The Increase in Autism Diagnoses: Two Hypotheses

A new study sheds more light on the question of what is causing the recent increase in the rate of diagnosis of autism. Professor Dorothy Bishop from the University of Oxford studied adults who were diagnosed in 1980 with a developmental language disorder. She asked the question – if these people were subjected to current diagnostic criteria for autism, how many of them would be diagnosed today as having autism? She found that 25% of them would. (Bishop 2008)

This epidemiological question has been at the center of a controversy over whether or not there is a link between vaccines (or the mercury-based preservative, thimerosal, that was previously in routine childhood vaccines) and autism. The primary evidence for this claim put forward by proponents of a link is that the number of diagnoses of autism increased dramatically at the same time that the number of vaccines routinely given to children was increasing in the 1990′s. They are calling this rise in autism an “epidemic” and argue that such an increase requires an environmental factor, which they believe is linked to vaccines.

That the number of new autism diagnoses is dramatically increasing is generally accepted and not a point of debate. The historical rate of autism is about 4 per 10,000 and the more recent estimates are in the range of 15-20 per 10,000 (30-60 per 10,000 for all pervasive developmental disorders of which autism is one type). (Rutter 2005) The controversy is about what is causing this rise in diagnoses. There are two basic hypotheses: 1) That the true incidence of autism is rising due to an environmental cause, 2) That the rise in incidence is mostly or completely an artifact of increased surveillance and broadening of the definition of autism. These two hypotheses make specific predictions, and there is much evidence to bring to bear on their predictions – this recent study only being the latest.

The confusion about the epidemiology of autism is one common to scientific medicine. Whenever historical comparisons are made it is possible that changing definitions and practices over time will distort those comparisons. This is why medical scientists are often reluctant to change nomenclature (disease names) and definitions – doing so immediately renders the literature obsolete. All subsequent literature must now have a footnote. But the progress of our understanding of biology and disease makes such changes unavoidable.

In the 1990′s the diagnosis of autism was changed to autism spectrum disorder (ASD) – the new name reflecting the changing concept of autism to include a broader spectrum of symptoms, including much more subtle manifestations. In particular a diagnostic entity known as Aspergers syndrome, which is essentially a subtle manifestation of autism features, was classified as part of ASD. Any time you broaden a category the number of individuals that fit into that category is likely to increase.

Autism researcher Eric Fombonne found that:

Recent epidemiological surveys of autistic disorder and other PDDs have heightened awareness of and concern about the prevalence of these disorders; however, differences in survey methodology, particularly changes in case definition and case identification over time, have made comparisons between surveys difficult to perform and interpret. (Fombonne 2005)

In addition to the broadening of the diagnosis, the social and medical network supporting ASD dramatically increased. There has been increased efforts at surveillance – scouring the community for hidden cases of autism. Further, parents have become much more accepting of the diagnosis, which may partly be due to the fact that is some states the label with facilitate access to special services. And clinicians have become more knowledgeable of ASD so are better able to make the diagnosis, even in subtle cases.

Rutter, in order to test this latter hypothesis that increased diagnostic rates were due largely to changes in diagnosis and surveillance, reviewed literature that contained sufficient information to assess true historical rates of autism. He found that applying modern criteria to these historical records yields similar rates of diagnoses: 30-60 per 10,000. Taylor did a similar review and found the following:

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)

Another prediction that flows from the second hypothesis is that if we compare apples to apples – meaning if we look at the same community and apply the same diagnostic methods that were used in the past as documented in a published study, then the incidence should be the same. In other words – if we control for any changes in the diagnostic criteria and surveillance methods the incidence of autism should be stable over time. Chakrabarti and Fombonne did exactly that, comparing the incidence of autism in 2002 (looking at a cohort of children born between 1996-1998)  to the same population using the same methods as a previous study looking at the cohort of children born between 1992-1995. They found:

The rate in this study is comparable to that in previous birth cohorts from the same area and surveyed with the same methods, suggesting a stable incidence. (Chakrabarti 2005)

If the broadened diagnosis hypothesis is true than it must also be true that as other diagnoses shifted over to autism they would decrease as autism numbers increased. This is exactly what Jick et al found when they reviewed a cohort of boys with and without autism. What was previously diagnosed as language disorder is now being diagnosed as autism, with a corresponding decrease in non-specific language disorders. Shattuck found the exact same effect, so called “diagnostic substitution,” when he studied the prevalence of disabilities among children in US special education from 1984 to 2003. He found that in locations where the prevalence of autism had increased there was a corresponding decrease in the prevalence of other disabilities. (Shattuck 2006)

This brings us to the current study by Bishop et al. They looked at adults who were diagnosed as children with a developmental language disorder. Applying current diagnostic criteria they found that a quarter of them (12) would have been diagnosed today with ASD. This also fits the hypothesis that diagnoses have been shifting over to autism from other developmental disorders over the past two decades. The authors, however, caution that the number of subjects in their study was small and therefore it is difficult to extrapolate from this to the general population.

It should also be noted that all of this research, while supporting the hypothesis that the rise in autism diagnoses is not due to a true increase in the incidence but rather is due to a broadening of the definition and increased surveillance, does not rule out a small genuine increase in the true incidence. A small real increase can be hiding in the data. There is no evidence upon which we can conclude, however, that true autism rates are increasing.

Of course the implications of this are profound. If there is no autism epidemic, if there is a “stable incidence” of autism over recent decades, then this alone is powerful evidence against the vaccine hypothesis – and in fact removes the primary piece of evidence for a vaccine-autism connection. Just as a true increase in incidence would have called out for an environmental factor causing autism, the lack of any increase argues strongly against any environment factor – especially when this is combined with the copious evidence for multiple genetic factors as the ultimate cause(s) of ASD.



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

Posted in: Neuroscience/Mental Health, Public Health, Vaccines

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29 thoughts on “The Increase in Autism Diagnoses: Two Hypotheses

  1. TsuDhoNimh says:

    Some examples of moving cases from one bucket to another – Legionnaire’s disease moved a bunch of pneumonias of unknown origin out of that bucket into a bucket of their own. No net increase in pneumonia cases.

    I can’t find a proper reference, but the number of cases of malaria in a southern US state went from several thousands to zero when the state required finding a malaria parasite in a blood smear for diagnoses. My epidemiology teachers loved that example.

  2. joseph449008 says:

    There’s another recent study, Matson et al. (2007) which found that 50% of a group of adults with intellectual disability could be diagnosed with PDD. Then there’s Shah et al. (1982) which is an old study of adults which finds something similar.

    Nylander & Gillberg (2001) screened adult patients at a psychiatric hospital in Sweden, and found that 89.5% (17/19) of autistics there did not have a prior autism diagnosis.

    There’s also Stahlberg et al. (2004), which probably suffers from referral bias, but it’s nevertheless interesting that it found 30% of adult patients with ADHD were also in the autism spectrum.

    Baron-Cohen et al. (2001) and Wakabayashi et al. (2004) are interesting in that they show what happens when you have random groups of adults take the AQ test.

    Additionally, if you look at, say, California data, the prevalence of assessed mental retardation is stable. The prevalence of MR in IDEA, where it’s treated as a category (a bucket), is declining. The number of institutionalized developmentally disabled individuals in California is declining slowly.

    I have looked at a lot of data over the years, and it’s quite clear to me that what we call autism is a stable, albeit often missed, categorization.

  3. joseph – thanks for the references.

  4. mike stanton says:

    If I may make a small correction, Chakrabarti and Fombonne (2001, 2005) did not compare cohorts 15 years apart. They compared consecutive birth cohorts (1992 – 1995 and 1996 – 1998) and found no change in prevalence. As late as 1997, according to Fombonne the literature supported a prevalence of no more than 15 in 10000 for all autistic spectrum disorders.

    Chakrabarti and Fombonne (2001) Pervasive Developmental Disorders in Pre-school Children
    Chakrabarti and Fombonne (2005) Pervasive developmental disorders in preschool children: confirmation of high prevalence.
    Fombonne, E. (1997) Prevalence of autism spectrum disorder in the UK. Autism 1:2 pp 227-229

  5. joseph449008 says:

    There’s a UK study on a cohort prior to the ones Mike mentions, albeit in a different part of the country. That would be Baird et al. (2007) which surveys children born between 1990 and 1991; practically pre-”epidemic”. The prevalence this study finds is high, at 1.16% for the entire spectrum.

    The key thing about Baird et al. is that they screen a lot of children with a possibly undiagnosed ASD.

    A prevalence higher than 1% was also found by Kadesjö et al. (1999) among 7 year-olds in the town of Karlstad, Sweden. Again, the key to this study was that the primary author personally screened 50% of the 7 year-olds in the town.

  6. BlazingDragon says:

    Thanks for the post.

    I hadn’t even thought about the “one bucket to another bucket” idea… but it makes a lot of sense. Of course, I have known for a long time that the “autism epidemic” is nonsense. Unfortunately, this will have no effect on the “autism-vaccine” nutjobs…

  7. msclark says:

    I think the biggest implication of the “high and stable rate” of autism is that we need to face the fact that somehow society has survived the number of autistics that it’s had up until now, so it’s not about to be destroyed by a tsunami, a new, vastly higher percentage of monstrous, tax-dollar sucking burdens. The fact is that over the years kids that started out looking really impaired sometimes turned out to be fairly self-supporting and certainly a good number of those who would be the famous “1 in 150″ (which may be an underestimate) are now taxpayers and lets not forget that a good number of them are now parents of autistic kids.

    We need to assess this situation of what has happened to “historical” autistics (like myself, an Asperger’s person, age 48) because we may find that we are terrorizing parents of newly diagnosed kids with ideas of “rapidly closing critical periods” and a life-or-death need for “early intervention” that is not in fact that life-or-death and could even hinder the development of autistic kids because it’s not scientifically based, but emotionally based. “Oh, this kid isn’t doing what I think a normal kid would do right now, so I will use behavioral methods to train him to act like a normal kid.” It’s simplistic in the extreme.

    Autistic kids don’t have the same developmental trajectory as typical kids and they may need to learn things on a different schedule in order to learn in the best way for them. They may need to have information presented to them in a way that behaviorists don’t. Who knows if everyone ignores what is being learned about autism from cognitive psych researchers?

    The people who are selling one kind of early intervention are also making an industry out of it. There’s big money in this, and the therapists are very self-serving in many cases. They take a kid who might be slightly delayed, cry “oh my gosh!!!! We must act now lest the child end up as a vegetable in an institution!!! Pay me and I will save your baby!!!!!” And since they are making this pre-diagnosis so young, they have a good chance of having panicked everyone for no good reason, the kid was just a little speech delayed…. but now he’s a walking advertisement for how their “intensive early intervention” has save a child from a fate worse than death… autism… cue the spooky music.

    And for the kids who are for instance, profoundly MR and autistic, too, all their early intervention isn’t going to make the kid normal, and though it might be beneficial, the science isn’t there to know what exactly needs to be taught, by whom, how and when to give the child the best advantage.

    One other point. Even if everyone agrees that there hasn’t been an increase in autism, or much of an increase if any… they could still look for the so called “environmental” causes, since an environmental cause might be more or less stable across many years. Dr. Irva Hertz-Picciotto, an autism epidemiologist from the MIND Institute used the example of some cases of schizophrenia possibly being caused by maternal exposure to the flu virus.

    But the big drive is to find imaginary environmental causes that are not just external to mom and dad’s most hallowed genes, but that are actionable. They don’t want to know about flu virus causing autism, they want to know that there’s someone they can sue. Even if it didn’t make their kid autistic so long as they can make someone believe that it did and they can cash in, that’s all that counts with some of these parents. I think these litigious parents are a small minority, but they are noisy and bad for autistics, period. And some of them are outright liars (like the people that “accidently slip” in WalMart and try to sue) and others are just deluded into thinking that they know what made their kid autistic.

    For these epidemic howlers they need to deny that there are probably a million or more ASD people who are age 20 to 80 in this country, and that is a direct threat to the very lives of some of these autistic adults who are currently undiagnosed and misdiagnosed.

  8. mike – You are correct, it is more meaningful to list the cohorts rather than the dates of the survey. I made the change to the entry.

  9. bcorden says:

    As a clinician who faces this problem almost every day, thanks for the wisdom. I especially appreciated msclark’s comments on what might call the “business” of autism.

  10. Jackson says:

    The autism/vaccine discussion came up again on the 4/28 Skeptic’s podcast.

    Last Nov. when I was in Atlanta for an IEEE meeting, I arrived a day early and found that Sunday was the closing day for an Autism conference. I went down to the conference area and bought a couple of the CDs they had for speakers. Some of my colleagues have autistic children — I agree with those who suspect it is more prevalent but aren’t sure.

    One session involved two speakers from NIH and I recommend that CD. Those guys had a tough audience and handled it pretty well. There is also a brochure on autism available from NIH and one would hope that a candidate whose position differed from NIH would have a really good story…

    WWW for brochure
    WWW for conference with CD of NIH speakers
    {I only see DVDs there. I got a CD.}

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