The prevalence of autism – the number of cases diagnosed at any age, has increased hugely.
But prevalence is not the same as incidence – the number of cases born with autism. There is no reason to believe that the incidence of autism has increased.
An increase in the prevalence of cases was inevitable given the recognition that the classic definition of autism was too narrow and there was a whole spectrum of autism (Wing).
The historic factors in the increase in diagnosed cases (prevalence) was the widening of the criteria, to fit them also to adults, and to be able to apply them to cases previously diagnosed merely with learning disability, where this meant less access to special support.
For instance, a study found that the average administrative prevalence of autism among children in the US had increased from 0.6 to 3.1 per 1000 from 1994 to 2003. But this increase was accounted for by a decline in the prevalence of mental retardation and learning disabilities.
This was not the only factor that drove up prevalence figures: Cases with only very mild symptoms previously not clinically diagnosed at all, were now also included in the autism category. It turns out that it is these cases which are the most likely cause of the increase in prevalence since 2000.