I thought that autism was primarily an inherited condition.
Both my parents and at least one of my siblings show some aspects of Asperger's syndrome behaviour and so did other relatives further back so that seems to suggest some hereditary component; particularly when I know of several other people who report that relatives also seemed to have the condition in some degree.
It could it be a combination of genetic and environmental factors.
Some other factors that no one has even thought of yet could be at work. If it was purely genetic than identical twins should express the same level of autism.