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July 16, 2013 12:45 PM How Autism Really “Spreads”

By Henry Farrell

The news that Jenny McCarthy will become a co-host of TV discussion show The View is generating a lot of controversy – people worry that McCarthy will be able to spread her controversial (for which read: crazy) views on autism, vaccination and chelation therapy to a much wider audience. Some of the theories as to what causes autism point to geographic clustering as evidence for some common physical cause. However, Columbia sociologist Peter Bearman and his collaborators build on a rich body of data from California to show that clustering of autism cases is plausibly caused by increased diagnosis thanks to the diffusion of knowledge across local communities. Ka-Yuet Liu, Marissa King, and Bearman find that at least 16% of the recent increase in diagnoses is down to the spread of knowledge:

One does not “catch” autism from someone else, yet a social diffusion process contributes significantly to the increased prevalence of autism. We observe a strong positive effect of proximity to other children with autism on the subsequent chance of diagnosis, robust to a range of individual- and community-level controls in both urban and less urban areas. In addition, close proximity to a child with autism was inversely associated with the likelihood of subsequent sole MR diagnosis, while it correlated strongly with the chance of autism-MR diagnosis. Proximity also increases the chance of autism rather [than] MR diagnosis given the same level of severity in autism symptoms. Social influence arises strongly for high-functioning cases of autism. The effect of proximity is also more prominent in younger children, when diagnosis is more difficult and parental resources are more important. Children who were diagnosed with autism have a similar mode of referral as that of their nearest neighbor with autism before their diagnosis. All of these findings are consistent with a mechanism of social diffusion of awareness of the symptoms and the benefits of treatment and are inconsistent with competing explanations. Social influence also accounts for the observed spatial clustering of autism. Such clustering could be caused by local environmental toxicants, the diffusion of a virus, or residential selection, but it is hard to see how a toxicant could cause a reduction in MR diagnoses, operate in all types of communities (urban or rural), and affect most strongly the high-functioning end of the severity distribution.

It’s likely that the increase in diagnoses reflects the difficulties in getting assistance within California (and the US education system more generally) for children who urgently need help, and who would be denied it without a full autism diagnosis:

Because the DDS provides services only to children with autism and not to children diagnosed with disorders on the autism spectrum, the importance of an autism diagnosis for parents striving to secure resources for their children is amplified. The steep and sudden cliff creates incentives that may not be present in other contexts, but pressure to do anything to help children is likely widespread and not limited to the California context. As Judith Rapoport of the National Institute of Mental Health told Grinker, “I’ll call a kid a zebra if it will get him the educational services I think he needs.”

Anecdotal evidence suggests that parents of kids with autism or related issues face grueling battles if they want local schools to acknowledge their needs and provide for them. It wouldn’t surprise me at all if much of the knowledge that is shared locally is knowledge of how to force action from a bureaucratic system where administrators have limited budgets and strong incentives to deny care if they think they can get away with it.

Update: This shorter piece by Bearman on why many parents believe that vaccines cause autism is also interesting.

[Cross-posted at The Monkey Cage]

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Henry Farrell is an associate professor of political science and international affairs at George Washington University.