One possible mechanism: One of the proposed reasons for the recent rise in levels of autism is that more autistic children are sent to the psychiatrist. You ne

One possible mechanism: One of the proposed reasons for the recent rise in levels of autism is that more autistic children are sent to the psychiatrist. You need to be relatively "rich" to be able to afford sending your child to the psychiatrist. Most programmers in Silicon Valley are relatively rich. Therefore, more children are sent to the psychiatrist in Silicon Valley than in the "rest" of the population, so more children with autism are diagnosed.