"The basic idea," she writes, "is that children who would previously have received another diagnosis or no diagnosis are now being identified with autism spectrum disorder (ASD)."
That's in part because we're now coming to recategorize what might once have been called retardation, semantic-pragmatic disorder, or something else. Our improving classifications for the brain disease have, in other words, gotten articulate enough that we can place kids with decent accuracy on a wide-ranging autistic spectrum. Since the definitions are relatively flexible, the fact that there are degrees of the disorder might be one explanation for why so many children seem to have "it."
Bishop also hints that our threshold for diagnosis is falling -- that is, doctors may be making calls based on a minimum of evidence. There are two big tests for diagnosing autism: ADOS-G and ADI-R. These two exams sometimes produce competing conclusions. In a 2008 study, Bishop found that in a given pool of test subjects, only 21 percent had ADOS and ADI results that agreed with one another. Sixty-six percent had differing results between the two surveys. If you diagnose autism based on the positive results of one test and not both, then you'd be diagnosing the disorder roughly three times as often.
There's a material incentive that may be driving up autism cases, too, Bishop says. Since autism is a well-supported disease with lots of resources, some kids may be getting diagnosed with it even if they really have something else.
In a study conducted in 2010, Bishop examined a group of children with an unrelated condition known as trisomy. Although autism isn't mentioned anywhere in the literature on trisomy, Bishop noticed that a far greater share of trisomy patients were being diagnosed with ASD compared to the general population, leading the researchers to suspect diagnostic substitution. It's not an airtight case, but two other studies have managed to replicate Bishop's results since her paper was published.
It is becoming clear that changing diagnostic criteria, increased awareness of ASD, and strategic use of diagnosis to gain access to services, have had a massive effect on the numbers of children with ASD. When I started studies in this area, I thought diagnostic substitution had happened but I did not think it would be sufficient to explain the increase in numbers of ASD diagnoses. But now, on the basis of studies reviewed here, I think it could be the full story.
And it isn't just with autism that our prevalence perceptions may be skewed. Aaron Carroll of The Incidental Economist passes along this chart from the British Medical Journal showing that mortality rates from diseases like melanoma, prostate cancer, and breast cancer have stayed the same over time even as the number of diagnoses skyrocketed. If the data are right, what we often think of as "epidemics" may not be so serious, after all, but rather just a sign of our attitude toward these afflictions, for better or for worse.