Monday, September 29, 2008


Found a link to a study about The Spread of Obesity in a Large Social Network over 32 Years on Neuroanthropology.

I have no idea what to think of it. Apparently having strong social ties to people as they become obese means that you are statistically more likely to become obese.

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.

Whoa, what?
To the extent that obesity is a product of voluntary choices or behaviors, the fact that people are embedded in social networks and are influenced by the evident appearance and behaviors of those around them suggests that weight gain in one person might influence weight gain in others. Having obese social contacts might change a person's tolerance for being obese or might influence his or her adoption of specific behaviors (e.g., smoking, eating, and exercising). In addition to such strictly social mechanisms, it is plausible that physiological imitation might occur; areas of the brain that correspond to actions such as eating food may be stimulated if these actions are observed in others.

I'm glad that statisticians are actually looking at the fact that social networks influence the choices of individuals (since I think asserting otherwise is pretty stupid), but this was something I hadn't expected.
We considered three explanations for the clustering of obese people. First, egos might choose to associate with like alters ("homophily"). Second, egos and alters might share attributes or jointly experience unobserved contemporaneous events that cause their weight to vary at the same time (confounding). Third, alters might exert social influence or peer effects on egos ("induction"). Distinguishing the interpersonal induction of obesity from homophily requires dynamic, longitudinal network information about the emergence of ties between people ("nodes") in a network and also about the attributes of nodes (i.e., repeated measures of the body-mass index).

The use of a time-lagged dependent variable (lagged to the previous examination) eliminated serial correlation in the errors (evaluated with a Lagrange multiplier test) and also substantially controlled for the ego's genetic endowment and any intrinsic, stable predisposition to obesity. The use of a lagged independent variable for an alter's weight status controlled for homophily. The key variable of interest was an alter's obesity at time t+1. A significant coefficient for this variable would suggest either that an alter's weight affected an ego's weight or that an ego and an alter experienced contemporaneous events affecting both their weights.

Do we have any statisticians in the house? I find this very interesting, but I'm not qualified to judge their methodology well.

If anybody is interested, there's a 28-minute video on social contagion of obesity. I'm just now watching it, so I can't tell you just yet whether it's incredibly offensive or what. It's got the scientists who did this study in it, so it's worth watching.

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