When higher rates are better?
An early-release from the journal Autism Research (subscription required to get past the abstract), by researchers at U.C. Davis, shows 10 population clusters of autism cases in California – places where the autism rates are much higher than average. These areas are strongly correlated with local education levels (higher) and race/ethnicity (Whiter). The story was also reported in the papers, and on NPR.
The suggestion from the authors – consistent with individual-level analysis in other studies – is that parents with higher education may be more likely to have their kids’ autism diagnosed. So the higher rates are good in these areas, relatively speaking.
The researchers failed to find any environmental explanation for the wide variation in autism rates – but they stress their method wouldn’t work for finding universal environmental factors that are not locally varying: “The findings from this study do not preclude a role for environmental exposures that cluster around nonpoint sources, such as traffic, or that are not clustered spatially because they are widely distributed, such as household products.”
The conclusion implies that the low-autism areas are underserved and therefore underdiagnosed. The association of education with autism rates has been found in other places, they note, but not Denmark, where everyone is screened for autism. Partly for that reason, they don’t believe Whites or people with high education are more likely to have children with autism. Still, that’s possible:
It remains possible that the associated demographic characteristics are surrogates for some other yet-to-be defined/confirmed risk factors, such as subfertility, accumulated exposures, genetic susceptibility, or access to optional medical interventions like assisted reproduction or scheduled casarean sections.
Their findings and interpretation seem plausible to me. But it’s not the whole autism story. First, they don’t suggest that this could account for the increase in autism rates in recent decades altogether (see figure). Second, there are other apparent cases of autism clusters, for example among Somalis in Minnesota, that don’t seem related to the parents high education or other pro-diagnosis factors. So some of the story remains to be told.