid-nodular hyperplasia, non-specific colitis, and pervasive developmental disorder in children. The Lancet351(9103), 637-641. Retrieved October 28, 2012, from

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