Their study was reported to have shown that you can “catch” obesity from having fat friends and that obesity is so contagious, it can be spread long-distance by email and instant messaging. Even healthcare professionals, who didn’t understand the etiology of true obesity or how statistics can be misused, failed to detect the implausibility of “second-hand obesity.” In fact, some doctors became so enamored with the new “science of networking” they believed it should be a new medical specialty: network medicine.
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Jason M. Fletcher, Ph.D., assistant professor at the Yale School of Public Health in New Haven, Connecticut, along with Boston economist, Ethan Cohen-Cole, Ph.D., designed an ingenious study. They selected conditions that no one would seriously believe were spread by social networking and online friendships: height, headaches and acne. They then applied the same standard statistical methods used in Christakis and Fowler’s social networking research to “find” that acne, height and headaches have the same “social network effect.”
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As they explained, patterns of association among people can lead to correlations in health conditions between friends that are not caused by direct social network effects at all.
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There is a need for caution when attributing causality to correlations in health outcomes between friends using non-experimental data. Confounding is only one of many empirical challenges to estimating social network effects.