Thursday, December 31, 2009

1 in 86: the prevalence of autism among adults

This post was chosen as an Editor's Selection for
"Autism rate in children has doubled, say doctors" ... "Autism 'more common than thought'" ... "Autism in children '10 times higher' than first thought" ... "Autism at a record high" ... "autism is 25 times more common than what researchers thought"...

This mess of headlines and claims was generated in response to one autism prevalence study, Baird et al. (2006), published in the Lancet.

All 56,946 individuals comprising the targeted population cohort in this study are, as of today, the last day in the decade, 18 years of age or older. They were born between July 1, 1990 and Dec 31, 1991 and they are now all adults.

Within this cohort, Baird et al. (2006) reported an autistic spectrum prevalence of ~116/10,000. That's 1 in 86, and all these autistics, originally assessed as such when 9 to 14yrs old, are now adults.

For those to whom this is a matter of pressing importance, Asperger's contributed exactly nothing to that prevalence figure. So none of those 1 in 86 assessed-as-autistic now-adults are, at least according to Baird et al. (2006), Asperger's.

I've previously rattled on about Baird et al. (2006), one of the most important autism epidemiological studies to be published to date, and one of the most misrepresented. The authors found that small differences in case definition produced prevalence figures ranging from ~25/10,000 (1 in 400) to ~116/10,000 (1 in 86), a 4.6-fold discrepancy--within the same cohort at the same time, using the same diagnostic criteria, diagnostic team, and "gold standard" diagnostic instruments.

In the context of an earlier closely related prevalence study (Baird et al., 2000), the authors additionally found that a change in case ascertainment method doubled autistic spectrum prevalence within very nearly the same cohort, even when diagnostic standards were equivalent.

But such provocative and crucial findings, and others similar, were lost in the predictably incoherent uproar (see representative headlines and quotes, above) over that one figure--the 1 in 86 who as of today are all adults. I suggest that if Baird et al. (2006) had been conducted in exactly the same manner with a cohort born five years earlier, the findings would not be much different. Or ten years earlier, or more, were that possible. Of course that's a wild guess, but not an entirely unfounded one.

Earlier this year, a small innovative UK study reported a prevalence of about 1 in 100 for autistics aged 16 and up--for autistic adults right up to and over age 75. There are only bad reasons for why this small study of autistic adults is unique and unprecedented.

The world would look very different now, for both autistics and nonautistics, if over the past decade or more there had been a rational discourse about autism prevalence in which the existence of older autistics was not automatically denied.


Baird, G., Charman, T., Baron-Cohen, S., Cox, A., Swettenham, J., Wheelwright, S., & Drew, A. (2000). A screening instrument for autism at 18 months of age: a 6-year follow-up study. Journal of the American Academy of Child and Adolescent Psychiatry, 39, 694-702.

BAIRD, G., SIMONOFF, E., PICKLES, A., CHANDLER, S., LOUCAS, T., MELDRUM, D., & CHARMAN, T. (2006). Prevalence of disorders of the autism spectrum in a population cohort of children in South Thames: the Special Needs and Autism Project (SNAP) The Lancet, 368 (9531), 210-215 DOI: 10.1016/S0140-6736(06)69041-7