I strongly suspect the modern "spectrum" is just grouping and labeling certain personality profiles that have always existed in roughly similar proportions.
Meaning that it's not the proportion that is increasing, it's that we chose to define certain combinations of human traits as ASD.
On the whole this labeling is probably useful since more people will get the support they need. But the point of delineation is mostly arbitrary.
Strongly heriditary. When you married within the local village the chance/risk of your spouse also being on the spectrum was lower than when you matchmake through online services and hundreds of dates.
/non-diagnosed GenX with two out of three kids diagnosed so far
So is it genetic background or is it your neurodivergent modeling of the behavior to them?
FYI it can't be determined without a much more in-depth genetic studies. This is especially true with complex multi-gene interaction traits like autism seems to be.