My joke: They haven't put the autism in the vaccines for fifty years, silly. That trope does work great to get the gullible to self-select their children to be sickly and weak, though.
My real theory: parents are in denial about their own autism
I don't think that the people who use this argument actually care about autism. I think it's just an anti-vax argument that claims that vaccines=bad assuming that autism=bad. So it's an unfounded claim with ableist presuppositions.
There's a fallacy that I keep making and I think it's because I'm #ActuallyAutistic: When I reach a conclusion in my mind based on logical deduction, I wrongly assume that everyone else must have reached the same conclusion, since they have access to the same data and logic is universal... Then when I tell other people my thoughts, I introduce them with "you probably already know it, but I realized that ..."
I mean, I know that different people have different experiences that make them reason in different ways and value certain pieces of information more than others, but still I keep falling in this mental trap...
I was already aware that adults and those who can communicate are under-diagnosed, that queerness and transness are much more likely, and it took me forever but also - that the typical female masking behavior that makes it difficult to diagnose applies to me too...
It's still hard for me to include myself with women, even in my own thoughts. I'm working on it and getting seen helps. And I understand what fierce little me had to do to survive and that involved a lot of masking.
Regarding "there's barely anyone non-autistic around", I know right? - but I also think that it's a bit biased by the fact that we tend to find each other and stick together.