The study was thought to be a prank at first, due to its peculiar methodology, but it's clear by now that it is not meant as a joke. Apparently, a lot of work went into it.
The authors of the study are economists, and this seems to be taken by many as a quick way to dismiss the study altogether. I would note that they are not the first authors of an epidemiological study on autism who are not trained in autism epidemiology. I think the study needs to be considered and critiqued on its merits.
Alternative interpretations have been proposed. Perhaps humidity causes autism, through mold and mildew, for example. Maybe spending time outdoors is a preventive measure. And so forth. These alternative interpretations, nonetheless, simply switch one hypothesis with another of equally unknown plausibility.
What I'd like to do in this post is propose an interpretation of the results based on a hypothesis known to be plausible, and which has been documented previously. Namely, that the results can be explained by a established correlation between degree of urbanization (or population density) with the administrative prevalence of autism. I already did something similar in a critique of the environmental pollution hypothesis.
That cable subscription rates are correlated with degree of urbanization or population density is a sure bet, and I won't consider that further. A potential correlation between population density and annual precipitation is non-obvious, however, although I had previously suggested that people generally don't build communities in the dessert.
I will use the state of Pennsylvania for this post, simply because there's readily available data on administrative prevalence with high regional granularity from that state. The authors analyze California, Oregon and Washington for precipitation. Readers can check if the analysis holds in those states. Fortunately, I didn't have to do much work to get the data I'm about to present, save for a few Google Image searches. Let me first post images modeling annual precipitation in the state of Pennsylvania.
Figure 1. Annual precipitation in Pennsylvania
(found at usda.gov)
Figure 2. Annual precipitation in Pennsylvania
(found at orst.edu)
Now, let me post an image with modeling of population density, and an image which shows the location of major cities in the state of Pennsylvania.
Figure 3. Population density of Pennsylvania counties
(found at dep.state.pa.us)
Figure 4. Pennsylvania road system, showing major cities
(found at smart-traveler.info)
Already we can see there is a fairly clear correlation between precipitation and population density in part of the state, perhaps even in 2/3rds of the state taken from East to West. Such a correlation is not at all clear in the West side of the state, but the correlation in the East side is probably enough of a confound.
Let me now post an image modeling autism prevalence across the state of Pennsylvania.
Figure 5. Pennsylvania IDEA autism cases per 10K enrolled
(found at fightingautism.org)
A correlation with either population density or precipitation is not that clear at first glance, is it? Let's focus on the bluest patches of prevalence, and let's compare with the Pennsylvania road map. You will note that high prevalence tends to coincide with urban areas, namely, Philadelphia, Harrisburg and Pittsburgh. Pittsburgh is particularly interesting, because it's an area with low precipitation. That's on the West side of the state, where I noted that a correlation between precipitation and population density was not clear.
Admittedly, there is an area near Erie (North-West corner) where there is high precipitation and high autism prevalence, but population density does not seem to be exactly where you would expect. This data point throws off my argument a bit, but it's not much to go on.
The correlation between administrative prevalence of autism and population density lends itself to false correlations between autism rates and innumerable proposed contributing factors in autism. It would be extremely easy, for example, to "prove" the autism fries hypothesis using this type of analysis. In the future, regions with equivalent population density and wealth should be compared. Additionally, studies should be devised to determine if differences in autism prevalence between regions are actual or simply administrative.
Note: I plan to email Dr. Waldman and refer him to this post.
I thought I would share a couple of remarkable images modeling annual precipitation and population density in the United States.
Figure 6. Annual precipitation in the U.S.
(found at ncgc.nrcs.usda.gov)
Figure 7. Population density in the U.S.
(found at wikimedia.org)
If you've ever wondered why population in the U.S. distributes the way it does, here's a big part of the answer. I have noticed that it's not so much that precipitation positively correlates with high population density, but that lack of precipitation correlates rather well with low population density. This is fairly obvious in retrospect.