Causes & Common Questions

What causes autism?

Researchers do not know the primary causes of autism. Studies suggest that a person’s genes and aspects of their environment may affect development in ways that lead to autism. CDC describes autism as a developmental disability caused by differences in the brain, and says there is not just one cause — scientists believe several causes act together. For many autistic people, the specific cause is not yet known.

NICHD notes that genes appear to be a main contributor: more than 100 genes on different chromosomes may be involved, to different degrees. Many autistic people carry changes in some of these genes, but most have different combinations — and many people who are not autistic carry some of them too. Researchers are also studying how brain connections, brain growth, metabolism and the immune system may play a part.

Factors linked to a higher likelihood

Some factors are associated with an increased likelihood of autism. According to NIMH and CDC, these include:

• Having a sibling with autism
• Having older parents
• Certain genetic or chromosomal conditions, such as Down syndrome, Fragile X syndrome or tuberous sclerosis
• Having a very low birth weight
• Complications at birth

The American Academy of Pediatrics adds that preterm infants are more likely to be identified as autistic, and that most autistic children have no specific medical or genetic condition that explains their autism. Not everyone who has these factors will be autistic, and many autistic people have none of them.

Do vaccines cause autism?

The World Health Organization states that extensive research shows the measles, mumps and rubella (MMR) vaccine does not cause autism. The study that first suggested a link was later shown to be wrong and fraudulent; the journal withdrew it. WHO also reports that other childhood vaccines, and the preservative and adjuvant ingredients that have been studied, have not been found to increase the likelihood of autism.

The American Academy of Pediatrics says decades of high-quality, large-scale studies show that vaccines do not cause autism, and describes vaccines as among the safest and most effective ways to protect children. The AAP also notes that studies do not show a causal link between acetaminophen (Tylenol) use and autism.

Is autism becoming more common?

CDC’s ADDM Network has tracked the number of eight-year-olds identified with autism in monitored U.S. communities since 2000. The identified rate rose from about 1 in 150 children (2000 data) to about 1 in 31 (2022 data). The American Academy of Pediatrics attributes the rise in reported autism since the early 1990s to greater awareness, more screening and a broader definition of autism under the DSM-5 in 2013. CDC notes that rates vary a great deal between communities — in the 2022 data, from 9.7 to 53.1 per 1,000 children depending on the site — and the AAP says access to screening likely contributes to those differences.

Is there a test for autism?

There is no medical test, such as a blood test, that diagnoses autism. Doctors look at a child’s behavior and development. CDC says autism can sometimes be detected at 18 months or younger, and that by age 2 a diagnosis from an experienced professional can be considered reliable — but many children are not diagnosed until much later, and some people are not identified until adolescence or adulthood. A screening questionnaire is not a diagnosis; it only shows whether a closer look is needed. How screening and evaluation work.

Other questions families ask

• Can autism be diagnosed in adults? Yes. NIMH explains that adults can ask a health care provider for a referral to a neuropsychologist, psychologist or psychiatrist experienced with autism. Adult identification.
• Does autism go away? CDC says autism begins before age 3 and can last throughout a person’s life, though characteristics may change over time. Supports and services can improve health, well-being and daily functioning at any age.
• Is it more common in boys? CDC reports that autism is identified more than three times as often in boys as in girls, and that it occurs in all racial, ethnic and socioeconomic groups.