Since the United States Centers for Disease Control released its latest statistics on autism prevalence last week, I have done a series of images, blog posts, and videos analyzing the new data. This post collects all of those things in one place for people who want an extensive analysis of what the new report means.
Although I have grown somewhat more concerned about the proposed changes in autism diagnosis for DSM-5, I think the variation from state to state and within different ethnic groups in the report proves that the current state of autism diagnosis is intolerably random and bad.
The other disturbing thing to me about both the report and the way it is being discussed is the fact that everyone seems to be ignoring the evidence from Britain that indicates that the rate of autism among children and adults is consistently about 1% of the population. There is a great deal of speculation regarding why the rate has changed so dramatically, and it seems to me that it should at least be informed by this information:
The only study to look for autistic adults in a national population was conducted in Britain and published in 2009. Investigators interviewed 7,461 adults selected as a representative sample of the country and conducted 618 intensive evaluations.
The conclusion: 1% of people living in British households had some form of autism, roughly the same rate that the U.S. Centers for Disease Control and Prevention estimates for children in America today.
The British study found it didn't matter whether the adults were in their 20s or their 80s. The rate of autism was the same for both groups.
Some speculation of my own:
I do not think the number of autistic people has changed very much, but I do think that more people are visibly disabled by their autism than has been the case in the past. Many people are investigating environmental causes such as various toxins and chemicals. I wish there would be more investigation into changes in the information environment as well.
Video version of this post at the end.
Here is what the new Centers for Disease Control report says about autism and intellectual ability:
Data on intellectual ability are reported for the seven sites having information available for at least 70% of children who met the ASD case definition (Figure 2). When data from these seven sites were combined, 38% of children with ASDs were classified in the range of intellectual disability (i.e., IQ ≤70 or an examiner's statement of intellectual disability), 24% in the borderline range (IQ 71–85), and 38% had IQ scores >85 or an examiner's statement of average or above-average intellectual ability. The proportion of children classified in the range of intellectual disability ranged from 13% in Utah to 54% in South Carolina. The two sites with the highest proportions of children classified above the range of intellectual disability (IQ >70) were Utah (87%) and New Jersey (73%). In all seven sites reporting data on intellectual ability, a higher proportion of females with ASDs had intellectual disability compared with males, although the proportions differed significantly (52% for females and 35% for males; p<0.01) in only one site (North Carolina). When data from these seven sites were combined, 150 (46%) of 328 females with ASDs had IQ scores or examiners' statements indicating intellectual disability compared with 608 (37%) of 1,653 males.
This is how I interpret this:
1) IQ is an antiquated concept. The idea that a person has a fixed amount of "intelligence" that will remain the same for his or her entire life does not match well with what we know now about the brain and learning. Teenagers' IQs can change as much as 20 points in a few years:
Professor Cathy Price and colleagues administered IQ tests and MRI scans to 33 healthy teens -- the first time in 2004, when the kids were 12 to 16 years old, and then a second time in 2007-08, when they were age 15 to 20. They found changes in individual subjects' performance on the tests, with verbal IQ, nonverbal IQ and composite IQ fluctuating up or down, in some cases around 20 points. In all, 39% of the sample had a change in verbal IQ, 21% in nonverbal IQ and 33% in composite IQ.
2) IQ tests are especially unreliable for autistic people. An IQ test is a snapshot, showing the subject's performance on one day, on one task. The capacity of autistic people to succeed on these tests varies more, in most cases, than the capacity of a neurotypical person from day to day and from one set of circumstances to another. This makes a somewhat unreliable process extremely scattershot in its effectiveness.
3) IQ tests may not match the communicative capacity of an autistic person. If you cannot communicate your ideas to another person, there is no way to test how intelligent they are. Rose Eveleth emphasizes the importance of using nonverbal IQ tests with autistic children, after explaining some of the differences between verbal and nonverbal intelligence tests:
The average child will score around the same percentile for all these tests, both verbal and nonverbal. But an autistic child will not. Isabelle Soulieres, a researcher at Harvard University, gave a group of autistics both WISC and the Raven test to measure the difference between the two groups. Although she expected a difference, she was surprised at just how big the gap was. On average, autistic students performed 30 percentile points better on the Raven test than on WISC. Some kids jumped 70 percentile points. "Depending on which test you use, you get a very different picture of the potential of the kids," she says. Other studies have confirmed this gap, although they found a smaller jump between tests.
4) We can assume that the scores in the CDC report are probably artificially low because of these difficulties.
5) There are still a significant number of autistic people who do have intellectual disabilities.
6) Autistic people with intellectual disabilities matter just as much as anyone else. They are people, and they are part of our community. I care very much about keeping them safe, creating opportunities for them, gaining from their contributions, and making them welcome.
Do the people who seek to measure the intelligence of autistic people need to get out of our heads?
Rose Eveleth has written an important article for the Scientific American about the hidden potential of autistic people and intelligence testing:
Researchers have long considered the majority of those affected by autism to be mentally retarded. Although the numbers cited vary, they generally fall between 70 to 80 percent of the affected population. But when Meredyth Edelson, a researcher at Willamette University, went looking for the source of those statistics, she was surprised that she could not find anything conclusive. Many of the conclusions were based on intelligence tests that tend to overestimate disability in autistic people. "Our knowledge is based on pretty bad data," she says.
The intelligence of autistic people is especially likely to be underestimated if it is tested by highly verbal instruments like the Wechsler Intelligence Scale for Children (WISC). The subject is something Eveleth knows a great deal about because of her autistic brothers. Her brother Decker was unable to finish an intelligence test based on verbal skills:
This year, as part of the test, the woman delivering the questions asked him, "You find out someone is getting married. What is an appropriate question to ask them?"
My brother's answer: "What kind of cake are you having?"
The proctor shook her head. No, she said, that's not a correct answer. Try again. He furrowed his brow in the way we have all learned to be wary of—it is the face that happens before he starts to shut down—and said, "I don't have another question. That's what I would ask." And that was that. He would not provide her another question, and she would not move on without one. He failed that question and never finished the test.
In order to accurately estimate the intelligence of autistic people, it's essential to use a test like Raven's Progressive Matrices, which does not emphasize verbal skills:
The average child will score around the same percentile for all these tests, both verbal and nonverbal. But an autistic child will not. Isabelle Soullieres, a researcher at Harvard University, gave a group of autistics both WISC and the Raven test to measure the difference between the two groups. Although she expected a difference, she was surprised at just how big the gap was. On average, autistic students performed 30 percentile points better on the Raven test than on WISC. Some kids jumped 70 percentile points. "Depending on which test you use, you get a very different picture of the potential of the kids," she says. Other studies have confirmed this gap, although they found a smaller jump between tests.
Intelligence testing issue can be a very painful and confusing issue for autistic people. Some of the smartest people I know score very low on a test like the WISC. I score in the low-average range for tests like the Raven and very high on verbally based tests. I'm not smarter than them, and I know this for a fact. We just have different kinds of brains.
Anyway, this is one of the best articles that's been published about autism this year, and you should read the whole thing.