Scientists don't know exactly what causes autism spectrum disorder (ASD).
Autism was first described in the 1940s, but very little was known about it until the last few decades. Even today, there is a great deal that we don't know about autism.
Because the disorder is so complex and no two people with autism are exactly alike, there are probably many causes for autism. It is also likely that there is not a single cause for autism, but rather that it results from a combination of causes.
Scientists are studying some of the following as possible causes of or contributors to ASD.
A great deal of evidence supports the idea that genes are one of the main causes of or a major contributor to ASD. More than 100 genes on different chromosomes may be involved in causing ASD, to different degrees.3,4
Many people with autism have slight changes, called mutations, in many of these genes. However, the link between genetic mutations and autism is complex:
- Most people with autism have different mutations and combinations of mutations. Not everyone with autism has changes in every gene that scientists have linked to ASD.
- Many people without autism or autism symptoms also have some of these genetic mutations that scientists have linked to autism.
This evidence means that different genetic mutations probably play different roles in ASD. For example, certain mutations or combinations of mutations might:
- Cause specific symptoms of ASD
- Control how mild or severe those symptoms are
- Increase susceptibility to autism. This means someone with one of these gene mutations is at greater risk for autism than someone without the mutation.
If someone is susceptible to ASD because of genetic mutations, then certain situations might cause autism in that person.
For instance, an infection or contact with chemicals in the environment could cause autism in someone who is susceptible because of genetic mutations.1 However, someone who is genetically susceptible might not get an ASD even if he or she has the same experiences.2
Researchers are also looking into biological factors other than genes that might be involved in ASD. Some of these include:
- Problems with brain connections
- Problems with growth or overgrowth in certain areas of the brain
- Problems with metabolism (the body's energy production system)
- Problems in the body's immune system, which protects against infections
- Landrigan, P. J. (2010). What causes autism? Exploring the environmental contribution. Current Opinion in Pediatrics, 22(2):219–225.
- Hallmayer, J., Cleveland, S., Torres, A., Phillips, J., Cohen, B., Torigoe, T., et al. (2011). Genetic heritability and shared environmental factors among twin pairs with autism. Archives of General Psychiatry, 68(11), 1095–1102.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development. (2014). Common gene variants account for most genetic risk for autism. Retrieved March 2, 2018, from https://www.nichd.nih.gov/news/releases/072114-gene-variants-autism
- Pinto, D., Pagnamenta, A.T., Klei, L., Anney, R., Merico, D., Regan, R., et al. (2010). Functional impact of global rare copy number variation in autism spectrum disorders. Nature, 466, 368-372. Retrieved February 23, 2018, from https://www.nature.com/articles/nature09146