Does Thimerosal Prevent Autism? Epidemiology Gone Wild

Human beings are born pattern recognizers, part of the repertoire of survival skills that separates us from the beasts. We are programmed to spot relationships that we are looking for, sometimes when they're not even there. This occasionally causes us to leap to questionable conclusions that can wreak all sorts of havoc.

The inability of the general public to distinguish between correlation and causation-aided and abetted by innumerate journalists employed by sensationalist media-has resulted in countless misfortunes, particularly when unfounded "scientific" conclusions are transformed into policy by an unholy alliance of advocacy groups, regulatory agencies, legislators, and the tort bar. Add to this the logical impossibility of proving that something does not exist once public alarm has been sounded-for example "sudden acceleration syndrome" in the electronic throttle controls of Toyota vehicles-and you have a recipe for hysteria.

Epidemiologists are supposed to be trained to avoid these kinds of mistakes. That's why a recent study linking maternal obesity to autism is raising eyebrows across the scientific community.

Published in the journal Pediatrics, the study "Maternal Metabolic Conditions and Risk for Autism and Other Neurodevelopmental Disorders," concludes that, "Maternal metabolic conditions may be broadly associated with neurodevelopmental problems in children." While this sounds like a pretty weak conclusion, with lots of wiggle room to back down should future studies prove contradictory, the press headlines screamed, "Autism Linked to Obesity!"

What could be better for reporters looking to alarm newspaper readers than an opportunity to talk about two "epidemics" in one story? Loop this alarm back through advocacy groups, and watch funding agencies roll out the red carpet for grant-seeking scientists looking to advance their careers by hitching a ride on the crisis du jour. What are the odds that at least a few convince themselves that they've found the connection they are looking for?

The last time this happened the villain was thimerosal, a mercury-based preservative once used in children's vaccines that was fingered as the cause of Autism. Thousands of lawsuits bloomed, driving the vaccine industry to remove thimerosal from children's vaccines despite the very real fear that this could lead to increased bacterial contamination. Over a decade and many scientific studies later, no causal linkage could be found. But by then, the damage had been done. Residual misinformation continues to delude some parents into avoiding vaccinations for their children.

Here's a tantalizing epidemiological observation as worthy as many others that get bandied about. Since thimerosal was removed from children's vaccines, autism diagnoses have increased by a factor of eight! Have you seen any articles (besides this one, with tongue planted firmly in cheek) that have used this undeniable fact to advance the theory that thimerosal prevents autism, and that removing it may have been a tragedy?

Taking a closer look at the world of epidemiology-based autism research is like traveling with Alice through her looking glass. Over the last 20 years Autism diagnoses have risen from less than one per 1,000 children to over 10 per 1,000. The media labels this an "epidemic" driving an increasingly frantic search for a "cause."

But observational studies, like the Pediatrics study quoted above, coupled with reliance on self-reported medical conditions (another favorite of epidemiologists when they find it too difficult or expensive to get their hands on objective data), pollute the scientific literature. Many of these studies contribute little to our understanding, while they divert funds from useful research into the fundamental nature of autism that might one day lead to a cure.

Here's a little known fact I stumbled upon while researching this column. The rapid increase in Autism diagnoses over the past twenty years has been almost exactly mirrored by a decrease in the number of diagnoses for Mental Retardation, a catch-all label that covers a wide range of developmental disorders, some of which have known etiologies and many of which do not. Could the entire autism "epidemic" be simply a case of reclassification? You would think this perspective would generate more sober discussion and analysis, although any public choice theorist would predict that few grant-seeking scientists will be pushing this hypothesis.

Young children exhibit a wide range of affects and developmental behaviors that cause concerned parents to worry. What can be easier than reaching for a medical diagnosis that demands third party intervention? A pill would be nice but expensive services will do, especially if they are paid for using someone else's money.

Before the advocacy group hate mail starts pouring in, no one would confuse a severe case of Autism or Asperger syndrome with overwrought parenting. But when the definition of a disease becomes an ever-expanding "spectrum disorder," and an entire public complex of interventional services becomes accessible to anyone armed with a diagnosis, who's to say where genuine illness ends and garden variety oddness begins? Sometimes raising an unusual or difficult child just takes patience and the understanding that every kid develops at his or her own rate.