Jan 08 2009

Is the Rise In Autism Rates Real?

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Comments: 35

It is without controversy that the number of autism diagnoses being made is on the rise. In 1991 there were about 6 cases per 10,000 births, and in 2001 there were about 42. This number continues to rise at about the same rate.

The cause of this rise, however, is very controversial. There are basically two schools of thought: 1 – that true autism rates are on the rise, and 2 – that the measured rise is an artifact of increased surveillance and a broadening of the definition. A new study published today in the journal Epidemiology lends support to the school claiming that autism rates are truly rising – or at least that is how proponents are interpreting it. After a closer look, I am not so sure.

Prior studies generally support the contention that autism rates are rising due to changes in diagnosis and surveillance. (see my list of references below) For example, Taylor, after reviewing the evidence, wrote in 2006:

The recorded prevalence of autism has increased considerably in recent years. This reflects greater recognition, with changes in diagnostic practice associated with more trained diagnosticians; broadening of diagnostic criteria to include a spectrum of disorder; a greater willingness by parents and educationalists to accept the label (in part because of entitlement to services); and better recording systems, among other factors. (Taylor 2006)

This is a fascinating epidemiological question – and one with huge implications. If autism rates are truly static, that would  compatible with the majority opinion that autism is dominantly a genetic disorder. If there is a true dramatic rise in the incidence of autism, then that strongly suggests an environmental cause or trigger.

As an aside, we need to avoid the false dichotomy of genetic vs environmental. Even if autism is dominantly genetic, as the evidence suggests, the effects of gene products ultimately interact with the environment. Complex genetic disorders, such as a neurodevelopmental disorder, is going to have environmental influences.

A “trigger”, on the other hand,  means that that the genes just set the stage, but the disorder does not manifest unless the environment pulls the trigger. This could be an infection, toxin, coexisting disease, or (for neurodevelopmental disorders) the social and cultural environment.

For further background on this discussion I should also mention that at present the majority opinion is that the epidemiological evidence does not demonstrate a true rise in autism rates, but a small true rise could be hiding in the data and has not been ruled out.

I take pains to make these points because critics of the mainstream opinion often attack oversimplified straw men, while scientists working on this question tend to have and express appropriately nuanced opinions.

The UC Davis Study

This new epidemiological study, which must be put into the context of all the other studies on this question, looked at the California database of autistic children. They wanted to specifically test the “increased surveillance and diagnostic range” hypothesis, so they looked at autism rates by age. They found that younger age at diagnosis only accounts for a 12% increase in the diagnostic rates. Meanwhile, autism diagnosis rates have increased by 500-600% since 1991.

They then controlled for two further specific variables. First they eliminated all children who were not born in California. This was meant to eliminate children who were brought into California in order to receive services. They also looked at the severity of the symptoms and concluded that the “inclusion of milder cases” resulted in a 56% increase in diagnoses. So in total they could explain only about a 68% increase in autism diagnosis, which is about 10% of the total increase.

They conclude:

Autism incidence in California shows no sign yet of plateauing. Younger ages at diagnosis, differential migration, changes in diagnostic criteria, and inclusion of milder cases do not fully explain the observed increases. Other artifacts have yet to be quantified, and as a result, the extent to which the continued rise represents a true increase in the occurrence of autism remains unclear.

And yet in the press release for this study lead author Irva Hertz-Picciotto is quoted as saying:

“It’s time to start looking for the environmental culprits responsible for the remarkable increase in the rate of autism in California,”

The disconnect between the appropriately conservative conclusion in her paper and her statements to the media is interesting This probably reflects the fact that peer-reviewed papers have to pass tight scrutiny, and so she could not get away with over-interpreting her results. But to the media she let her true biases be known.

The key to putting this study into context is the phrase “other artifacts”. This study did not control for all possible artifacts resulting in higher diagnosis rates. Specifically, it did not address surveillance, which is likely the dominant factor. It also did not control for shifting diagnosis. In other words, 20 years ago a child may have been diagnosed with a non-specific speech disorder, and today they would be diagnosed with autism, so-called diagnostic substitution as was found by Bishop in 2008.

Another factor is that physicians, teachers, and parents have increased awareness not only of the symptoms but of the autistic label. How many parents who notice that their child is socially withdrawn are going to seek out services or medical attention?

This study did nothing to assess these potentially huge factors. So what this study really did was account for 10% of the increase in autism diagnosis. But it did not show anything about the other 90%, nor rule out the leading contenders for diagnostic artifact. I will add it to my list of references on this question, but it certainly does not overturn all the prior studies listed.

The Vaccine Connection

Of course, the anti-vaccine crowd can be counted on to grab hold of this study, cherry pick it from the other studies whose conclusions they don’t like, and overinterpret the results.  David Kirby has already done so over at the Huffington Post. He wrote a piece today titled: UC DAVIS STUDY: “Autism is Environmental” (Can We Move On Now?) He wants to declare premature victory and “move on,” even when, officially, the study authors had to admit that the question is “unclear.”

He writes:

Autism is predominantly an environmentally acquired disease, the study seems to conclude. Its meteoric rise, at least in California, cannot possibly be attributed to that shopworn mantra we still hear everyday, incredibly, from far too many public health officials: It’s due to better diagnosing and counting.

Compare that to the actual conclusions of the study.  I wonder if Kirby read past the press release.

And, of course, Kirby wants to bring this all back to vaccines.

 (It is important to keep in mind that almost every child born in 2000 would have received many vaccines that contained the mercury preservative thimerosal, which was not completely phased out of most – but not all – childhood vaccines until at least 2003.)

Even if it were ultimately found that autism rates are truly rising, and that this rise was due to an environment factors – vaccines would still be a very poor candidate.  There is already sufficient independent evidence against a significant link between vaccines and autism. If vaccines were causing a 5-6 fold increase in autism that would be a huge signal the studies to date would have picked up.

Also, Kirby fails to mention that the same data base used in this latest study is the one that showed that after thimerosal was removed from the childhood vaccine schedule autism rates continued to rise without any change – pretty much destroying the thimerosal hypothesis he clings to.

As usual the most mindless aspect of Kirby’s commentary is his casual and self-serving assumption of moral and intellectual superiority, even over dedicated medical researchers. He writes:

Now, it’s always been easier and more reassuring to tell ourselves that autism was almost purely genetic, that it was always with us at the rate of 1 in 90 men (1 in 60 in New Jersey) and that, gee, weren’t doctors doing a great job these days of recognizing and diagnosis this disorder.

This pathetic groupthink has helped create hugely lopsided funding priorities in autism, where genetic studies get lavishly funded, while environmental ones are lucky to even pick up the dollar scraps left behind.

It is cheap and easy to portray and hard-won consensus built upon years of research and evidence as “groupthink.” Also, Kirby appears to have insufficient familiarity with the real world or medical research to understand that grants are not earned and careers not made by following the herd. There are many researchers tackling these tough questions from many angles and perspectives. Young researchers hoping to find a niche for themselves are more likely to question dominant beliefs.

In fact, while I disagree with Hertz-Picciotto in her bottom line interpretation of the evidence, I think it is a very healthy thing for researchers with a minority opinion to challenge the majority. This happens all the time. But, of course, the burden is on her to make her case.

Kirby insists that the scientific mainstream disagrees with him, not because maybe they understand the research better, but because they are afraid of the truth. They want to avoid any research that could ultimately point back to vaccines. But this is just conspiracy-mongering self-righteous nonsense.  And it is a huge non-sequitur.

The notion that scientists disagree with someone because scientists are generally unimaginative and afraid of the truth is huge red flag. This is the mantra of the crank.


This latest study is interesting, but was too limited in scope to significantly alter the evidence as a whole. I would not be surprised if some portion of the increase in autism diagnoses were due to environmental factors. But I don’t think current evidence lends much support to this notion either. The current state of evidence strongly suggests that the dominant reason for the increase in numbers is due to changes in diagnostic behavior.

There are many similar epidemiological questions in the world of medicine. I am content to let the chips fall where they may, and will gladly alter my opinion as new evidence comes in. The same does not appear to be true for the Kirby’s of the world. The autism question in particular is mired in a fake controversy promulgated by an ideological anti-vaccine movement that causes no end of mischief.



Hertz-Picciotto, Irva a,b; Delwiche, Lora a. The Rise in Autism and the Role of Age at Diagnosis. Epidemiology. 20(1):84-90, January 2009.

Bishop DV, Whitehouse AJ, Watt HJ, Line EA. Autism and diagnostic substitution: evidence from a study of adults with a history of developmental language disorder. Dev Med Child Neurol. 2008 Mar 31

Chakrabarti S, Fombonne E. Pervasive developmental disorders in preschool children: confirmation of high prevalence. Am J Psychiatry. 2005 Jun;162(6):1133-41.

Fombonne E. Epidemiology of autistic disorder and other pervasive developmental disorders. J Clin Psychiatry. 2005;66 Suppl 10:3-8.Click here to read

Jick H, Kaye JA. Epidemiology and possible causes of autism. Pharmacotherapy. 2003 Dec;23(12):1524-30.

Rutter M. Incidence of autism spectrum disorders: changes over time and their meaning. Acta Paediatr. 2005 Jan;94(1):2-15.Click here to read

Paul T. Shattuck. The Contribution of Diagnostic Substitution to the Growing Administrative Prevalence of Autism in US Special Education. PEDIATRICS Vol. 117 No. 4 April 2006, pp. 1028-1037

Taylor B. Vaccines and the changing epidemiology of autism. Child Care Health Dev. 2006 Sep;32(5):511-9.Click here to read

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