Home > Got Questions? > Has the rate of autism truly increased in the last 50 years or so or is it just that the classification of autism has gotten broader and as such the prevalence seems larger?

Has the rate of autism truly increased in the last 50 years or so or is it just that the classification of autism has gotten broader and as such the prevalence seems larger?

“Got Questions?” is a new weekly feature on our blog to address the desire for scientific understanding in our community.  We received over 3000 responses when we asked what science questions were on your mind. We answered a few here and the Autism Speaks Science staff will address the other themes we received in this weekly post.

In the last two decades reported autism prevalence has increased by more than 600% and a number of studies have sought to investigate the cause(s) of this dramatic increase in autism prevalence over time. Recent findings suggest that at least a portion of the increase in prevalence can be attributed to changes in diagnostic practices, earlier age of diagnosis, and increased awareness of autism over time. However, converging evidence also suggests that while these factors account for a portion of the increase, they cannot alone explain the dramatic rise in autism prevalence. Thus genetic and/or other environmental factors likely play a role and are the subject of numerous research projects currently supported by Autism Speaks.

  1. December 22, 2010 at 4:15 am

    If expanded autism disorder definitions, increased awareness and other social factors can only explain a portion of the “dramatic” rise then environmental factors are presumably involved and studies exploring potential environmental causes or triggers of autism should receive more funding than they currently do. Even the IACC has acknowledged that funding dollars are disproportionately directed towards genetic focused autism research at the expense of environmentally focused autism research. Government funding agencies and organizations like Autism Speaks should restore the balance and direct more funding dollars towards environmentally based autism research.

  2. December 25, 2010 at 7:29 pm

    There are a number of other factors tat have not been factored in, mainly because the last study done would not tackle them due to the lack the ability to statistically quantify them.
    Diagnostic substitution over time almost certainly has played a statistically significant role in the rise in the past 20 years. Data shows a steady decrease year over year in the diagnosis of mental retardation, Even the name change from mental retardartion to intellectual disability suggests a shift in the charactarizations of those with disabilities.
    Another factor that is often overlooked which I also feel is a significant factor over the last 20 years is the availability of services in school systems and government in general due to legislation like the Americans with Disabilities Act. The 1990’s marked a watershed decade where disability rights were more prevelent than ever before and the resultant changes have meant unpresidented avaialability of services, especially for those with developmental disabiliies.

    All these factors may also not fully explain an increase in autism or any other developmental or neurological disabilities for that matter. But taken as a whole and in perspective, it may help us to clarify and focus our efforts from exclusively looking at factors in the past 20-30 years to looking at a broader area of evironmental factors as well a potential inter generational issues. Genetics, in my view, is not limited to exclusively to finding genetic differences, but also at the much more difficult and potentially more eventful areas of epigenetics, where the interplay of genetics, environment and stressors may help to explain much in the area of developmental/intellectiual disability…

  3. Joe Brown
    December 31, 2010 at 9:20 pm

    Just be certain you haven’t fallen for the “times” trick in your prevalence claims.

    There is a tendency, based on the desire to be emphatic, to overstate differences of all sorts. You are likely as guilty as the ones I’ll cite.

    Watch Wendell Potter do it at http://www.msnbc.msn.com/id/3036677/vp/34386829#34386829 (about 4 minutes 5 seconds into the video) There, he states that the price of a stock had “increased 300%” between two dates. He does not mention the actual prices.

    The price of Cigna on the specified beginning date (Obama’s meeting with insurance executives, March 5, 2009) was 12. What would it have been when he spoke if the price had truly increased 300%? Please don’t be lazy, do the math.

    The price as Potter spoke was 36. He was clearly in error, and the reason was that he did something you probably do as well: he used idiomatic rather than literal English to conceptualize the relationship. Specifically, he thought of the number 36 as “three times greater” than 12, when, literally, it is actually three times *as great as* 12. “Greater than” and “as great as” are in no other context equivalent, only in this type of “times” expression.

    Likewise, Macworld magazine published (July 1989) a review of a new computer in which the 24 MHz clock speed was referred to as “150% faster” than its 16 MHz predecessor. Is there any doubt that the immediate cause of this error was thinking “one and a half times faster” be equal to “one and a half times as fast”?

    This misconception has repercussions for *all* reported percentage differences, not just obvious ones such as the one at http://theautochannel.com/news/2006/07/10/014219.html (where the headline writer mindlessly converted the story author’s “six times less likely” statement into “600% less likely.”)

    Since it is often impossible to know who is being literal and who idiomatic, I recommend avoiding “times” comparisons altogether, or at least the ones that combine “times” with any “-er” adjective. Even if you decline to deprecate them in your own usage, be aware that when most people say “n times greater/faster/heavier/…” *you cannot convert n to a percentage by simply moving its decimal point.*

    Once you have been made aware of this discrepancy, failing to be alert to its consequences is tantamount to negligence and can even represent intentional deceit.

  4. picarules
    December 31, 2010 at 10:05 pm

    Where in fact do you come up with “a 600 percent increase in just the past 20 years* as stated at


    when the 1990 figure was 1:2000 and the 2010 figure was 1:110?


    “Studies suggest boys are more likely than girls to develop autism and receive the diagnosis three to four times more frequently. ”


    Did you mean “three to four times” AS frequently? If your zeal overrides your accuracy, you are no more than a propagandist.

    A good cause doesn’t exculpate bad math.

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