But what is often not mentioned is that there has been an equally large (if not larger) increase in the number of middle and lower functioning children as well. This pattern is plainly visible in many of the recent estimates of autism prevalence (I hope to have a post up about that soon, time permitting).
Of course the reason or reasons for the sharp increase are still unknown; whether it be more awareness, better services, an expanding definition of autism, an actual increase in the number of cases, or some other reason entirely.
So, in light of all of this, I found this recent re-examination of a historical data set from 1980 to be of interest. I haven't had a chance to look at the full text of the paper yet, but it does raise some questions. The immediate thing that jumps out at me is that the "missed" group is comprised of children who would likely be very low functioning, having an average IQ of 35 (!) .
The original group was solidly in the intellectually disabled range (i.e. low functioning) and yet the group identified in this re-examination had an even lower average IQ than that. For reference, an "average" person's IQ is about 100 and intellectual disability starts at about 70.
The abstract is below.
Autism Spectrum Disorder Reclassified: A Second Look at the 1980s Utah/UCLA Autism Epidemiologic Study.
The purpose of the present study was to re-examine diagnostic data from a state-wide autism prevalence study (n = 489) conducted in the 1980s to investigate the impact of broader diagnostic criteria on autism spectrum disorder (ASD) case status. Sixty-four (59 %) of the 108 originally "Diagnosed Not Autistic" met the current ASD case definition. The average IQ estimate in the newly identified group (IQ = 35.58; SD = 23.01) was significantly lower than in the original group (IQ = 56.19 SD = 21.21; t = 5.75; p < .0001). Today's diagnostic criteria applied to participants ascertained in the 1980s identified more cases of autism with intellectual disability. The current analysis puts this historic work into context and highlights differences in ascertainment between epidemiological studies performed decades ago and those of todayOf course, the real question about this result is what exactly is the "current ASD case definition" that was used and how it was determined whether an original participant met this definition. As we saw with other recent studies, it is not always easy (or possible) to map the data collected years ago into what the criteria for autism look like today.
Miller JS, Bilder D, Farley M, Coon H, Pinborough-Zimmerman J, Jenson W, Rice CE, Fombonne E, Pingree CB, Ritvo E, Ritvo RA, McMahon WM. Autism Spectrum Disorder Reclassified: A Second Look at the 1980s Utah/UCLA Autism Epidemiologic Study. J Autism Dev Disord. 2012 Jun 13. [Epub ahead of print]
PubMed PMID: 22696195. DOI: 10.1007/s10803-012-1566-0