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Convener in chief:


David Lazer
(Methodology, Networked Governance)

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Stanley Wasserman
(Current Trends, Methodology, Social Networks)
Guy Stuart
(Economic Sociology, Finance)
Allan Friedman
(Simulations)
Nathan Eagle
(Technology, Social Computing, Powerlaws, Current Trends)
Ben Waber
(Technology, Social Computing) Ines Mergel
(Knowledge Sharing, Social Computing, Social Software, Current Trends)
Maria Binz-Scharf
(Qualitative Methodology, Knowledge Sharing, eGovernment)
Alexander Schellong
(Admin, eGovernment, Citizen Relationship Management)

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    31 July 2007

    The contagion of obesity

    Remarkable article from NEJM last week:

    Nicholas A. Christakis, M.D., Ph.D., M.P.H., and James H. Fowler, Ph.D.
    The Spread of Obesity in a Large Social Network over 32 Years, New England Journal of Medicine, Volume 357:370-379 July 26, 2007

    quoting from the summary of results:

    Results Discernible clusters of obese persons (body-mass index [the weight in kilograms divided by the square of the height in meters], 30) were present in the network at all time points, and the clusters extended to three degrees of separation. These clusters did not appear to be solely attributable to the selective formation of social ties among obese persons. A person's chances of becoming obese increased by 57% (95% confidence interval [CI], 6 to 123) if he or she had a friend who became obese in a given interval. Among pairs of adult siblings, if one sibling became obese, the chance that the other would become obese increased by 40% (95% CI, 21 to 60). If one spouse became obese, the likelihood that the other spouse would become obese increased by 37% (95% CI, 7 to 73). These effects were not seen among neighbors in the immediate geographic location. Persons of the same sex had relatively greater influence on each other than those of the opposite sex. The spread of smoking cessation did not account for the spread of obesity in the network.


    This paper received enormous attention, with front page articles in a variety of newspapers, and the ultimate sign of popular attention, jokes by late night talk show hosts.


    What Jay Leno did not bring up, however, was the deft way this paper handles the issues around causality (see my earlier posting on issues around causation and social networks).

    In particular, the paper uses several approaches to rule out explanations for their findings other than social contagion. First, the analysis leverages the longitudinal nature of the data to distinguish contagion from (obese) birds flocking together. Second, auxiliary hypotheses (in particular, (a) showing that asymmetric friendships—I think that you are a friend and not vice versa—have asymmetric effects, and (b) that friendship effects matter even for geographically distant dyads) undermine alternative explanations based on omitted variables. All in all, a very nice piece of analysis.

    I am sure Leno was waiting to get to these finer points until this week…

    Posted by David Lazer at July 31, 2007 10:16 PM