Here's another response from a well-respected Toronto geneticist to our question "Is there an autism epidemic?" We will be posting more in the coming days:
Stephen Scherer, Director, University of Toronto McLaughlin Centre and The Centre for Applied Genomics, Hospital for Sick Children:
The Autism and Developmental Disabilities Monitoring Network of the U.S. Centers for Disease Control (CDC) has surveyed the prevalence of autism spectrum disorder (ASD) among 8-year-olds from up to 14 U.S. centres every two years since 2000. The most recent analysis pertains to the 2008 cohort, estimating overall prevalence as 1 in 88 children — almost double that of the original, 2000 cohort. So I suppose this substantial observed increase would indeed support the possibility of an autism epidemic, with the caveat that these data cannot distinguish the proportion of the increase attributable to changes in case ascertainment, as opposed to a true increase in ASD prevalence.
Our colleagues in Montreal, Drs. (Mayada) Elsabbagh and (Éric) Fombonne, undertook a comprehensive survey of epidemiological reports to catalogue the global prevalence of ASD and found that it is often under-recognized, particularly in the developing world. Variation in prevalence estimates is largely accounted for by variability in ascertainment and measurement, but environmental exposures may also play a role.
All of these data and my own observations lead me to believe that ASD prevalence is rising incrementally, but that so far there has not been an outbreak, nor is it pandemic. We now know from both the genetic and clinical sense that ASD is highly heterogeneous, so there are many targets or paths that can trigger or lead, respectively, to a diagnosis of ASD. Add to this mix the many new selective pressures being imposed on our brain’s development by the information-based society we now live in, and these phenomena too may account for some of the increase we see in ASD prevalence.