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.
There are some evidence that hints to genetic factors. For example, we know that boys are at much higher risk than girls, but we don't know why. Similarly, we know that older parents are more likely to have autistic children—but again, we don't know why.
Also the boy/girl factor could be explained by different behaviors, so different exposures.
Not saying I know anything about this, or that this is the case, just that the increased family risk and boy/girl risk doesn't mean it is genetics.
That's correct, but looking at concordance rates between monozygotic (identical) and dizygotic (fraternal) twins demonstrates that there is at least a partial genetic connection.
Identical twins share the diagnosis in 77% of cases, while fraternal twins only share the diagnosis in 31% of cases.[1] Given that identical twins share more genetic material, the conclusion is that autism has a genetic component to its incidence.
That said, most research on the subject, including the paper I'm citing, establishes that there is most likely a dual nature-nurture component. In particular, this study was conducted on twins within the same household, so the genetic component would have been augmented by shared environmental factors.
The implication seems to be that the parents also exhibit traits of the autism spectrum, so it's hardly surprising that their kids would do the same.
Despite the quantity of autism research, most of it is quite bad and not that helpful on questions like this.
"Environmental" can include "parenting", but in this case the distinction is between "got it somehow from autistic-leaning parents" vs got from something else.
Also, the claim that autism might be somehow learned from parents is extraordinary and not widely claimed or supported by existing evidence. It was a discredited theory of the past though, that "cold" parenting created austism.
So we've seen that shift. The diagnostic criteria changed a lot, not just a little, over time.
By coincidence, Wakefield launched his fraudulent crap about vaccines a few years after the criteria were broadened, and that became the popular explanation rather than the fact that the criteria changed. Belief in his BS is perhaps fading but it's still around.
The "epidemic" was predicted and an intentional outcome of the DSM criteria change, though.
If you hang out with a bunch of families with autistic kids, it's very obviously inherited in a large fraction of cases, because the kids are so much like their parents.
Another large fraction is likely genetic in the sense of "de novo" mutation, rather than inherited genes. There's likely also a group of environmental cases, mostly prenatal environmental factors.
Because social function depends on many coordinated aspects of human cognition, any condition that has a fairly global effect on the brain can create traits meeting the autism diagnostic criteria. The criteria are very broad, with many different ways to meet them.
It's important to realize that autism is a broad category, not a single condition, and no proposed single gene or environmental factor ever seems to relate to more than a few percent of cases. https://intellectualizing.net/2013/12/17/book-rethinking-aut...
Some of those causes could well be new to the modern world, but the changing diagnostic criteria do explain most of the expansion in diagnoses already, so there isn't a lot of evidence that we need to be looking for something novel to explain the increased diagnoses.