Respectful Insolence

Evidence against an “autism epidemic”

One of the key arguments by advocates claiming a link between mercury in childhood vaccines is that there is an “epidemic” of autism. They’ll claim that autism was unknown before the 1930’s, when thimerosal was first introduced into vaccines. (Never mind that there are plenty of descriptions of autism-like conditions dating from as far back as the 18th century.) They’ll then claim that there is an “epidemic” that accelerated in the 1990’s, when additional vaccines were added to the recommended childhood schedule, and that it was the additional mercury from those vaccines that was responsible. It has been argued that a lot of that apparent increase was actually due to a widening of the diagnostic criteria for autism in the early 1990’s.

Now there’s more evidence to support that view. Just published in the April issue of the journal Pediatrics is a study by Paul Shattuck at the University of Wisconsin, entitled, The Contribution of Diagnostic Substitution to the Growing Administrative Prevalence of Autism in US Special Education. (Hat tip to the “source” who informed me of this article.)

In the paper, Shattuck analyzes special education figures that are being used to bolster claims of an autism “epidemic” and finds them wanting. In essence, diagnostic substitution can explain nearly all of the apparent increase of autism as recorded by the number of children receiving special education services. What that means is that children that would have been diagnosed with something else 15-20 years ago are now being diagnosed with autism. Dr. Shattuck starts with an example from a different condition, mental retardation as one of his reasons for suspecting diagnostic substitution as a cause of the perceived “epidemic”:

Second, prior research has established a precedent of diagnostic substitution in special education enrollment. From 1976 to 1992 the number of children in the mental retardation (MR) category decreased by 41%, whereas the number in the learning disabilities (LD) category increased 198%. There is considerable evidence that suggests this was because of a growing likelihood that schools would use the LD label for children with mild MR, presumably because a label of LD was increasingly seen as carrying less stigma than MR. Finally, a recent epidemiological study depicted a downward deflection in the incidence trend of other developmental disorders just as the trend for autism made a sharp upturn in the early 1990s, again suggesting the possibility of diagnostic substitution.

I was somewhat surprised to learn that autism was first listed as a primary diagnostic category for special education in the early 1990’s, as a result of being mandated by part of the Individuals with Disabilities Education Act in 1990 as part of Public Law 101-476.28. Before Public Law 101-476, there was no separate autism category. What that meant is that and children with autism enrolled in special education were lumped together for administrative purposes in the legal definition of the “other health impairments” (OHI) service category. Shattuck states: “However, no reliable data exist that would indicate how the enrollment of children with autism was actually distributed among other enrollment categories before the 1990’s.” States were required to begin using the new category in 1993, and 1994 was the first year that all states used it. Using a variety of statistical analyses, Shattuck looked at the rates of autism and a variety of other diagnostic criteria reported as special education recipients over a 20 year time period. He also used a method to correct for the possibility that diagnostic substitution was taking place simultaneously among several diagnostic categories. The findings were:

The average administrative prevalence of autism among children increased from 0.6 to 3.1 per 1000 from 1994 to 2003. By 2003, only 17 states had a special education prevalence of autism that was within the range of recent epidemiological estimates. During the same period, the prevalence of mental retardation and learning disabilities declined by 2.8 and 8.3 per 1000, respectively. Higher autism prevalence was significantly associated with corresponding declines in the prevalence of mental retardation and learning disabilities. The declining prevalence of mental retardation and learning disabilities from 1994 to 2003 represented a significant downward deflection in their preexisting trajectories of prevalence from 1984 to 1993. California was one of a handful of states that did not clearly follow this pattern.

So, in nearly all states, as the reporting of autism cases for special education administrative purposes increased, the reporting of mental retardation and learning disabilities declined correspondingly, suggesting that children who once would have been classified as mentally retarded or learning disabled were now more appropriately being classified as having autism or ASDs. He further comments:

The mean administrative prevalence of autism in US special education among children ages 6 to 11 in 1994 was only 0.6 per 1000, less than one-fifth of the lowest CDC estimate from Atlanta (based on surveillance data from 1996). Therefore, special education counts of children with autism in the early 1990s were dramatic underestimates of population prevalence and really had nowhere to go but up. This finding highlights the inappropriateness of using special education trends to make declarations about an epidemic of autism, as has been common in recent media and advocacy reports.

Indeed. Using such estimates is fraught with peril even using good statistics. If you use bad statistics, as the Geiers do, you can make such numbers seem to show almost anything you want.

Shattuck then issues a stinging rebuke to those who would use these numbers to support the concept of an “epidemic”:

Steep growth in administrative prevalence after introducing a new category is a common pattern that was also seen in the other 2 reporting categories newly introduced in the 1990s (TBI and DD). As with autism, in the first few years these categories were used it was not uncommon for states to
report very few children with a primary diagnosis of TBI or DD. The prevalence for these categories also had nowhere to go but up. Suggestions that special education trends substantiate the existence of an autism epidemic would logically also have to either claim an epidemic of brain injury and DD or explain why the same pattern of growth in these 2 categories does not represent an epidemic as it does for the autism category.

(Emphasis mine; note TBI= traumatic brain injury and DD=developmental delay.)

While pointing out the number of studies looking for a link between mercury and autism that failed to find a link, Shattuck does emphasize that the results of his do not disprove a connection between environmental factors and autism. Rather, they simply show that you can’t legitimately use numbers of children assigned to administrative categories for special education as a reliable estimate of changes in the true prevalence of autism, because you’re comparing apples and oranges. The categories didn’t even include autism before 1993. Consequently, for purposes of special education, autism did not exist as a major diagnostic category, and in the vast majority of states children diagnosed with autism had to be assigned a different category. Of course, autism rates as listed for administrative purposes in special education children skyrocketed after that.

As Dr. Shattuck stated in an interview (hat tip to Autism Diva):

In the case of special education counts for children with autism, the administrative prevalence is simply the number of students with a primary classification of autism divided by the total number of students in that given region, whether it’s a state, district or county.

This means, among other things, that data collected in this manner often underestimate “the true population prevalence because, for instance, schools do not go out into the community and actively seek out and evaluate all kids for autism,” Shattuck said.

For instance, consider data collected in Wisconsin: In 1992, 18 children were counted in special education programs as being autistic. By 2002, that number had jumped to 2,739.

“The conclusion is that the prevalence of autism has grown by 15,117 percent. This is ridiculous,” Shattuck said. “No credible clinician or scientist in the field would ever suggest there were actually only 18 children with autism in all of Wisconsin in 1992.”

The key word phrase here is “no credible clinician or scientist.”

Clinicians have long suspected that the the apparent autism “epidemic” as detected using numbers of of children receiving special education in different categories was a sham. Now they have some hard data to support that view. Of course, this study is not the final word and is not without some problems. A commentary by Craig Newschaffer accompanying the article this issue points out some potential problems with the model that Shattuck used but in the end seemed to come to similar conclusions. The point of difference was that the rate of diagnoses of mental retardation may not be falling as fast as the rate of increase of diagnoses of autism. Dr. Shattuck includes a point-by-point response.

What both end up agreeing on is that epidemiologists may never be able to figure out if there truly has been an increase in the prevalence of autism over the last couple of decades. Near the end of his commentary, Dr. Newschaffer laments:

I also believe that the time has come to accept that, given the behavioral basis of the autism diagnosis, the lack of knowledge about autism’s underlying etiology, and the limitations of retrospective analyses, we are not likely to develop a conclusive body of evidence to either fully support or fully refute the notion that there has been some real increase in autism risk over the past 2 decades.

Dr. Shattuck more or less agrees but points out that it is still important to study the epidemiology and gives reasons:

Second, Newschaffer concludes with a plea to turn from the intractable, and perhaps unanswerable, question of whether autism’s true prevalence has increased over the past 2 decades toward a greater prospective focus on conducting epidemiologic and genetic research related to autism. I agree that we may never be able to fully understand the dynamics behind historical changes in autism prevalence and of course agree that we need more research into etiology. However, autism diagnosis is based on observing behavior rather than clearly identifiable biological markers, diagnostic agreement among clinicians is not perfect, black children tend to be identified later than white children, and clinicians have reported a willingness to give a diagnosis of autism for children with ambiguous symptoms if they know it will result in the child obtaining more services. Therefore, I also believe that we need to continue complementing research into etiology with careful examination of the sociopolitical context wherein processes of identification, referral, diagnosis, and intervention occur for children with autism and their families.

The bottom line is that, even if Newschaffer is correct and there has been an increase in the prevalance of autism, it is almost certainly not of the magnitude suggested by advocates claiming that mercury in childhood vaccines cause autism, who sometimes dismiss suggestions that the apparent increase during the 1990’s was due to improved diagnosis and broadening of the diagnostic classification with ridiculous statements such as, “No child with autism ever went undiagnosed.” The “autism epidemic” (or, as it is sometimes more offensively dubbed, “the autism tsunami“) is nothing of the sort.

How long do you think it will be before the mercury crowd starts attacking Dr. Shattuck as biased or a pharma shill?

Comments

  1. #1 Lorlee Bartos
    April 4, 2006

    It is my understanding that there is a substitute for thimersol … Isn’t the simple answer just to go ahead and use it and be on the safe side.

    There is always sound and fury on both sides of an argument when one could hope for a solution to avoid that confrontation… isn’t the elimination of thimersol such an answer.

    Similary with abortion — no one is for abortion… yet the wrangling goes on — Too bad there can’t simply be an agreement for everyone to join together to try and prevent as many as possible.

    We waste so much political capital arguing rather than finding solutions.

  2. #2 Drew Coombs
    April 4, 2006

    Don’t go putting words in my mouth, Lorlee. I’m both for abortion and against being on the “safe side.”

  3. #3 JP
    April 4, 2006

    There are two problems with Lorlee’s suggestion as I see it. 1. the anti-thimersol case has no sound science to back it up. We can’t settle fear vs. science debates by caving into fear every time — it’s a slippery slope.
    2. Compromising on thimersol without evidence actually gives more weight to the quacks who advocate treating autistic children with dangerous/bogus mercury poisoning treatments.

  4. #4 HCN
    April 4, 2006

    Actually the “substitute” for thimerosal has been implented for the last several years: Costly single use vials. So that is a non-issue (especially for the MMR vaccine, which never ever had thimerosal).

    The point is that there really has not been an increase of austism, but an increase in EDUCATIONAL diagnoses. It is not a coincidence that the start of the increase is at the point it was included into the Individuals with Disabilities Education Act. So instead of being placed into other catagories, the students were placed in a more appropriate catagory.

    Also, there are variations in the numbers based on each service area’s diagnostic criteria. For instance the states of Oregon and Washington are similar in geography, climate and environmental impacts… but have very different levels of kids diagnosed with autism. Check it out with this essay: How “Educational Assessments” Skew Autism Prevalence Rates

  5. #5 ShyrlAnn Cone
    April 4, 2006

    I would simply like to point out that “diagnosis” is a term often overused by education professionals who do not diagnose anything but rather certify a child as eligible for special education based on their observation of characteristics and on testing conducted (generally in the school setting). In my school district and county, I believe that is where much of the “overdiagnosis” comes in. The criteria used do not strictly adhere to the DSM IV guidelines for autism spectrum disorders, either.

  6. #6 ebohlman
    April 4, 2006

    ShyrlAnn is right: the IDEA statistics should properly be labeled “educational assessments” rather than “diagnoses.” Some states require a DSM-IV diagnosis of autism in order to qualify for an educational assessment of autism, but many don’t.

    Also note that students don’t receive educational assessments of any condition unless they’re found eligible for special education, which is the outcome of a process that has to be initiated by the parents. This will not necessarily happen in “milder” cases of autism (Asperger’s and PDD-NOS) where the child may not be having serious difficulties, at least not in the early grades. Remember that the 1 in 166 figure is for all autism spectrum disorders, not only for Autistic Disorder (what the antivaxers call “full spectrum” or “full syndrome” autism).

  7. #7 Joseph
    April 4, 2006

    I also believe that the time has come to accept that, given the behavioral basis of the autism diagnosis, the lack of knowledge about autism’s underlying etiology, and the limitations of retrospective analyses, we are not likely to develop a conclusive body of evidence to either fully support or fully refute the notion that there has been some real increase in autism risk over the past 2 decades.

    I would disagree with that. It’s pretty clear in the data that no noticeable epidemic has occurred since 1992. People just aren’t aware of it because they haven’t analyzed the data. In any case, the burden of proof has shifted, as I argue in my latest blog entry.

  8. #8 Lorlee Bartos
    April 4, 2006

    To Drew….

    there is a difference between being for the right to choose an abortion and being for abortion. Semantics are important in framing many of these debates. I am steadfastly behind the right of women to control their bodies and their destinies.

    I am not a scientist, but believe that there is much evidence that mercury is toxic to humans. If that is the case, why would we continue to want to inject it into babies.

    Again, it is semantics — and for the great unwashed among us, simply saying we don’t use it any more solves the problem for most of us. Takes away the argument and those who posit that it is thimersol have to find another bogey man.

  9. #9 Hank Barnes
    April 4, 2006

    This is a surprisingly well-thought out post.

    One of the problems when science/medicine gets politicized, the definitions of the disease in question changes several times, either to maximize or minimize the numbers — depending on the circumstances.

    When I read this:

    For instance, consider data collected in Wisconsin: In 1992, 18 children were counted in special education programs as being autistic. By 2002, that number had jumped to 2,739.

    my skeptical radar is triggered.

    There are really only 2 possibilities: Wisconsin has suffered a remarkable, abrupt, massive increase in austistic kids or they’re simply sweeping more kids (dyslexia, poor spelling, problems at home, etc) into the autism mix.

    I fear the latter.

    Again, I pass no judgment on whether thimerasol causes autism or not, and remain extremely sympathetic to the parents of autistic children.

    But, the numbers are being distorted for some reason.

    Hank Barnes

  10. #10 Jennifer
    April 4, 2006

    “How long do you think it will be before the mercury crowd starts attacking Dr. Shattuck as biased or a pharma shill?”

    It has already happened.

    http://www.nationalautismassociation.org/press040306.php

    “NAA has learned that he was a Merck Scholar Pre-doctoral Trainee from 1999-2003, and in 2003-2004 he successfully applied for $530,000 from the Centers for Disease Control and Prevention (CDC).

    “Given the rocky history of the CDC and the autism community, failing to mention the author’s ties to this agency is a glaring omission that requires an explanation,” commented NAA board chair Claire Bothwell. ”

    It’s all so predictable.

  11. #11 Ali
    April 4, 2006

    Re: “diagnoses”

    Do you really think that matters to the altie population? Heck, they’d rather have an education professional’s “diagnosis” than that of someone who’s part of the big, bad medical industry. One only has to look as far as the raging, ridiculous success of Airborne.

  12. #12 Nat
    April 4, 2006

    Dear Hank

    I can offer three more explanations for the 18 to 2739 increase in autism diagnosed children in Wisconsin Schools. But the data presented in Orac’s post indicate that your second option is viable. By using “autism” I mean the true population of children with the condition, not simply those who have been diagnosed by some method (accepting the posts above who dispute the use of the term diagnosis in this case).

    1. More kids with “autism” go to school now.
    2. More kids with “autism” actually get diagnosed now.
    3. Schools now get extra money/resources for having more kids at school who have been diagnosed as having “autism” (i.e. they were always at school -but why go through the hassle of official notification if you don’t get extra resources?)

    Nat

  13. #13 Ivan
    April 4, 2006

    Lorlee- As has been pointed out, here in the US, we don’t have Thermisol in the vaccines anymore. As a rich country, we chose to do single use vials instead. Poor countries don’t have that luxury, so for them it’s a choice: Thermisol or not vaccinating everyone. Which would you choose?

    Ivan

  14. #14 Kev
    April 5, 2006

    “There are really only 2 possibilities: Wisconsin has suffered a remarkable, abrupt, massive increase in austistic kids or they’re simply sweeping more kids (dyslexia, poor spelling, problems at home, etc) into the autism mix.”

    Or, there always were this prevalence of autistic people and we’ve got better at recognising and diagnosing it. Thats the broad scientific consensus.

    “Again, I pass no judgment on whether thimerasol causes autism or not, and remain extremely sympathetic to the parents of autistic children.”

    Thanks, but we don’t need sympathy. What we need is for people to stop misrepresenting autism as mercury poisoning.

  15. #15 Paul Power
    April 5, 2006

    Coming at this from Ireland: I was going to post a slightly angry warning about the previous history of Paul Shattuck (see e.g. http://www.kevinleitch.co.uk/wp/?p=341) when I realized that he is not the same man as the British Paul Shattock (note the -ock) who is a promoter of the “MMR causes autism” line.

    So if you’re out on the Internet looking for more on either man, be careful about the surnames as some sites seem to confuse the two spellings which could lead you to confuse the reliability of their respective bodies of work.

  16. #16 David Harmon
    April 5, 2006

    I can totally buy diagnostic substitution just from personal experience. Way back when I was in kindergarten, I was “diagnosed” with MBD (“Minimal Brain Dysfunction”). Over the next 15 years, I watched that diagnosis go from MBD to Learning Disability, to Hyperactivity, to Attention Deficit Disorder, and finally to ADD+H (combining the prior two terms). Unfortunately, I was well past college before my *other* learning disorder hit anyone’s radar screens…. Today the ADD would probably have been considered a sideshow, to my Non-Verbal Learning Disability (NLD) (see http://www.nldontheweb.org/). So, that’s six different descriptions of the same kid… none of them actually erroneous, but “still it moves”.

  17. #17 Catherina
    April 5, 2006

    someone should tell RFKjr that his numbers do not hold up. In any case, this is supposed to come out as a full time ad tomorrow:

    http://www.putchildrenfirst.org/media/ad.060404.pdf

    which vaguely reminds me of what my grandmother used to say: “who shouts is lying”

  18. #18 Minnesota Skeptic
    April 5, 2006

    Craig Westover – who pushes the mercury/thimerison connection has a column that describes a committee hearing on this topic – requiring thimersol free vaccines for children.

    http://craigwestover.blogspot.com/2006/04/column-favorites-fell-but-its-only.html

  19. #19 Hank Barnes
    April 5, 2006

    Nat,

    Fair enough — maybe there were thousands of autistic kids in Wisconsin circa 1992, who were being missed.

    Seems little a post hoc to me, but sure, it’s a possibility.

    Hank B.

  20. #20 BronzeDog
    April 5, 2006

    Fair enough — maybe there were thousands of autistic kids in Wisconsin circa 1992, who were being missed.

    Seems little a post hoc to me, but sure, it’s a possibility.

    And probably labelled “retarded” or something else. Wasn’t there a time when ADD/ADHD kids were simply called “bad kids”?

    Oh, and you’re probably thinking ad hoc.

  21. #21 Jonathan Semetko
    April 5, 2006

    Hank,

    It is a little ad hoc, but remember that in 1992 the I.D.E.A. revision that estbalished autism as a special ed service category was brand new and that reporting the number of students served in the autism service category that year was optional.

    Also, more States back then used the guess-timate system of counting their kids. Delaware experienced a phenomenal autism increase from 2004-2005 school year as they switched from estimations to actual child count.

  22. #22 Nat
    April 5, 2006

    Dear Hank

    As an epidemiologist I’m puzzled as to what other type of reasoning I can use for interpreting historical data other than post hoc?

    The massive increase noted seems consistent with all 5 possibilities (your two and my three). It doesn’t have to be one or the other- it could be all five for instance.

    Given the extra analyses presented it seems misclassification of kids with whatever “autism” is into other categories at baseline explains at least some of the increase.

    Nat

  23. #23 Hank Barnes
    April 5, 2006

    As an epidemiologist I’m puzzled as to what other type of reasoning I can use for interpreting historical data other than post hoc?

    Hah! You guys are stumbling over simple things.

    First principle:

    How many cases?

    Number of cases then (18), number of cases now (2739)

    Observation: There is a huge increase of autism cases. What does this mean?

    Hypothesis one: Many more kids are developing autism.

    Hypothesis two: The definition of autism has changed over time. Then, too few austistic kids were counted, now, too many are counted. Hence, the 15,000% increase in autism cases.

    I genuinely don’t know the answer. I genuinely would like to ascertain the cause of autism. But, first one must sift thru numbers, which prima facie (more Latin for you boys) seem inflated to me. They may not be. I concede that. I am making no arguments, pro or con, about putative causal connections.

    I’m just saying Shattuck, I think, has done a fine job examining whether the numbers are distorted or not. That’s all. Not to controversial (I hope).

    Hank

  24. #24 Jonathan Semetko
    April 5, 2006

    Hi Hank,

    I get what you are saying. I dig it.

    I just want to add, that the autism descriptive epidemiology, is pretty insane in terms of complexity and that it is a real chore to sort it all out.

    Case in point; via the ed data, the data may be inflated (as you note) but if so, they are not inflated enough. The prevalence rate in 2005 was 1 per 473, the epi in 2005 puts it at 1 per 166.

    Just an example

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