Genetic contributors to autism include:

1. common variants. Common variants are genetic variants weakly associated with autism that we all have to some degree or another, and that only result in autism given a sufficient number of them are found in a person..or at least that's a common idea and pretty well supported. These variants are inherited.

2. rare de novo variants: These are spontaneous genetic variants that the parents did not have, but there was a mutation during production of sperm, for example. Many times, rare de novo variants that occur in highly conserved genes cause tremendous neurodevelopmental problems, spanning death often prenatally, other conditions, and for, autism, most frequently with severe intellectual disability.

Common variants are heritable and common, and are part of the normal spectrum. Only with a very large load do these result in the severest conditions, I think, like intellectual disability.

Occurrence of rare de novo variants are associated with things like age of father (older, less reliable production of sperm without genetic mutations), pesticide exposures, etc.

There is also a real immune factor involved, as well as environmental exposures, which can interact with genes.

Is there an understanding of the relative prevalence of each type in modern society? Thinking about changing rates of diagnosis, with common circulating variants, it seems like changes to detection and classification would be the only explanation for rate change.