This article in the Kansas City Star alerted me to a study published in PLoS ONE on July 12, giving the first indications I've seen that the conventional wisdom about autism and social class --- that autism is mainly a disorder of the upper classes --- might have more to it than just the simple fact that people with unfettered access to health care are a lot more likely than people without such access to get diagnosed with autism-spectrum conditions.
The way I had understood it, there was no big mystery there: people who can afford to see a doctor in non-emergency situations are going to get all sorts of things diagnosed at a higher rate than people who can't afford that kind of preventive care. In that framework, a higher rate of autism diagnoses among children from wealthy families says nothing about autism per se; it just serves as one more example of the huge inequality of access to health care here in the U.S.
The study I mentioned --- carried out by Dr. Maureen S. Durkin and her colleagues --- tries to address how much of the association between autism and higher socioeconomic status (which Durkin says shows up about half the time in the existing literature on the topic) is due to this gap in access to health care and how much may be due to other factors yet to be uncovered.
To try to include autistic children lacking formal diagnoses in their analysis, Durkin et al. relied on the CDC's Autism and Developmental Disabilities Monitoring Network, whose methodology is described here* on the CDC's website:
In 2000, CDC established the Autism and Developmental Disabilities Monitoring (ADDM) Network to track the prevalence and characteristics of ASDs in the United States. The ADDM Network is a multiple-source, active, population-based surveillance system that reviews developmental records at educational and health sources and employs a standardized case algorithm to identify ASD cases.There is also a flowchart showing where they get their data from and how they decide which children are actually autistic; unfortunately, the flowchart is still pretty vague about which "educational and health sources" they use. It does mention that they use "multiple" of each, though.
So, in the years 2002 and 2004, the ADDM Network collected data on 407,578 (in 2002) and 172,335 (in 2004) eight-year-old children living in fourteen areas around the country: the northern half of Alabama; the central region of North Carolina; the Coastal and PeeDee regions of South Carolina; the entire states of Arkansas and West Virginia; parts of the cities of Denver, Colorado; Atlanta, Georgia; St. Louis, Missouri; and Salt Lake City, Utah; and the cities and surrounding areas of Phoenix, Arizona; Baltimore, Maryland; Newark, New Jersey; Philadelphia, Pennsylvania; and Milwaukee, Wisconsin**. Of these children, 2,685 met diagnostic criteria for an ASD in 2002; in 2004, the number was 1,376.
Durkin et al. took the data on all the children determined to be autistic --- minus the ones living in Utah and West Virginia, since those sites didn't provide enough information to determine socioeconomic status for the children living there --- and compared prevalence rates for ASDs across three socioeconomic "tertiles": high, middle and low socioeconomic status, as determined by three factors: 1) percentage of people in the area living above the poverty line, 2) percentage of adults 25 and over in the area who have at least a bachelor's degree, and 3) median household income for the area. They also compared those factors between the autistic children and the general population of eight-year-olds living in the same areas at the time of the 2000 census.
Compared with the general population of eight-year-olds living in the same areas, autistic eight-year-olds are somewhat less likely to live in "poverty areas," or areas defined in the 2000 census as having 20% or more of its families with children living on poverty-level incomes, with 16.8% of autistic children living in such areas and 25.8% of all children living in them.
Similar small-but-noticeable differences show up with respect to the other two factors, median household income and proportion of adults with college degrees. Among the general population, the median household income was $42,898; among families with autistic children, it was $50,114. Autistic children, on average, tended to live in census blocks where a larger proportion of adults 25 and older had bachelor's degrees; 30.3% had them compared with the general-population figure of 24.8%.
These differences kept showing up, even when the comparisons were set up differently: besides comparing autistic children with their (approximate) peers on measures of socioeconomic status, the researchers also compared autism prevalence rates across the three socioeconomic-status categories they'd created, and found that the richest group had the highest proportion of children meeting criteria for autism, the middle-income group had the second-highest, and the poorest group had the lowest. This pattern showed itself across all the racial categories included in the study, too.
Now, the question is, does the design of this study correct sufficiently for (well-established) class differences in access to specialized medical and educational services? Can its findings of autism prevalence increasing with socioeconomic status be taken at face value?
Part of the reasoning behind the authors' contention that at least part of the association between autism and wealth is real lies in their use of school-based, as well as clinic-based, documentation. Theoretically, all children in the U.S. going to public schools are going to be monitored by their teachers and screened for learning and developmental disabilities if they start to have problems with classwork, behavior or social interaction. But not all schools have the resources to make this sort of individualized attention a reality. It probably won't surprise you to hear that schools in districts where poor people live have less money than schools in districts where richer people live, and accordingly the poorer school districts are less likely to employ psychologists, paraprofessionals or even college-educated teachers. With less space and less staff, also, even very dedicated, insightful and observant teachers aren't able to give individual attention to every student they see struggling.
The U.S. educational system might be more equitable than its health-care system --- last I heard, public schools were still legally required to educate every student living in their districts --- but similar issues of accessibility plague both systems. Even if our educational system is technically socialized --- available for free to everyone, paid for with taxes --- there are still huge differences between the kind of education (including special education) you can get at a well-funded school in a primarily upper- or middle-class district and the kind of education you can get at a school in a poor district. The richer districts can levy additional taxes to give more money to their schools, and students going to those schools also pay student fees to cover equipment and activities.
All of this leads me to suspect that ascertainment bias can be just as big a factor in an educational setting as in a medical setting.
The study authors tackle this issue in their Discussion section:
The other important factor that somewhat mitigates the effect of differential access to the educational and medical professionals who can diagnose autism and provide autism-related services on this study's outcome is the inclusion of a subgroup within the group of autistic children they studied who had no prior diagnosis of autism; there were enough of these children (1,244) to do a separate statistical analysis of this group to determine whether the association between autism and socioeconomic status still holds up among children who had never been evaluated. The association did still stand: even among those children with no previous diagnoses of ASDs, the ratio of autism prevalence as determined by the CDC of low- to middle- to high-SES children was 0.78:1:1.09. (That is, the poorest children were somewhat less likely to be autistic than the middle-income children, who in turn were somewhat less likely to be autistic than the wealthiest children). It's worth pointing out that the ratio for children with previous ASD diagnoses is stronger in both directions, at 0.70:1:1.25.
An important limitation of this study was that the ADDM Network surveillance system relies on information for children who have access to diagnostic services for developmental disabilities. We could not rule out the possibility that the quantity and quality of evaluations and the information available for case ascertainment might have varied by SES. We looked for evidence of this by examining the number of evaluations per child with ASD recorded in the ADDM Network surveillance system, reasoning that if the higher prevalence of ASD among children of higher SES was due to increased access to diagnostic services, high SES might be associated with a higher number of diagnostic evaluations per child. However, we found no association between the number of evaluations per child and SES. We also examined the mean ages at diagnosis by SES and found that children of high SES received an ASD diagnosis at an average age of 58.0 months, 1.1 month earlier than those of middle SES (p = 0.2838) and 2.7 months earlier than those of low SES (p <>
While I'm not sure this study goes far enough to correct for huge systemic inequities in availability of services, those two findings --- that number of evaluations per child does not vary with SES, and that autism prevalence does vary with SES among never-diagnosed children --- introduces some doubt in my mind where there had been none before. Maybe there *is* more to this socioeconomic-status thing than just access to diagnostic services. I remain skeptical, but no longer absolutely convinced that ascertainment bias explains everything.
*A more complete description exists in this 2007 article in Pediatric and Perinatal Epidemiology, for which I cannot find the full text anywhere online for free.
**The list of fourteen areas applies to 2002; in 2004, only eight of those areas participated --- Alabama, Arizona, Georgia, Maryland, Missouri, North Carolina, South Carolina and Wisconsin. That's why the numbers are so much lower for 2004.
Durkin, M., Maenner, M., Meaney, F., Levy, S., DiGuiseppi, C., Nicholas, J., Kirby, R., Pinto-Martin, J., & Schieve, L. (2010). Socioeconomic Inequality in the Prevalence of Autism Spectrum Disorder: Evidence from a U.S. Cross-Sectional Study PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011551