Autism Spectrum DisorderCauses and Risk Factors

Causes of Autism #

The exact causes of autism aren’t yet known. However, scientists believe that genetics play a big role in determining who is at greatest risk of developing the condition. Environmental factors may also contribute.

Autism Prevalence #

Researchers currently estimate that autism may occur in approximately 1-3 in every 100 children, with the condition at least 4 times as likely to arise in boys as in girls.¹ While this prevalence is much higher than what was recorded decades ago, the definition and diagnosis of autism have also changed, significantly broadening the scope into a much wider spectrum of conditions collectively termed “autism spectrum disorder.”  

Around the globe, autism prevalence rates vary widely, in some places as low as 1 in 1000 or as high as 4 in 100. These measurements are complicated by differences in the definition of autism and mechanisms of diagnosis. A child that would be considered autistic in one country may not reach the diagnostic criteria elsewhere in the world.² In addition, in areas where services such as behavioral therapies and learning assistance are available to children with autism, parents are more likely to ask for a diagnosis for their child, which can affect prevalence estimates.³

Autism Risk Factors #

Most autism risk is driven by our genes. Researchers estimate that upwards of 65% -- perhaps as high as 95% -- of autism risk is due to genetics.⁴ The remaining risk is what scientists call “environmental,” meaning events and conditions outside of our genetics. In the case of autism, environmental risk factors may include pregnancy or early-life exposures, or perhaps other factors that we have not yet discovered.⁵

Boys are much more likely to be diagnosed with autism than girls, and girls tend to have different symptoms than boys. While some of this disparity may be because our diagnostic tools are designed to pick up boys’ symptoms, some scientists also believe that even given the same risk genes and environment and better diagnostic tools, a girl would be less likely to develop autism than a boy.⁶ This is often called a “protective effect” in girls. Researchers are still trying to understand what drives this protective effect, or whether it even exists.⁷
 

Autism Heritability #

Researchers have identified more than 100 common genetic variants that contribute to the development of autism. Generally, these variants are passed down in families over many generations. Because of this, people whose close family members have been diagnosed with autism are much more likely to develop autism or have some autistic traits. 

Scientists believe that most cases of autism are driven by many mild variants working together to raise the risk of the disorder. However, up to 10% of children with autism have a rare genetic condition that conveys a high level of risk on its own.⁴ Examples of rare genetic conditions that carry a high risk of autism include Prader-Willi/Angelman syndrome, fragile X syndrome, Cohen’s syndrome, and 16p11.2  and 22q11 deletion syndromes.  

Environmental Risk Factors #

While genetics play a major role in the development of autism, certain environmental factors may also contribute to whether a person with a given genetic background develops symptoms of autism. Scientists have explored many different factors including medications, maternal health, vaccines, and more.⁴ The strongest evidence for autism association includes parental age, gestational diabetes,⁸ infections or maternal autoimmunity during pregnancy, birth complications, and use of the medication valproate during pregnancy. The scientific evidence is weaker for an association between autism and air pollution, pesticide, and heavy metal exposure.⁹