You wouldn't know it from the conclusions, but the researchers also admit that they didn't consider a factor that ought to be important in a database like that of California DDS: awareness.
What I want to discuss in this post is something else. It's interesting that the researchers don't mention how much of the rise in "cumulative incidence by 5 years of age" (prevalence at age 5 really) is explained by the factors they did consider, taken together. You have to do the calculation yourself. Let's take a look at the numbers, shall we?
First, what's the extent of the rise? The conclusions of the paper say 7- to 8-fold. But the results from the abstract actually say the following.
Cumulative incidence to 5 years of age per 10,000 births rose consistently from 6.2 for 1990 births to 42.5 for 2001 births.
That's a 6.85-fold increase. Now, how much of this is explained by the factors the researchers considered?
Quantitative analysis of the changes in diagnostic criteria, the inclusion of milder cases, and an earlier age at diagnosis during this period suggests that these factors probably contribute 2.2-, 1.56-, and 1.24-fold increases in autism, respectively, and hence cannot fully explain the magnitude of the rise in autism.
If we multiply 2.2, 1.56 and 1.24, we get a 4.26-fold increase. That's not too bad. With all the problems the paper has, it actually explains 62% of the rise. It doesn't admit to that anywhere, but it does.
(If it's unintuitive why you have to multiply the factors, try the following mental exercise. Suppose the rise in "full syndrome autism" is 3-fold due to changes in criteria, and there currently are also 3 times as many autistics due to inclusion of Asperger's and PPD-NOS. Obviously, you have a 9-fold increase total.)
The interesting aspect of this has to do with the 2.2-fold factor due to changes in diagnostic criteria. I say it's interesting, because the researchers choose this one number, based on a single Finnish study, in favor of the results of a meta-study, Williams et al. (2006).
A meta-analysis of 37 studies of autism prevalence found a 3.6-fold higher risk from DSM-IV or ICD-10 criteria versus other criteria, but this figure would have been confounded by the year of study.
I'm not sure what they mean by the year-of-study confound; they don't explain it. They could very well be right. I don't know.
But just for kicks, let's see what would've happened if the researchers had chosen the 3.6-fold factor instead of the 2.2-fold factor. That is, we multiply 3.6, 1.56 and 1.24.
In this case, the study would explain a 6.96-fold rise. The actual rise was 6.85-fold. In other words, the study would have explained 102% of the rise. Does anyone else find that kind of suspect and hilarious at the same time?