We can't be sure yet:
The earliest prevalence studies from 1966 to 1979 showed around 4-5 cases per 10,000 children in England and the USA. However the latest rates from the Center for Disease Control and Prevention showed 113 cases per 10,000 American children. … Let’s be clear that we don’t definitely know all the causes of the increasing numbers of children diagnosed with autism as a whole, or which of the underlying disease entities or subgroups are involved. Possibilities include expansion of the diagnostic criteria, greater awareness of autism by physicians and the public, secondary diagnoses added to individuals with intellectual disability, enhanced survival of preterm infants, older age of the parents, and as-yet-unproven environmental toxins.
The Dish 