The percentage of people with both autism and mental retardation has dropped significantly, a trend that may provide clues for those trying to solve the autism puzzle.
That's not stated accurately, but in fact there has been a gradual drop in the prevalence of mental retardation within the population of recognized autistics, at least for the last 17 years. See page 20 of the report.
This is a well known fact, or at least I'd like to think it is. I've discussed it several times previously (here, for example.) It is mentioned in Gernsbacher et al. (2005) as well.
I'm sure broadening ascertainment denialists can find ways to rationalize this finding. But what if I told you that the phenomenon is not only a time-based phenomenon? It can also be observed when you compare one regional center to another. That is, regional centers with a higher administrative prevalence of autism will tend to have a lower proportion of autistics who also have a classification of mental retardation.
You know a chart is coming, but as usual I'd like to be clear as to where the data comes from, so anyone reading can double-check if they so wish. I'm using a file provided by California DDS upon request, named CDERQtrData.zip. It only goes up to January, 2006, but it contains more autism-specific information for each regional center than you normally find in the regular report. I don't have population data for each regional center (and gathering that would be a bit much for a blog post) but I will use the Autism-Epilepsy ratio as a proxy of administrative prevalence. The administrative prevalence of epilepsy in California is roughly stable (just below 0.1%) so dividing the autism caseload by the epilepsy caseload of a regional center should provide us with an adequate proxy of the administrative prevalence of autism.
The figure demonstrates an inverse association between the Autism-Epilepsy ratio and the proportion (%) of autistics who also have an MR (or unkown MR) classification. The downward trend is statistically significant with 99.6% confidence.
Now, you'll note the distribution of the dots in the chart is fairly random. I wondered why that might be, especially why the Central Valley regional center would have a low administrative prevalence of autism and a low proportion of autistics with MR.
The Central Valley RC has 9,284 clients with mental retardation as of January, 2006. That's comparable to other big RCs. Of all individuals with MR, only 247 (2.7%) also have an autism classification. This is ridiculously low. I'm sure that if someone went to the Central Valley RC and screened the individuals with an MR classification, they would find that a lot more than 2.7% of them are also autistic.
It's generally understood that there's been increasing recognition of autism in the population without mental retardation. This is what they call HFA. What's not so intuitive is that there's also been increasing recognition of autism in the population with mental retardation. Clearly, regional centers have different levels of recognition across both populations. In average, 7.7% of persons with mental retardation will have a classification of autism in California (which is still rather low.)
Let's look at what the result would be if we were to adjust the autism numbers under the assumption that every regional center should have a level of recognition of 7.7%.
That's a lot more clear, isn't it? This suggests that differences in the recognition of autism in the population with MR across regional centers are an artifact.
Imagine there's a hypothetical regional center where recognition of autism has gotten out of hand, to the point where every person who resides in the area served by the RC is classified as autistic by it. In this case the Autism-Epilepsy ratio would be about 1,000.
You'll note the first chart above has a power regression model (the formula on the upper right.) The type of model is theoretically justifiable, and I can discuss that on another occasion.
y = 32.235 x-0.4525
In the model, x is the Autism-Epilepsy ratio and y is the proportion of autistics with MR.
So what if x is 1,000? This model predicts that the proportion of autistics who have MR would then be 1.41%. This is basically what you'd expect for the general population. In other words, DDS data is entirely consistent with a cultural explanation of the differences in administrative prevalence of autism between regions.