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So say researchers from Harvard Medical School and the University of California, who published research in a July 2007 edition of the New England Journal of Medicine. After analyzing data from 12,067 individuals, they found that when one individual becomes obese, the chance that his or her friend will become obese increases by 57 percent—even if their friend lives far away, and especially if their friend is of the same sex—71 percent in that case. Other members of people's "social network" were also affected: their siblings’ risk increased by 40 percent and their spouses' by 37 percent. In contrast, a neighbor, if not a part of their social network, experienced no increase in risk.
Obviously, weight gain isn’t contagious in the same way a cold is contagious. Rather than being spread through the transmission of bodily fluids, like a virus, obesity is “socially" contagious—it can be spread through the transmission of behaviors and social norms. People within a social network often engage in health-impacting behaviors together, such as Friday night parties with too much wine and cheese or working lunches of fatty restaurant fare. These behaviors may result in weight gain, especially if they become habits. Even more importantly, each person within the social network serves as a standard by which others in the network may compare themselves. The 10-pound weight gain your best friend is wearing makes you feel a little less guilty about the extra five pounds you’re sporting, and if fast-food is an acceptable meal for your sister-in-law, you may develop a more lackadaisical attitude about dinner in your own house. Continued ›