January 28th, 2009

We still hear a lot about the “autism epidemic”. I suppose it’s only to be expected, since it is repeated so often and by so many people. And clearly, there has been an increase - a steady increase - in the number of children reported as “autistic” by educational sources and social service providers.

But is it a real epidemic?

This is a serious question because, if the rise in autism prevalence is real, then we need to find out why? Likewise, if the “autism epidemic” is an artifact of changes in awareness, diagnostic criteria, social implications, etc., then we should stop expending time, effort and - most of all - money trying to find the cause.

Here is a graph - similar to many that you may find all over the Internet - of the autism prevalence reported from 1993 (the first year “autism” was a mandated IDEA category) to 2007 (the last year USDE data is available).

As you can see, the rise in autism - at least the USDE definition of “autism” - has been spectacular and continuous. There appears to be no slackening in the rise, no plateau in sight.

But is this an accurate view of autism?

Here is another graph. This one shows both autism and mental retardation prevalence - as reported to the USDE - over the period 1993 to 2007.

The first thing that struck me about this graph is that autism doesn’t look so “scary”. The second is that the rise in autism coincides with a decline in mental retardation. More about that later.

The third graph shows autism and mental retardation along with all children served under the IDEA. I have not changed the autism numbers a bit.

Clearly, autism is not as significant a part of the total IDEA “picture” as some might imagine. You will note that over 10% of all US children are served under the IDEA.

In the next graph (number four), I have normalized the prevalence of autism and mental retardation by dividing the number of children in those categories by the total number of children served under the IDEA that year.

The reason for doing this is that the proportion of children classified under one or more IDEA disability categories has shown a slow but steady rise over the years. The possible reasons for this are many, but removing it by normalizing to the total number of children served under the IDEA eliminates one source of artificial (or administrative) drift in disability prevalence.

As you can see, the prevalence of autism has shown a rise that rather closely parallels the decline in mental retardation.

Over the years, a number of people supporting the “autism epidemic” hypothesis have derisively asked those who question the “epidemic” where the “hidden hordes” of adult autistic people are. Their argument being that if autism prevalence weren’t actually rising, we should expect to find larger numbers of adults - even the elderly - with autism.

I may have found the “hidden horde”.

In the final graph (number five), I’ve added the autism and mental retardation prevalence and normalized to the total number of children served under the IDEA. This will more readily show how closely the decline in mental retardation is paralleled by the rise in autism.

As you can see, the rise in autism is almost exactly paralleled by the decline in mental retardation (slope = 0.017, r = 0.999), indicating that the total prevalence of autism and mental retardation - as a fraction of all children served under the IDEA - has not changed significantly since 1993.

Does this mean that all of the rise in autism is simply a shift away from “diagnosing” children with mental retardation? Unfortunately, these data can’t make that determination - all they can show is that the “epidemic” rise in autism has been accompanied by an…. what would you call an inverse epidemic?… an anti-epidemic of mental retardation. However, a number of studies - looking at different data sets [Shattuck (2006) and Coo et al (2007) come readily to mind] have shown that diagnostic substitution is a large part of the “autism epidemic”.

All these data show is where the “hidden horde” might have been hiding.

Of course, I don’t expect the folks who are heavily invested in the ”autism epidemic” idea to be very happy with this information. I expect that they will cast all sorts of aspersions on me, claiming that I am a “tool” of “Big Pharma” or even that I am in the pay of a sinister conspiracy of “the government”, “Big Pharma” and the “AMA”. Maybe they’ll even throw in the Rosicrucians, for good measure.

Regardless of my presence or absence on the payrolls of “Big Pharma” (or the Illuminati), I’d be interested in hearing an alternative explanation for these data that supports the idea of an “autism epidemic”.

Remember, in order to play, you have to be able to account for all of the data - you can’t just ignore the parts that don’t work for you. In addition, you can’t use large government/industrial conspiracies or aliens as part of your explanation.

Prometheus

Filed under: Autism Policy, Autism Science, Critical Thinking

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