The exact cause or causes of autism are still unclear, and some theories have sparked controversy, such as the role of vaccinations (see our article on vaccination safety here) or the importance of dietary factors such as gluten (see our article on gluten here.) There is clear evidence for a possible genetic link.
Autism and related conditions seem to run in families; if one identical twin is diagnosed with autism, the other twin has a much higher chance of also having autism.
We also know that children who are born prematurely have a greater chance of being autistic, and children with older fathers are at slightly higher risk of autism. 
While genetic causes of autism are still being investigated, other studies suggest that certain environmental exposures in the womb may increase the chances of a child developing autism.
In addition, certain geographical areas of the country have much higher rates of autism, such as California, Texas, North Carolina and Utah.
Are doctors or parents in these states quicker to suspect and diagnose autism, or is something else going on that is causing more kids to develop autism?
States that require a physician or psychologist to diagnose autism for families to qualify for special education benefits tend to have lower overall rates of autism.
It’s also true that children who live in urban areas and whose parents are better off are more likely to be diagnosed with autism, but again that finding doesn’t explain the higher rates of autism in California, Texas, North Carolina and Utah.
Some scientists have suggested that higher amounts of environmental toxins in these areas may explain the relatively high rates of autism.