I have studied this quite a lot.
Researchers have been busy pinning down genetic causes but that cannot possibly be the whole story.
Yes certain genes are definitely associated with autism. But you cannot have a genetic epidemic that causes one specific disease.
If it were a mutagen one would expect all sorts of deformities, not just autism, to be manifest.
Something(s) in the environment have to be behind this, possibly making compensating mechanisms unable to work.
One clue (but definitely not the whole story) is increase in autism among Somalis in Minnesota (low sunlight) and the amount of time kids spend indoors (1994-1995 with the advent of the Hypertext Markup Language and widespread internet usage). Both implicate vitamin D playing perhaps a protective role.
It’s interesting that wildlife doesn’t have autism. So, whatever is causing it is unique to humans.