To clarify, I'm not trying to argue definitively that these traits are all genetic vs. acquired by watching one's parents. Perhaps it's some mix, though mostly-genetic seems likely to me. I _am_ trying to say that often the traits run in families, which argues against external causes such as vaccines and chemicals.
My son is the third or fourth generation to be likely-diagnosable, for example.
Currently-middle-aged parents without intellectual disability had almost zero chance of being diagnosed as children, because when we were kids autism was defined such that you usually had to be nonverbal or intellectually disabled to qualify.
A very commonly-reported experience is for parents to be diagnosed after their children are diagnosed.