It’s a phrase that is heard a great deal in online discussions about autism: there are no genetic epidemics. Genes don’t change quickly enough for a genetic condition to see an increased prevalence over a single generation, right? Well, yes and no.
Consider Down Syndrome. A condition almost everyone would agree is genetic. Per the NIH website : Down syndrome is a genetic condition in which a person has 47 chromosomes instead of the usual 46. Most Down Syndrome children are born to non-Down Syndrome parents. I.e. the genetic condition is not inherited. DS risk increases significantly with the age of the mother. Here is a graph showing estimated DS risk as a function of maternal age ( source ):
Thus there are at least two social factors which impact Down Syndrome incidence: maternal age and parental choice. Average maternal age is going up, and there are indications that termination rates are going down. These social factors and possibly other factors have led to–a genetic epidemic. Down Syndrome prevalence has been steadily climbing for years:
From 1979 through 2003, the prevalence of DS at birth increased by 31.1%, from 9.0 to 11.8 per 10000 live births in 10 US regions. In 2002, the prevalence among children and adolescents (0–19 years old) was 10.3 per 10000. The prevalence of DS among children in a given age group consistently increased over time but decreased with age within a given birth cohort. The pooled prevalence of DS among children and adolescents was lower among non-Hispanic black individuals and other racial/ethnic groups compared with non-Hispanic white individuals; it was also lower among females than males.
What does this mean for autism? At present there is no commonly used test for autism, but there are strong indications that parental age (both maternal and paternal) increase autism risk. Peter Bearman at Columbia calculated that about 11% of the increase in the administrative autism prevalence in California was due to increased parental age. Groups that promote autism as vaccine injury have been antagonistic towards the idea that parental age, especially paternal age, increases autism risk. The reasons are fairly obvious: they indicate that the idea of “there can be no genetic epidemic” is, at best, a much more complicated statement than they would have us believe.