Sociologist Peter Bearman of Columbia and colleagues have completed a major study attempting to explain why the rate of autism has increased more than 500 percent over the past 20 years. They found that they could explain more than half of the increase with the following three factors:
Better diagnosis, that is, differentiating autism from mental retardation. They estimate this accounts for 26% of the increase.
Awareness. By comparing the rate of autism in children who live near another child with autism to the rate among those who don't, they estimate that increased awareness of autism accounts for 16% of the increase.
Older parents. The rate of autism is higher among children of older parents, and the age of parents is increasing. They estimate this accounts for 11% of the increase.
That adds up to 53%, leaving nearly half of the increase unexplained. I don't think this study is anything like the last word, but it is a serious attempt to quantify the skeptic's sense that the rise in autism is just about changes in diagnosis. That these statisticians were not able to account for all of the rise in autism leaves the lingering suspicion that something about modern life is the cause. With vaccines pretty much ruled out by a whole raft of studies, that leaves the disconcerting possibility that almost anything about our lives could be the culprit.