Multiple studies link parental age to autism. Teenage mothers, and to a much greater extent older parents (especially fathers) have a higher chance of autistic children.
It doesn't seem to be enough to account for the whole difference, but given the increasing parental age at birth, it's an appealing theory. Lots of developmental disorders, e.g. Down's syndrom, have already been linked to older parents.
I know "45 is the new 25" is popular, but biology is harder to convince.
Aspie here, my parents mid 20's when I was conceived and I was a week late being born.
Would in retrospect say my farther was also somewhat upon the spectrum, so genetics are a factor. If those genetics can be activated or induced by other factors, I can not say though like all DNA we have which is dormant in some and not in others, external factors can come into play by that stress, chemical (environmental or self induced exposure) and other factors we are still learning about. Heck recently identified an organ in the humanbody that we had overlooked for all this time ( http://www.independent.co.uk/news/science/new-organ-mesenter... )
So who knows. All we do know is once a trait becomes the predominant trait in humanity then that is classed the new normal.
Since the recorded increase is large and for births within the last 25 years I suspect that changes in diagnosis play a bigger role in the increase than more gradually-changing birth ages and pre-term survival rates (much as I'd like to believe the latter was of huge importance and not just a potentially relevant factor since the relationship between preterm birth and inattention was the subject of my sister's PHd research...)
Except most kids with Down syndrome are born to young mothers... the risk is higher with older mothers but young women still make up the majority of new mothers.
The underlying biology of spermatogenesis is also involved. As males age, the germ cells (~stem cells) that produce sperm are continuously dividing. One of the daughter cells from the division becomes sperm, the other continues on as a germ cell [0]. Over time and with aging, the germ cells accumulate new mutations (primarily single nucleotide changes), which are then also passed on in the newly created sperm. While it is also possible that a mother's eggs carries a new mutation, these do not increase in frequency with aging because her eggs are created in utero and do not undergo the lifetime of regeneration that the father's sperm do.
These "de novo" mutations can sometimes affect or "break" genes important for normal development, thus resulting in autism. They can be studied by sequencing father/mother/child families and comparing the genetic sequence of each. The de novo mutations show up in the child only. We can differentiate if they arose in the father's sperm or the mother's eggs by examining the "haplotype" around each of these mutations (basically, looking for other nearby mutations that were inherited by the child from either mom or dad).
That wouldn't explain why the rate is increasing, unless these autistic fathers are having higher than average children despite starting at a higher age.
Would in retrospect say my farther was also somewhat upon the spectrum, so genetics are a factor. If those genetics can be activated or induced by other factors, I can not say though like all DNA we have which is dormant in some and not in others, external factors can come into play by that stress, chemical (environmental or self induced exposure) and other factors we are still learning about. Heck recently identified an organ in the humanbody that we had overlooked for all this time ( http://www.independent.co.uk/news/science/new-organ-mesenter... )
So who knows. All we do know is once a trait becomes the predominant trait in humanity then that is classed the new normal.