Why autism at 2.5% isn’t surprising.

29 Nov

Let’s get one thing out right away–autism prevalence studies undercount. Not all autistics are diagnosed. That’s just a fact. Consider the recent CDC study. They look at school and medical records. In many cases, they find children are autistic based on their records–but the schools and doctors hadn’t diagnosed those children.

Combining data from all 11 sites, 81% of boys had a previous ASD classification on record, compared with 75% of girls (OR = 1.4; p<0.01).

Yeah, more than 20% of the kids counted in their prevalence had no diagnosis. They and their families didn’t know.

And, if there isn’t enough in the records to show a kid is autistic? That kid gets uncounted altogether.

So, when people look at the CDC prevalence estimates from over the years and cry “epidemic”, well, there’s a reason why those people usually have some causation theory that they believe in. The irony is that they are usually wrong that their theory needs an epidemic to support it. But, heavily biased people are not usually the best sources of reliable analyses.

What would be a better method of counting how many autistics are in a population? Sounds obvious–test all the kids in a given population. Equally obvious–this is a much more expensive and difficult task. One such study was published in 2011. Yes, 7 years ago. In Prevalence of autism spectrum disorders in a total population sample, the autism prevalence in Korea was found to be 2.63%. A study performed in South Carolina and reported at IMFAR last year found a prevalence of 3.62%.

This all said, we had another autism prevalence come out this week–The Prevalence of Parent-Reported Autism Spectrum Disorder Among US Children. This study found a prevalence of 2.5%.

Now here’s a nice thing about this recent study–OK, two nice things. First, they don’t just look at kids of one age. Second, you can obtain the data. Which I did. Let’s look at the autism prevalence broken down by birth year.

Do you see autism prevalence increasing with birth year? I don’t. I see some scatter, but in general the autism prevalence is about 2.5% from birth year 2000 to 2010. For what it’s worth–the scatter is due to the small numbers of kids in each year making the value uncertain. It’s statistical noise.

If you are wondering about how the autism prevalence drops off above birth year 2010, keep this in mind: kids aren’t diagnosed at birth. In the CDC study, 1/2 of the kids didn’t get diagnosed until after 52 months (4 years 4 months) of age. For this type of reason, the more recent study didn’t count kids under 3 years of age.

People are very fond of graphing autism prevalence data from various years and claiming these are accurate, full counts of autism prevalence (they aren’t) and, from that, claiming an epidemic. Here are the CDC data:

The numbers go up. Steadily up. I’d have to be a total denialist to not see that as evidence of an epidemic, right?

Consider this–the CDC autism prevalence for birth year 2000 is 1.1%. The study just out gives an autism prevalence more than double that (2.9%). For the same birth year. Both are good studies, for what they are. Both are limited. But, for one thing, the CDC study was performed in 2008. 10 years ago. Since then a lot has changed. For one thing, the kids got older and had more chances to get diagnosed. They didn’t just suddenly become autistic in the past 10 years.

So, yeah, we have an autism prevalence estimate of 2.5%. I’m not surprised and I’m not taking this as evidence of an epidemic.

The unfortunate thing in this discussion is that with all the work in this study, all the potential for advocacy, the only number that usually gets discussed is the overall prevalence. Watch the video abstract (which I can’t get to embed). One of the authors goes into a lot of detail about the other findings. Findings I hope to discuss soon.


By Matt Carey


6 Responses to “Why autism at 2.5% isn’t surprising.”

  1. doritmi November 29, 2018 at 05:11 #

    Thank you for going through this – it’s incredibly helpful.

  2. Science Mom December 5, 2018 at 15:02 #

    One aspect of autism prevalence fluctuation is the identification of those with lower needs that account for the majority. You know, those who wouldn’t have been identified in years past.

  3. Ed December 15, 2018 at 00:42 #

    You just aren’t making comparisons far back enough.

    • Sullivan (Matt Carey) December 17, 2018 at 17:45 #


      Tell me, which study from far enough back gives us a comparison to today’s numbers? Which study, using the same methods and understanding of autism, from the past shows evidence of an actual increase?

      I’ll answer that for you–there isn’t one.

      However, this study shows, that from 2000 on, the autism rate is basically flat.

      It also shows, that for kids born as early as 2000, the autism rate as higher than previous studies would indicate.

      All these are, of course, within the limitations of the study. BUT–the idea of an epidemic is based on such a limited logic that it is miles away from being as clear as this example.

      Sorry–I know the epidemic is important to those who push the idea that vaccines cause autism. I know it’s the weapon of choice for instilling fear in vaccines. I know it is the way a community of science denialists use and abuse the autism community.

      It’s just not an idea based in real fact.

    • Sullivan (Matt Carey) December 17, 2018 at 17:46 #

      You just aren’t making comparisons far back enough.

      great example of the depth of logic behind the epidemic. “I don’t like your analysis. I won’t take the time to actually analyze it and discuss it. Just dismiss it”

    • Chris December 17, 2018 at 19:27 #

      “You just aren’t making comparisons far back enough.”

      Under which DSM? Also, what level of autism as defined by DSM V. Should we limit it to Level 3, or is it okay to include Level 2? How about type of autism with known genetic sequences? Which ones on this list (which is about to get bigger) should we include:

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