"Autism" is a diagnosis reached through a battery of tests administered by professionals. The diagnosis encompasses the presentation of a range of cognitive and behavioral symptoms. However the underlying causes of this presentation can be quite varied, ranging from the inheritance of specific genetic alleles, chromosomal events, probably epigenetic factors, brain injury and who knows how many other non genetic developmental factors.

I do not believe that there are firm stats, but only a small fraction of cases can be ascribed to a specific cause.