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.
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.