“There is no getting around the data on age. A mother over 40 has a 77% higher risk of having a child with autism than does a mother under 25.
If age was irrelevant, that risk differential would not exist or would be so small as to be irrelevant. Maternal age has no effect on color blindness or hemophilia, but infant gender does because they are x-linked recessive disorders.”
Back to my original question. If the studies point to an interaction of genetics and environment, what genetics are different in 40yr olds that haven’t had prior pregnancies compared to those that have? Because it looks like the entirety of that difference is the exposures between age 25 and age 40. Ie, the environment. The older you are, the more ‘exposed’ you are. Else there would also be age differences in strictly genetically heritable diseases as well.
This does not compute. There is NO age differences in some strictly genetic diseases because there is no environmental component in them like the previously mentioned x-linked disorders. Age itself is an environmental factor, not just because of possible cumulative exposure to some third factor.
I think the issue here is percentages. As you mentioned earlier, women who survived childbirth, accidents and epidemics could be expected to produce children for years and years before hitting their late 30’s-40’s. That means the vast majority of the population was born earlier in their mothers’ lives. Since so many women today put off motherhood until that borderline-risk age, that large pool of people born to young mothers is shrinking. Therefore the percentage of children with autism is climbing. Not because there is more of some mystery ingredient floating around but because there are fewer children born to young mothers who would otherwise be keeping the ratios constant.