Dan Olmstead, a journalist of "no autism among the Amish" fame, is a case in point, with his three new Age of Autism articles titled Something Wicked -- 1, Something Wicked -- 2 and About Those "Old Dads". In them he puts forth the hypothesis that autism may be caused by chemicals parents are exposed to. The mechanism or its plausibility are not clear, but let's leave that aside for the time being.
Olmstead cites work by Thomas Felicetti, who had found a higher prevalence of chemists among the parents of autistic children compared to controls. This confirmed prior work by Mary Coleman. He further goes into the case histories of Kanner's 11 autistic patients born in the 1930s. Let me quote from his "Old Dads" article:
We've also pointed out a possible connection in the very first cases described in the medical literature -- 11 children born in the 1930s -- with new chemical compounds, plausibly including mercury-based fungicides. One of those 11 kids was the son of a plant pathologist, another the son of a forestry professor at a southern university, and a third grew up in an area being heavily planted with seedlings to create a national forest in Mississippi (there's that southern forestry connection again -- in fact, his home town is named Forest).
A fourth was the son of a mining engineer. And the very first case to come to medical attention, at Johns Hopkins University in 1935, was the son of a 30-year-old chemist-attorney at the U.S. Patent Office. A chemist.
Anyone with more than a passing interest in autism research will recognize the obvious confound in this hypothesis. I would also point out that these studies are from a different generation, and we might find things have changed. For example, I would wager computing might be a relatively common occupation among parents of autistic children nowadays. (Readers are encouraged to provide anecdotal data in the comments section).
A series of articles (e.g. Baron-Cohen et al. (1998)) have shown that mathematicians, computer scientists, physicists and engineers are over-represented among the parents of autistic children. Jarrold & Routh (1998) analyzed the data and found that the parents of autistic children were also over-represented in accountancy and science, suggesting that perhaps there's a general professional over-representation. Wheelwright & Baron-Cohen (2001) argued that even taking into account a possible over-representation of professionals, engineers were still over-represented.
Further evidence of this confound comes from an AQ test assessment by Baron-Cohen et al. (2004). The researchers found that "scientists (including mathematicians) scored significantly higher than both humanities and social sciences students" and that "within the sciences, mathematicians scored highest." Further, the group of Mathematics Olympiad winners scored significantly higher than the male Cambridge Humanities students.
Dan Olmstead does try to address the confound by pointing out that there is no correlation between social class and autism, which is correct. But social class is clearly not what the confound is about.