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The number of diagnosed cases of autism has increased substantially in the last decades as seen in the following graph.

enter image description here

Bar chart of the number (per 1,000 U.S. resident children aged 6–17) of children aged 6–17 who were served under the Individuals with Disabilities Education Act (IDEA) with a diagnosis of autism, from 1996 through 2007. Image from Wikimedia Commons


How can this increase be explained. Are there environmental factors that contribute to a higher rate of autism today? Or did we just get better at diagnosing autism?

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You talk about decades, but the graph just shows one decade + a year. –  user unknown Feb 28 '11 at 1:52
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Who says we're getting better? An increased number of diagnosed cases could also mean we're getting worse at diagnosis and that there are many more "false positives". Especially with something that gets as much press and money as autism, there are many incentives for a false positive. –  Russell Steen Feb 28 '11 at 19:14
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Pedantry alert I don't care how much the rate is going up... it's not an "epidemic" unless there is an infectious vector! Grrr.... –  dmckee Oct 2 '11 at 15:11
    
@dmckee The meaning of 'epidemic' has long since transcended just infectious diseases. –  Fomite Oct 4 '11 at 21:34
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@dmckee: If only it were infectious, we could develop a vaccine for it. And wouldn't that be a dilemma for the anti-vaccination crowd! –  Bruce Alderman Dec 13 '11 at 16:00

2 Answers 2

up vote 38 down vote accepted

First of all if you look at the original data, things are way more suspicious. Why are they suspicious? Because most mental illnesses are actually also increasing dramatically during that period.

One can speak about epidemiology of a single disease, but when there are many diseases with the same trend, producing that graph is clearly cherry-picking the data to show that there must be something wrong with autism in particular!

So this leads us clearly to systematic causes, like improved diagnostics, changed diagnostic criteria and age at which the diagnosis can be produced, etcetera.

Obviously this doesn't disprove that autism has increased. It might have increased as well, but it's hard to say in a situation in which a proper study hasn't been conducted.

Let me quote Wikipedia:

  • More children may have autism; that is, the true frequency of autism may have increased.
  • There may be more complete pickup of autism (case finding), as a result of increased awareness and funding. For example, attempts to sue vaccine companies may have increased case-reporting.
  • The diagnosis may be applied more broadly than before, as a result of the changing definition of the disorder, particularly changes in DSM-III-R and DSM-IV.
  • Successively earlier diagnosis in each succeeding cohort of children, including recognition in nursery (preschool), may have affected apparent prevalence but not incidence.
  • A review of the "rising autism" figures compared to other disabilities in schools shows a corresponding drop in findings of mental retardation.

The article also contains a pretty damning conclusion:

The reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness.


The following studies are referenced (emphasis mine):

More children are being diagnosed with ASDs today than in the past. Some of the prevalence increase is undoubtedly attributable to changing diagnostic tendency; however, there are insufficient data to determine whether this can explain the entire increasing trend.

source

The prevalence of autism in metropolitan Atlanta in 1996 for children aged 3 to 10 was 3.4 per 1000. This overall rate is 10 times higher than rates from 3 other US studies that used DSM-III or ICD-9 criteria to identify children with autism and pervasive developmental disorders in the 1980s and early 1990s. Our rate is closer to that found in a recent prevalence study in Brick Township, New Jersey, that used DSM-IV criteria (4.0 per 1000 for autistic disorder and 6.7 per 1000 for the entire autism spectrum). Our findings also are similar to rates from several recent European studies that used ICD-10 or DSM-IV criteria (2-6 per 1000 for autism).

source

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+1, pretty damn to the point. DSM-IV was published in 94, though. Surely, the new definitions would've caused a massive increase, but is it reasonable that it should explain any of the changes we're seeing in the time span the chart covers? –  David Hedlund Feb 26 '11 at 11:25
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The article credits three studies for "pretty damning conclusion." Rather than quote Wikipedia on this, you should take a look at those studies. –  Borror0 Feb 26 '11 at 13:58
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Wikipedia isn't really a great source to be used to reach a conclusion. If Wikipedia cites a source for its statement, we should use that source. If it doesn't it's just some random guy's opinion. –  DJClayworth Mar 4 '11 at 21:59
    
Isn't a good point also that population is increasing? –  Chris Dennett May 12 '11 at 1:14
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@Borror0 added studies –  Sklivvz Oct 2 '11 at 8:39

A South Korean study, that was recently covered by Steven Novella at the Neurologica blog found that - if you actually look carefully - the incidence of autism is around 2.6% (i.e. 26 on your chart), which is much higher than the normal diagnosis rate, and around double what was than previously thought.

This has a number of consequences, including:

  • providing support for the idea that we will continue find many more cases as we look harder (even without the underlying rate of incidence changing), and
  • also supporting the idea that there is a spectrum of symptoms and severity for autism. This makes the prevalence very dependent on our definitions of when autism is considered a disorder.

These ideas are discussed further by Steven Novella, in the above article.

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1  
When I was a kid in the 50s, there was no "Autism". There were only varying degrees and types of mental retardation. We even carefully classified such children as "idiots" or "cretins" or "morons" depending on their perceived IQ or equivalent development by age. I don't think I encountered the word "autism" till the 70s. We had a friend in "special education" at the time, and there was a brief period where autism became a sort of fad diagnosis as it sounded better than simply being "retarded". –  M. Werner May 12 '11 at 23:34
    
Just so we are clear: Mental Retardation (en.wikipedia.org/wiki/Mental_retardation) is a different diagnosis to Autism (en.wikipedia.org/wiki/Autism) or being somewhere on the Autism Spectrum (en.wikipedia.org/wiki/Autism_spectrum). That autism was rarely diagnosed in the 1950s (after being coined in 1910) but is commonly diagnosed now is the question being looked at. If symptoms that were formerly classed as mental retardation are now classified as autism, that would explain the data, but we need more than an anecdote to support it. –  Oddthinking May 13 '11 at 9:33
    
That was precisely what the guest on NPR's Talk Of The Nation said in a recent program; that they were now more carefully able to diagnose the Autism spectrum of syndromes, whereas previously they just tended to get lumped in with the general "retardation" diagnosis. –  M. Werner May 13 '11 at 16:01
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not just "more carefully", but more profitably. With drugs, therapy sessions, etc. all generating more income for autism now, there's more incentive for practitioners to diagnose it than simply "mental retardation" for which there is afaik no treatment. Same reason the "diagnoses" of ADHD among teens have exploded at around the same time. –  jwenting Oct 3 '11 at 9:16
    
Both A and B, in my opinion. We're seeing more autism because we're LOOKING for it more. It's also a topic which is well funded - and you have to have funding to do research. I think it's also terribly short-sighted of people to focus on one 'cause' of a problem and not try to think 'outside the shot'. If I had the funding, I'd like to examine the rates/histories/occurrences of pharmaceuticals and POP's in our waters/soils. –  Darwy Apr 1 '12 at 8:35

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