There  is no known single  cause for autism spectrum disorder, but it is  generally accepted that  it is caused by abnormalities in brain  structure or function. Brain  scans show differences in the shape and  structure of the brain in  children with autism compared to in  neurotypical children. Researchers  do not know the exact cause of  autism but are investigating a number of  theories, including the links  among heredity, genetics and medical  problems.

In many families, there appears to be a pattern  of  autism or related disabilities, further supporting the theory that  the  disorder has a genetic basis. While no one gene has been identified  as  causing autism, researchers are searching for irregular segments of   genetic code that children with autism may have inherited. It also   appears that some children are born with a susceptibility to autism,  but  researchers have not yet identified a single “trigger” that causes   autism to develop.

Other researchers are investigating the  possibility that under certain  conditions, a cluster of unstable genes  may interfere with brain  development, resulting in autism. Still other  researchers are  investigating problems during pregnancy or delivery as  well as  environmental factors such as viral infections, metabolic  imbalances  and exposure to chemicals.​ Genetic Vulnerability
​Autism   tends to occur more frequently than expected among individuals who  have  certain medical conditions, including fragile X syndrome, tuberous   sclerosis, congenital rubella syndrome and untreated phenylketonuria   (PKU). Some harmful substances ingested during pregnancy also have been   associated with an increased risk of autism.​ Environmental Factors
​Research   indicates other factors besides the genetic component are contributing   to the rise in increasing occurrence of autism – for example,   environmental toxins (e.g., heavy metals such as mercury), which are   more prevalent than in the past. Those with autism (or those at risk)   may be especially vulnerable to such toxins, as their ability to   metabolize and detoxify these exposures might be compromised.