In the 1980’S a major epidemiological study was performed by UCLA researchers focused on the population of Utah. This resulted in six publications. One study, the prevalence paper, has now been revisited recently with results indicating that the broadening of autism criteria with the shift from DSM III to DSM IV had a major impact on prevalence. In particular, on the prevalence of individuals with lower IQ’s.
Yes, the the DSM III missed a large number of individuals with low. IQ.
Here is the abstract from the 1990 study:
The UCLA-University of Utah epidemiologic survey of autism: Prevalence
Ritvo ER, Freeman BJ, Pingree C, Mason-Brothers A, Jorde L, Jenson WR, McMahon WM, Petersen PB, Mo
Division of Mental Retardation and Child Psychiatry, University of California School of Medicine, Los Angeles.
The authors conducted an epidemiologic survey in Utah using a four-level ascertainment system, blind current diagnostic assessments, and DSM-III criteria. Of 483 individuals ascertained, 241 were diagnosed as having autism. The best estimate for the prevalence rate was 4 per 10,000 population. Autism was not associated with parental education, occupation, racial origin, or religion. Sixty-six percent of the autistic subjects scored below 70 on standardized IQ tests, and females scored proportionately lower than males. Twenty (9.7%) of 207 families had more than one autistic sibling, which supports the authors’ previous finding that there may be a familial subtype of autism
The recent study is:
Here is the abstract:
Abstract 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 today.
Emphasis added.
This goes counter to the common perception that the expansion of diagnostic criteria was confined to adding so-called “higher functioning” autistics.
Comparing these results to the recent CDC results show that much of the increase in Utah was due to inclusion of higher IQ individuals. The recent prevalence estimate for Utah was 212/10,000 with most with IQ >70. One has to note that the prevalence estimate from the recent CDC report is roughly 20/10,000, about 5x higher than the UCLA report from 22 years ago. (further note that the CDC data for Utah are based on only 45 autistics so the error bars are very big).
It would be interesting to go through the screening process from the UCLA study to see how well they might have been able to capture individuals without intellectual disability.
This study doesn’t explain a substantial fraction of difference between the 1990 prevalence and the most recent estimate. It does point to a shift in diagnostic standards for low IQ individuals.
Left Brain Right Brain 
As I recall, the authors commented that since they reexamined the original study subjects (rather than, say, returning to examine a birth cohort) they would not have seen an increase in “higher functioning” diagnoses because, according to the original criteria, “higher functioning” individuals would not have been referred for inclusion in the original study population.
Very good analysis here
http://www.disabilityscoop.com/2012/06/29/autism-surge-analysis/15957/
Don’t forget,that there were those,like myself,who were obviously not normal,and obviously seriously disabled,but not intellectually disabled,who were given the diagnosis of “childhood schizophrenia”.This was often considered a distinct disorder from “autism”,where you had to be nonverbal,and intellectually disabled.
This goes counter to the common perception that the expansion of diagnostic criteria was confined to adding so-called “higher functioning” autistics.
I think one reason for this preception,is that families of more seriously disabled autistics,have not been as vocal,or as out there to the public,to the degree higher functioning people are.There is no sense of balance.I see this as a real problem.
I m in agreement that awareness is the driving factor of the increase in autism diagnoses. However there are things I cannot square with the reasons given in ther article
The proportion of those diagnosed with intellectual disability as well as autism as decreased markedly. It used to be about 2/3 of those with autism also had intellectual ability, but now only about 1/3 do. I’ll see if I can find a source, as I giving these figures from memory. One would think that if it’s all a matter of the intellectually disabled being reclassified the proportion would be going up and not down.
It may be with services tied to autism that children who have intellectual disability and autism only receive the autism diagnosis rather than both whereas in the past they received both. I am actually not sure what services are available to a child with autism which are not available to a child with only the diagnosis of intellectual disability. Perhaps ABA?
It may be that autistics who are higher functioning received a diagnosis of being intellectually disabled incorrectly.
Even if the increase is solely tied to children trading a diagnosis of intellectual disability for autism, it does not mean they have received an incorrect diagnosis Studies looking at DSM V which tightened up the criteria for an autism diagnosis have shown a far greater percentage of people with autism and no intellectual disability no longer meet the requirements than those who also have intellectual disability. Indeed a very legitimate concern was raised over children who would lose services, and it was decided to “grandfather in” those children.