Researchers have known that there is a strong genetic component to autism but autism is also found in clusters around the country. Certain communities and states have a much higher rate than the rest of the country http://graphics.latimes.com/usmap-autism-rates-state/. Autism rates are highest in the Northeast and on the West Coast and lowest among the Southern and Plains states. For example, a child born in California is several times more likely to be diagnosed with autism than a child in Alabama, but the question is why?
The fact that clusters are found in certain geographical areas seems to imply a local, environmental cause. Looking at environmental factors is difficult because much of that information is undocumented. One would have to consider air quality, water quality, and energy resources. Several other factors to consider are income levels and ethnicity. One group of researchers compared the rate of male genital malformations with the autism rate with the suggestion that environmental toxins in the area were responsible for the malformation due to the high rate. They found the greater the amount of malformations in boys, the greater the autism rates in the area, suggesting a correlation between environmental factors and autism diagnosis. But then how does one explain the phenomenon of identical twins which have the same genetic and environmental information but one twin has autism and the other does not.
When looking at such correlations, it is important to keep in mind that correlations do not mean causation, particularly when looking at the broader pictures. For example, one cannot assume that just because alcoholism rates are high in a city with high suicide rates that one is causing the other.
The theories on causation and rise of autism rates is complex and as research continues to make strides in the treatment, diagnosis and cure for autism it is important to keep in mind that autism is multi-factorial.