In very few cases it is purely genetic, like these de novo cases they are finding, but the majority of cases are not.

I want to throw this out there, not as something I believe, but as an idea to be discussed, that one factor in the rise of Autism might be the sudden rise in wealth of a new type of person (programmers, engineers, etc) which led to different mating patterns and a sudden change in the genetics of the children born to these couples. Or even just a change in how they raise the kids and what food they eat.