Cities and Autism! Diagnostic Rates are not Incidence Rates

Over at Just a 'Lil Blog (a blog I recommend reading) there is the following:

Interview with a Unicorn

How did we come to be in front of the camera? Well, it started with the University of Pittsburgh. See, they're doing a study on the effects of environment on the prevalence or symptoms of autism in the surrounding area. Apparently Pittsburgh is quite a little hotspot for it, and so this study is geared toward…well…something about the environment...in their words, "The Research Study of Environmental Risk Factors for Childhood Autism is a multi-year study which began in 2010. It is being conducted in southwestern Pennsylvania (Allegheny, Armstrong, Beaver, Butler, Washington and Westmoreland Counties) by the University of Pittsburgh's Graduate School of Public Health. The aim of the study is to help identify environmental and other factors that may put children at risk for developing conditions within the ASDs."
— from http://blogginglily.blogspot.com/2013/02/interview-with-unicorn.html

When I read these stories about "autism prevalence" I cringe. There are a number of potential complications, represented by past studies and their questionable "findings" about autism prevalence — which tends to confuse diagnostic rates with actual incidence rates of autism.

Simply because autism is more likely to be diagnosed and labeled in certain places, does not mean the incidence is higher than elsewhere.

Consider the following "rates" of autism as a percentage of disabilities among students from 2005 IDEA statistics:

Calif.	Kent.	Minn.	Ohio	Texas	Wisc.
4.36%	2.06%	7.23%	3.12%	3.13%	3.97%

An interesting trend can be detected in most autism prevalence studies in the United States:

• Cities have higher reported rates of autism than rural areas.
• Cities with more universities and health research centers have higher rates of autism diagnoses.
• The education level of parents correlates to diagnostic rates… and more people with advanced degrees live in cities.
• Proximity to freeways was correlated to autism diagnostic rates… and cities have more freeways.
• Higher rates were found among some minority populations… and again, you can guess the urban vs. rural split.

Now, we could conclude any of the following:

• Universities correlate to autism.
• Tech centers correlate to autism (for many years, Asperger's was called "The Geek Syndrome").
• Cities correlate to autism.

So, avoid cities, universities, and freeways?

The lowest diagnostic rates are in the rural Deep South and in the rural Great Plains. The highest diagnostic rates include the areas around Los Angeles, San Francisco, Chicago, Salt Lake City, Minneapolis, Pittsburgh, and parts of New York.

Ask yourself the following:

• Aren't places with autism specialists more likely to have accurate diagnostic statistics?
• Aren't educated, middle-class and upper-class families more likely to be able to obtain diagnoses?
• Aren't the schools and other institutions in higher-income areas more likely to provide screening and supports?

Rural areas have a physician shortage. They also have a serious mental health care deficit. Clinicians and researchers work at… universities and large hospitals. These institutions are in cities. We could easily overlay population maps with demographic maps of income, higher education, and technology companies.

And while the "Amish don't have autism" nonsense has been discredited (I've blogged on that myth several times), there is a simple truth: rural residents are less likely to be diagnosed with medical and mental health conditions because they lack access to specialists.

The German-speaking Amish do have autism, at a rate of approximately 1:250 to 1:200, which is lower than the national average, but oddly close to the rates in other parts of rural PA, OH, and WV. So, that leads to other questions about clusters of autism and why some areas have substantially lower diagnostic rates -- with or without vaccination. 

Never assume that diagnostic rates corresponds to actual incidence of any medical diagnoses. Consider all the variables.