But I think the issue linking “nerds marrying nerds” and all the more specific reports linked to that results from the classic element of change in input causing change in output.
Women have been older mothers for years. When “pioneer” women had 10-12 children, spread out over 20 years, there was appreciably less autism.
The change was men and women from the same specific disciplines marrying each other much more frequently. Women who were strong in math met and married men they met in math graduate school. Lawyers, with the mental skills required form that profession, met in law school and married each other, as opposed to a school teacher or nurse.
That was the change in the input, not parental age.
Remember, Autistics are often brilliant in one area, but totally deficient in all others. Are their areas of brilliance from the same input from both parents? I think so.
Now we're back to the "incidence vs. diagnosis" question and the issue of defining "autism."
Apart from that, though, I think the idea of people with the same very specific qualities producing offspring is probably one of many factors.
As described in post 201 and the last paragraph of post 195, one can't compare the pregnancy and birth of a 12th child delivered at 40 to an experienced woman against the first and only chlld born to a woman of 40 whose body had not been "primed" by previous pregnancies.
We also cannot compare today's intrusive record-keeping by government of births and health of infants to the conditions of children in the frontier environment, where a severely disabled child unable to work could be killed and buried on the family farm and no one would know what really happened but the parent(s).