The underlying assumption of these hypotheses is an "impossible mutation model where deleterious mutations build up to form haplotypes and syndromes" which can "explain" autism "spectrum" "disorders." "It is impossible because it goes against evolutionary theory (survival of the fittest) and genetic models (fixation and genetic drift)."
"If a mutation exists long enough in the genome, it must either go towards fixation or get lost by genetic drift. If the mutation is positive, it has magnitudes higher chance of reaching fixation than a mutation that is neutral or negative. Therefore, few deleterious mutations with survive over time. Only new ones will, and they are not linked to each other" as is claimed. "The level of deleterious mutations is always kept low in the genome, much lower than the prevalence of most neuropsychiatric disorders."
You can have a higher prevalence of some relatively "deleterious" trait than you would expect from "deleterious mutations" by simple things such as these traits not being caused by a single, dominant, mutation, but rather depending on a complex interaction of multiple genes, "epistasis", and "additive variation".
Such phenomena "protect" some deleterious traits from natural selection in a way that invalidates this adaptationist prediction that they shouldn't exist if they were purely genetic. Not only they such genes can persist unexpressed as deleterious traits, but also in less deleterious points of a "spectrum" (somewhat like height or "pure" skin color, unaffected by tanning), and perhaps in some cases they can even be
beneficial variation (the textbook example is sickle cell anemia, but the principle could apply to conditions related with cognitive functioning as well).
Furthermore there's the additional complication that humans care for each other and so any deleterious trait is not as deleterious as it would be in less social and less technological species, further decelerating the negative selection they would otherwise suffer.
As I see it, there are only two plausible explanations for autism. Both are based on environmental toxins. The difference is that one looks to chemical toxins, while the most likely explanation looks to social toxins that cause depression and complex post-traumatic assault and battery. Autistics are victims of violent crime, not disease. This was known before the truth was "discredited" through politically-motivated brainwashing by perps who wish to avoid responsibility.
Now you're entering into conspiracy theory territory.
Child abuse isn't something ever posited to be harmless by any biologist or geneticist. If such conspiracy theory were true, that's what we would be expecting to hear. "Beat all you want, as long as you don't physically hurt, there's no such thing as psychological trauma, much less something that could have psychiatric implications. So relax and beat your children at will, that's the way to go".
In fact we observe the opposite trend, even the traditionally thought harmless spanking is being condemned by scientists of the relevant disciplines.
There's no secret campaign to tolerate child abuse, to make it seem harmless. It's only something that does not seem to be the cause of autism in particular. Just like it is not the case for Down syndrome, it does not make any good to posit that it was somehow the fault of the parents for something they apparently have no control at all. If you have really concern towards unjust psychological trauma, that's something you ought to find really bad.
That's not to say that there's no environmental factor at all. I don't know about factors concerning the interaction between parents and children, but there's some literature on certain types of food, apparently autists have a more "leaky" brain-blood barrier (perhaps it's something more indirect, I'm not sure) and some ingredients of some foods or of the digestion of such foods are to be avoided.
