A New Autism Study May Not Say What You Think It SaysQuote:
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The key to understanding autism epidemiology is to examine the data by birth year. Your year of birth is one of the few things about you that can never change. Your age, on the other hand, always increases by one every year.
When you compare people who were a certain age in different study years, you are comparing different people born in different years. Many papers show data organized by study year while few explicitly show birth year. You can easily convert one to the other: study year minus age equals birth year. For example, children who were eight years old in 2020 were born in 2012. This JAMApaper’s reported data are in terms of age, not birth year. But we can easily convert it to birth year.
To understand what the data has to say, we have to know what the numbers mean. Is it about rates that people were diagnosed? Or is it about prevalence—that is, how many autistic people there are at a given time? Those are different things. The paper uses both “diagnosis rates” and “prevalence.”
While the correct interpretation is not obvious, Grosvenor confirmed my guess. In fact, the data shows the proportions of members of the participating health networks who had an autism or ASD diagnostic code in their electronic health records in each study year. A diagnostic code does not mean that someone was diagnosed recently. It doesn’t necessarily mean they were ever diagnosed. Doctors treating patients decide what diagnostic codes to put in patients’ records. An autism or ASD diagnostic code might indicate that the patient was diagnosed at some point in the past, but that’s not entirely correct. We can treat it like prevalence.
The 26-34 group in the 2011 data was born in 1976 through 1984, while the 26-34 group in the 2022 data was born in 1987 through 1995. (In this paper’s data, the ages refer to the year before each study year.) Greater diagnosed prevalence in the later-born group vs. the earlier one could be due simply to greater prevalence among the later group.
But does the later-born group actually display a greater prevalence? We can answer that by looking more closely at the data.
The paper’s data has birth year prevalence information, but it takes a bit of effort to dig up. This JAMA paper’s eTable 7 gives the proportions of autistic patients (those with diagnostic codes) for each study year grouped into ranges of ages. I used an Excel spreadsheet to study the diagnostic code prevalence by individual birth years. The result is not as precise as we’d like, because of the age grouping, but it is still quite informative.
The following graph shows the birth year prevalence for birth years 1976 through 1995, encompassing both the earlier and later groups mentioned above. As I said, it’s not exactly prevalence but it’s the closest we can get with this data.
As you can see, there was a dramatic exponential increase in birth year prevalence: 12.3 percent per year. Note that the graph looks lumpy because the data is grouped into ranges of years.
The later-born group, born in 1987–1995, has an average birth year prevalence that is 3.82 times that of the earlier group born in 1976–1984. So, most, or possibly all, of the increase found between those two groups of adults likely came from the increasing birth year prevalence. That shows that the “recently diagnosed adults” interpretation is not correct.
Other data sources consistently show that as people from each birth year age, more are diagnosed, and this trend is similar for all birth years. So while the health systems may have diagnosed some members as adults—we can’t tell from this data—that is not something new. It has been going on for decades.
We can also examine the trend in birth year prevalence for all birth years.
This shows an increase of 7 percent per year of birth, which strikes me as fairly significant. It's almost the same increase as in the set of CDC autism reports on children born 8 years before each study year. The CDC reports show an increase of 7.4 percent per year from birth years 1992 through 2012.
Two scientific papers in the last few years purport to show there are vast numbers of undiagnosed adults. Both rely on the assumption that birth year prevalence has been constant all along.
I argue that both the data in this JAMA paper and the CDC reports contradict that assumption, meaning that there are relatively few undiagnosed adults now but there will be many more autistic adults in the future as the younger autistic people age. That says a lot about where public policy and research should focus.
A problem with this whole idea of "birth year prevalence": It assumes that all actually autistic people get diagnosed eventually (or at least sufficiently suspected, at some point, for their doctor to use the relevant ICD code).
But a lot of older, probably-autistic adults never do seek a diagnosis. For many of us, seeking a diagnosis is just not worth the hassle and money, due to the lack of services for autistic adults. And it seems to me that the older someone is before it finally occurs to them that they might be autistic, the less likely it is to seem worthwhile to seek a diagnosis.
Yesterday, 2:54 am
To sum up, I wish I was diagnosed, but with something less stigmatising than autism/Asperger's. It was a blow to my confidence even as an adult, but let's be honest, adults are more emotiononally resilient and have a more stable self-esteem.
