Aug 17 2010

More On Autism Prevalence

Published by under autism
Comments: 22

In order to teach science to the public it is better to tell a story about how questions are resolved in science, rather than simply to teach authoritatively the current findings of science. The process is interesting – often more so than the facts.

One such story is the true cause of the increasing autism prevalence over the last 20 years. There is no question that the number of people being diagnosed with autism is increasing. There are various theories as to why, the best, in my opinion, is that the increasing numbers are an artifact of broadening the diagnosis and increased surveillance. Meanwhile, a true increase may also be hiding in the numbers, and that would present an interesting (and important) mystery to solve.

I have written about this before (here and here on SBM, and again here on NeuroLogica). Now there is a new study which may shed some light on the question, but to quickly summarize where the evidence is: There are several ways to address the question of whether or not there is a true increase in autism numbers. One is to assess autism prevalence at various age groups. If true autism incidence is increasing, that younger age groups should have more diagnoses of autism than older age groups – when the same diagnostic criteria are applied. When this kind of analysis is done it appears that autism incidence is stable over time, which is powerful evidence against a true increase.

Another approach is to look at the same cohort today and 20 years ago (where data is available, or by comparing to historical studies) and apply a thorough survey of consistent diagnostic criteria. When this is done again we find a consistent rate of autism diagnosis.

The third approach is to look for factors that could lead to an increase in diagnosis with a true increase in incidence. When this kind of analysis is done many sources of artifactual increase are found, but the deeper question is – are they enough to account for the entire increase. This is tricky because unless you count up all the factors you will underestimate this type of increase in diagnosis rates.

Studies looking at this question have found that there is strong evidence of expanding the diagnosis of autism, diagnostic substitution (what was diagnosed 20 years ago as something else, like a language disorder, is now diagnosed as autism), increased surveillance (partly caused by increased services provided), and culture (for example, autism is more likely among the neighbors of others diagnosed with autism, suggesting that access to information about autism increases the chances of being diagnosed). However, the few studies I have seen that have attempted to quantify these effects usually focus on a few, and have not been thorough.

A recent study is another attempt to quantify the increase in autism diagnoses. Bearman and his colleagues found that 26% of the increase in autism can be accounted for by expanded diagnostic definition. A further 16% can be explained by increased surveillance. They also concluded that 11% can be accounted for by increasing parental age. This is a likely cause of a true increase, as the risk of autism increases with parental age (more definite for paternal age), and people are having children older.

These three factors add up to 53%, which means that 47% of the increase in not accounted for by Bearman’s analysis. However, this analysis does not seem to account for all possible factors, including the fact that a diagnosis of autism was a stigma to be avoided 20-30 years ago, while more recently it has lost this stigma. This means that parents are more likely to seek out the diagnosis (when appropriate) in order to get the services their child needs. In addition diagnosis falls prey to the availability heuristic – non-experts (and even experts to a lesser degree) will tend to gravitate toward available diagnoses to explain ambiguous cases. I have seen this in action personally – patients and others will fit symptoms to the limited number of diagnoses that are available to them because they are in the news or in the popular culture. There has been a dramatic change on this score for autism, and this social factor is hard to quantify.

The weakness of Bearman’s analysis is that there is uncertainty in the numbers, and therefore it is difficult to quantify exactly how much of an effect each factor has, and there are many social factors that are difficult to quantify at all. We are therefore left without a compelling reason to conclude that there is another true increase in autism rates hiding in the numbers (although again, this can also not be ruled out).

But put into the context of the other approaches to the question of autism rates, it seems that the best conclusion we can make at this time is that most of the rise in autism numbers are caused by an increase in the process of making the diagnosis, with some increase from increasing parental age. There may or may not be another factor, but if so it is likely not large.

There is also research that indicates that the true rate of autism has always been much higher than the rate of diagnosis. Thorough surveys of cohorts have found that true autism rates are about 1% of the population. We are just now approaching that true rate. If this is correct, then rates should start to level off. We are still in the middle of this story, and it is likely to be clarified further over the next decade or so.

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22 responses so far

22 Responses to “More On Autism Prevalence”

  1. superdaveon 17 Aug 2010 at 9:33 am

    A lot of people ask why they have never noticed autism before, if the rates today are really about the same as they were years ago. I think this is just a testament to the power of human pattern recognition. We have all had moments in our lives where suddenly introduced to a new idea or topic, we start to see it everywhere. Learning about evolution was like that for me. Once I developed an understanding of how evolution works, I started seeing evidence for it everywhere. Of course, as pointed out in earlier posts, this can go wrong and lead you down the path of conspiracy thinking, which is exactly what happens with the anti vaxxers.

  2. Karl Withakayon 17 Aug 2010 at 12:08 pm

    “We have all had moments in our lives where suddenly introduced to a new idea or topic, we start to see it everywhere”

    Like when you buy a new car and then start seeing other people driving the same model everywhere you look.

  3. Todd W.on 17 Aug 2010 at 12:47 pm

    @Karl Withakay

    That’s just what I was going to say. A couple years ago, I bought a new car. When I had settled on a couple likely options, I suddenly noticed people driving those cars all over the place, when I swear that I had never seen so many before. Clearly, then, something happened around the time that I started looking for a new car that caused all manner of other people to get the same car…

    Or, I just had it more readily to mind, and the prevalence was always there; I just didn’t notice because it wasn’t important to me.

  4. Steven Novellaon 17 Aug 2010 at 12:56 pm

    I had the same experience when I started birding. Suddenly I noticed many different species of birds on a daily basis. They were always there in my environment – I just never noticed them. I don’t think the prevalence of black-capped chicadees has actually increased in CT.

  5. locutusbrgon 17 Aug 2010 at 1:22 pm

    Police Blotter theory.
    Why does the crime rate go up every time you hire police officers? The crime rate is not truly increasing you are just catching more criminals.
    Simple you train more people to look for Autism you are going to find more.

  6. catgirlon 17 Aug 2010 at 2:01 pm

    Sometimes I wonder if the increase is also partially due to better care of newborns, especially babies that are born prematurely. I’ve heard that being premature can increase the risk of all kinds of diseases, like autism and food allergies. Maybe in the past, children that would have been diagnosed with autism simply didn’t survive long enough for the problem to even prevent itself. Of course this is pure speculation right now, but I would love to see if there have been any studies to back up this hypothesis.

  7. Rikki-Tikki-Tavion 17 Aug 2010 at 2:28 pm

    How have Bearman et al quantified how much influence the different factors have?

    Ok, parental age influence is understandable. They correlated study data on the relative Influence with census data.

    But how did they quantify, for example, the increased surveillance?

  8. CivilUnreston 17 Aug 2010 at 4:30 pm

    Dr. Novella, I would like your secret to being a Superhuman. If I’m not mistaken, you are a practicing neurologist, a researcher, an active blogger on at least two sites, a parent and now a BIRDER!!?? Way to make the rest of us look like slackers.

  9. eeanon 17 Aug 2010 at 4:46 pm

    @CivilUnrest I’m sure Dr. Novella must use some sort of magic mineral oil, available to you for only three easy payments of 19.95.

  10. passionlessDroneon 17 Aug 2010 at 5:01 pm

    Hello friends –

    A recent study is another attempt to quantify the increase in autism diagnoses. Bearman and his colleagues found that 26% of the increase in autism can be accounted for by expanded diagnostic definition. A further 16% can be explained by increased surveillance. They also concluded that 11% can be accounted for by increasing parental age.

    These three factors add up to 53%, which means that 47% of the increase in not accounted for by Bearman’s analysis.

    I saw this posted somewhere else and found myself wondering why we feel like we can add these numbers up and get to 53% with any confidence?

    If we have more awareness, that in turn, leads to more accurate diagnosis of the same child, aren’t we counting that child twice with this model? Likewise, if our older parent has a child that subsequently gets accurately diagnosed, or diagnosed at all due to increased awareness, aren’t we counting that child twice, or three times?

    Other groups have found a far less pronounced effect of older mothers and fathers in California. Independent and dependent contributions of advanced maternal and paternal ages to autism risk [PMID: 20143326] reports less than half the increase can be attributed to advanced parental age. [4.6% as opposed to 11%]

    I thought this was supposed to be a site for skeptical analysis.

    - pD

  11. daedalus2uon 17 Aug 2010 at 5:24 pm

    Civil, I think it is just that keeping track of reality is easier than keeping track of all the myths and just-so stories that people make up.

    When you don’t know something, you just admit it and say “I don’t know”, rather than try and make something up or keep track of something arbitrary that someone else made up.

  12. superdaveon 17 Aug 2010 at 6:36 pm

    Catgirl,
    That’s a pretty reasonable hypothesis. Another thing to keep in mind is that psychiatric evaluation of children of any kind has had a tremendous increase. We don’t know how many people went undiagnosed in the first place.

  13. sonicon 18 Aug 2010 at 2:26 am

    It seems that these numbers (47 and 53%) are probably inaccurate enough to say that we can’t account for about half of the increase in autism (Perhaps even a little more than half.).

    While I would agree that in all likelihood the increase is due to factors like those mentioned, I’m not sure that the conclusion
    “There may or may not be another factor, but if so it is likely not large.”
    follows from the evidence presented.
    (After all, the effect might be half or more of the increase- that’s large…)

  14. Dawnon 18 Aug 2010 at 10:20 am

    @eean: I’m sure Dr. Novella must use some sort of magic mineral oil, available to you for only three easy payments of 19.95.

    I’m not even going to go there….

    Yeah, the car thing. When I bought my car, I swore I rarely saw Suzuki cars on the road. All of a sudden, they were all over, including my model (which I would have sworn was rare, too).

    I agree that more people are being diagnosed who wouldn’t have been 40 years ago when I was in elementary school. There were several kids who probably would have fallen under HFA or Asperger’s in my school. We just considered them “weird”. And I have often wondered how many of the kids in the “retarded room” (believe it or not,that was the name posted on the wall by the door to the room!!) were actually autistic and not mentally deficient?

    Going back 70 years, my mom had a cousin who was characterized as “a problem”. He was intelligent, but very impulsive, perservated on things (he could name every neighbor in every house for blocks) and could be violent. He liked to turn on the gas burners of the stove and watch them flame. He was institutionalized at about 20 years of age and died in the institution (diagnosed as schizophrenic). Today, I sincerely believe that he would be diagnosed as autistic and given help and training to function.

  15. SpicyCupcakeon 19 Aug 2010 at 9:55 am

    @passionlessDrone Hello friend, most likely this would be covered in the study under their methodology. I have not read the study and as you can tell by me commenting two days after the post, I am just catching up on my reading. You bring up a legitimate concern for discussion that has an easy first step to getting answered: Look at the paper Dr. Novella is talking about. Bringing up a reasonable question and then turning around and making a snide ad hominem aimed at both Dr. Novella and those involved here is not an appropriate way to be involved in discussion. In my eyes, it also cuts away at your credibility.

    @sonic I believe that Dr. Novella directly linked to the previous posts that deal with factors that have lead to an increase in diagnosis. My take away from this post was this: 1) This study accounted for 3 factors that increase diagnosis, but not occurrence of Autism. 2) It did not consider all of the factors that come into play for increasing the diagnosis of Autism without an increase in occurrence. 3) Even with an incomplete set of effects in play the study managed to explain 53% of the increase in diagnosis. 4) If you were to take into consideration all the factors that were excluded, there may be no effect or a small effect left that could be a legitimate increase in the occurrence of autism, but there is no way to know this from the data available. 5) Specific prediction: If occurrences of autism are not increasing over time, as we approach a diagnosis rate of 1% of the total population, the increase in diagnosis will begin to level off to that of population growth.

    To which I add, for the United States or specific nation. I guarantee as developing nations begin to have better awareness and health care in their cultures they will increase their diagnosis. That increase will likely be held by cranks as definitive evidence that a scapegoat of their choosing causes autism. Get ready for the correlations to flow forth in decades to come.

  16. passionlessDroneon 19 Aug 2010 at 11:16 am

    Hi SpicyCupCake –

    I love your handle!

    You bring up a legitimate concern for discussion that has an easy first step to getting answered: Look at the paper Dr. Novella is talking about.

    It is actually a news article that takes together three papers, all written by the same author. Here’s to hoping that a single link fits through the spam filter:

    http://www.newscientist.com/article/dn19316-autism-explosion-half-explained-half-still-a-mystery.html?DCMP=OTC-rss&nsref=health

    Take a look at the title of this article:

    “Autism explosion half explained, half still a mystery”

    Dr. Novella has written several postings that decry with much fanfare the state of scientific reporting and the problems with headlines and content that cannot be validated once the details of the underlying science are examined.

    Bringing up a reasonable question and then turning around and making a snide ad hominem aimed at both Dr. Novella and those involved here is not an appropriate way to be involved in discussion. In my eyes, it also cuts away at your credibility.

    That is unfortunate, but I still feel that the amount of skepticism applied in this case falls far, far short of the caution usually employed at this blog. The fact that other groups have found far, far less pronounced effects of advanced age was extremely simple to find.

    As another example, the 16% value, taken from a spatial proximity study (also blogged by Dr. Novella @ SBM), is the first and only study of that type ever published, and yet, it’s results are being taken as so strong that we have no problem with the headline: “Autism explosion half explained”.

    This is the exact opposite of the type of dispassionate analysis that usually goes on at this blog.

    - pD

  17. sonicon 19 Aug 2010 at 2:28 pm

    SpicyCupcake-
    My take from the article is the same as yours.
    So while I would agree that there is a likelihood that the difference lies in the unquantified, unaccounted for, untested reasons given; I would suggest that 47% leaves plenty of room for a possible explanation that has yet to be accounted for.
    In other words, I see reason to maintain doubt.

  18. Woodyon 19 Aug 2010 at 4:53 pm

    Given the number of studies that suggest that autism prevalence is actually stable and the increasing number of cases is accounted for by improved case ascertainment, I wonder where all the adults with previously undiagnosed autism are? If they are out there, what were they misdiagnosed as?

  19. ChrisHon 19 Aug 2010 at 5:58 pm

    Woody, some of them have been found in homes for disabled adults. See:
    http://leftbrainrightbrain.co.uk/2010/07/more-unidentified-autistic-adults-found/

  20. Woodyon 19 Aug 2010 at 8:23 pm

    Thanks for the link, ChrisH. If the two studies mentioned there are representative of global trends, there are a large number of unrecognized individuals out there that could help research efforts to better pinpoint the true causes and natural history of the ASDs.

  21. ChrisHon 19 Aug 2010 at 10:37 pm

    You might also want to read Not Even Wrong by Paul Collins and Unstrange Minds by Roy Grinker. They are both fathers of autistic children. Paul Collins is a kind of literary historian, and Roy Grinker is an anthropologist who does research on autism in other countries.

  22. terry the censoron 25 Aug 2010 at 10:47 pm

    There’s a lot of discussion of improved awareness and older parents but, from personal experience, I suggest false diagnosis might be a very strong factor. As Steve alluded to, child care and healthcare workers are now very aware of autism and — not wanting to miss a case, like in the bad old days — they see it everywhere. These reports gets handed up the ladder, confirmation bias doing its thing, until a doctor receives a thick file of “evidence.”

    I’d like to see how many kids being diagnosed today still have the diagnosis 10 years from now.

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