What the CDC Report Actually Says

The CDC just issued a press release saying that the average rate of ASD among 8-year-olds increased by nearly 60% between 2002 and 2006, such that almost 1% of 8 year olds have an ASD. The anti-vaccine community's response has been predictable. A post on my twitter page says "Can Neurodiversity ideology survive latest #autism rising CDC facts?"

This CDC study is important. But most of the conclusions being drawn from it are either overblown or erroneous. Nothing in the CDC study itself supports the claims of the anti-vaccine community.

Some background...
According to the CDC, data was collected in 2006 from 11 states (Alabama, Arizona, Colorado, Florida, Georgia, Maryland, Missouri, North Carolina, Pennsylvania, South Carolina, and Wisconsin). The 2006 data was compared to data from 2002 from all of the above states except Florida.

Children were identified through "screening and abstraction" of existing health and education records from health-care or education facilities. For the purposes of this study, to have an ASD, children's records had to "document behaviors consistent with the DSM-IV-TR criteria for autistic disorder, prevasive developmental disorder-not otherwise specified (PDD-NOS), or Asperger disorder." An "evaluation record" was defined as "the documented record of an assessment conducted by a community professional to determine the need for special education services or the presence of a developmental disorder. The assessments could be conducted at any time in the child's life through age 8 years." These could be conducted by a professional with special training in developmental disabilities, such as a developmental pediatrician, child psychiatrist, pediatric neurologist, clinical or developmental psychologist, or speech/language pathologist. The CDC says such an evaluation must be made showing behaviors consistent with an ASD, but does NOT seem to specifically state that these professionals must actually diagnose the child with an ASD.

Indeed, the key evaluations were made by a team of doctors, psychologists, and speech/language pathologists assembled by the CDC, not by the professionals who had actually evaluated the child. All the documents about the children in the study were sent to this team. They used a coding guide based on the DSM-IV-TR to determine whether the evaluations suggested either autism, Asperger's, or PDD-NOS.

What can we conclude from this?
1. Only 10 states are fully represented, with a beginning and end point. These states may or may not be representative of autism rates for the nation as a whole.
2. Skewed downwards? One of the states with high reported rates and many resources for helping kids with ASD, California, was not included, which may have shifted rates downwards.
3. ...Or skewed upwards? Notice that children did not have to actually be diagnosed by professionals as having an ASD. Their records merely had to show "behavior consistent with" an ASD diagnosis, according to the DSM-IV. Since ASD is a spectrum and many neurotypical people have ASD-like traits, the CDC might have reported higher rates than actually exists. This is NOT a trivial problem.
4. Skewed upwards? We are at a large remove from the children themselves. Consider that there are many overlapping symptoms between ASDs, sensory processing disorders (SPD), ADD, nonverbal learning disability (NVLD), and gifted children. Much sensitivity is required to determine what certain behavioral symptoms indicate, as we've discussed. Good clinicians say that the first step is to rule out other possible disorders. The CDC's professionals haven't done this. They didn't check for other possible disorders that might better explain the child's symptoms, they checked only for an ASD. This is understandable, since the researchers had to process large amounts of data, but it could lead to inaccurate diagnoses that overestimate the rates of ASD.
5. The years being compared are 2002 and 2006. On the one hand, these years are very close together, so a dramatic jump in the number of diagnoses would be noteworthy. On the other hand, even the earlier measure occured since thimerosal was removed from vaccines, iirc. Also, not enough time would have passed for major environmental or lifestyle changes to have swept through the US population.
6. No information about causes was collected. This was not an experiment, with manipulations and control groups. It was a purely demographic measure. Therefore, this study does not tell us whether better diagnostic practice, environmental or dietary changes, or assortative mating between ASD people might have caused the results. Statements from researchers are just that--statements of opinion. They are not evidence of anything.

Armed with this information, let's consider what the researchers found.

In Age of Autism, David Kirby writes:
According to the latest CDC figures, the ASD rate at 11 sites among 8-year-olds in 2002 (kids born in 1994) was 60-per-10,000, and 94 per-10,000 in 2006 - among kids born just four years later, in 1998. Some of this increase was explained by better access to school records and other factors, the CDC said, though it added that a "true increase" could not be ruled out.
Note the CDC's statement. The study does not investigate causes. Therefore, it can't rule out anything. The fact that we can't disprove a "true increase" in autism rates does not mean we have an epidemic on our hands. We probably can't disprove that you just killed someone yesterday--should we believe that you did?

Comparisons between different racial and ethnic groups also do not support the anti-vaccine movement's claims that we have an environmentally caused epidemic. David Kirby writes:
Another surprise was the difference between some of the racial and ethnic categories. In 2006, the rate among non-Hispanic white children was 102-per-10,000, but among black children it was 76-per-10,000, a 34-percent difference, and among Hispanic children it was 61-per-10,000, a difference of 67 percent. Inexplicably, the rate among Hispanics in Alabama actually plummeted during the period in question, by 68%, from an already low 19-per-10,000 in 2002 to an almost rare 6-per-10,000 in 2006. Meanwhile, with the exception of Arizona, the CDC said, "prevalence among Hispanic children did not change significantly within any of the other 10 sites.”
Black and Hispanic kids are more likely to be economically disadvantaged. They are more likely to confront broken homes, crime, and other situations that may disrupt their social and emotional development. They receive less support and enrichment inside and out of school. Cheap urban housing, and urban life in general, likely exposes kids to toxins, so you would think black and Hispanic kids would have more exposure to environmental toxins and thus higher rates of autism. The facts don't support that.

The majority of the Kirby article consists of quotes from Dr. Thomas Insel, Director of the National Institute of Mental Health and Chair of the federal government's Interagency Autism Coordinating Committee (IACC). This man is tasked with recommending funding priorities for autism research and services. Some representative quotes here:
He added that he never saw a single case of autism during his training in the mid-1980s, including a full year's rotation in child psychology. "I wanted to see children with autism. I couldn't find them," he said. "Now I wouldn't have to go any further than the block where I live to see kids with autism today."

"Yes," Insel said. "I don't think anybody is arguing that it is 100-percent genetic. And I don't think in those terms, exactly, that it's either genetic or it's environmental. From my perspective, it's almost always going to be both. And the only question is: How do you nail down this interaction, how do you go after it?"

Insel said, "It's quite believable to me that there are many children who develop autism in the context of having severe gut pathology, of having autoimmune problems, of having lots of other problems. And some of these kids really do recover. And that is quite different from the autism that was originally described in the 1940s and 50s - where it looks like you have it and you are going to have it for the rest of your life."
Notice that none of these are drawn from the actual results of the study. They are just his opinion. The fact that he is director of NIMH and chair of the IACC does not make his opinion anything more than an opinion. Nowhere does he actually cite any studies. His evidence is his own experience. Experience is okay for formulating hypotheses about individual children, but not so great for population-based causal studies or for determining funding priorities.

Furthermore, I believe that David Kirby is either accidentally or wilfully misunderstanding Dr. Insel and taking his remarks out of context. First, note that "environmental factors" doesn't necessarily mean things like exposure to mercury. If we're talking nature vs. nurture, the way the brain develops always depends on an interaction between genes and the environment--including the social interactions and school experiences one has. Neurodiversity advocates would not disagree with this interpretation of Dr. Insel. Second, he says, "I think that we're approaching autism as if it is a single thing, as if it is a syndrome that will have one cause, one treatment"--an approach with which he disagrees. I would consider myself a moderate believer in neurodiversity, and this entire blog is devoted to the idea that there are multiple "autisms." The "neurodiversity advocates" who supposedly believe that only genetics cause autism and who are supposedly hurt by the CDC study are straw men.

Finally, let's look at the most important and widely quoted fact: the prevalence rates themselves.

The average rate for all 10 of these states is a little less than one percent. I would bet you more than one percent of kids in the US have the flu right now, and we're not worrying about a flu epidemic. For comparison, a reputable researcher, Shaywitz, estimates that between 5 and 15 percent of the US population has a reading disorder. Is anyone frantic about an epidemic of reading disorders? No, but the affected percentage of the population is five to fifteen times higher. The frenzy over the so-called "autism epidemic" combined with indifference to reading disability is caused by the same reasoning errors that lead us to fear airplanes but not cars, even though the death rates for driving are much higher.

In other words: the CDC study is not evidence for some sort of vaccine/environmental toxin-caused epidemic. Rather than panic, let's continue to research the nature and causes of ASDs.


  1. Thanks for reading and commenting.

    "We're old too fast and smart too late?" True... (I hope I translated your comment correctly!)