From Scientific American:
The people we associate with can have a powerful effect on our behavior—for better or for worse. This holds true for human health and body mass, too. The heavier our close friends and family, the heavier we are likely to be. This correlation, described in 2007 by a team that analyzed data from the longitudinal Framingham Heart Study, is well established. But just how this transpires—whether via shared norms, common behavior or just similar environments—has been the subject of much debate. The authors of the 2007 study proposed that social norms shared among friends and relatives might be a strong determinant of body mass index (BMI). And a new study, published online May 5 in the American Journal of Public Health, drills down to see just how these social forces might be at work. The study of more than 100 women—and hundreds of their friends and family members—however, suggests that social attitudes might not be key in determining obesity clusters after all.
“Going in as anthropologists we assumed that the norms would have a strong influence” on BMI, says Alexandra Brewis, executive director of the School of Human Evolution and Social Change at Arizona State University in Tempe. She and her colleagues found themselves surprised how small an effect the norms had on a person's BMI. Just one type of social dynamic seemed to play a statistically significant role—and that was only about 20 percent. But a small effect is not no effect, points out James Fowler, a professor at the University of California, San Diego School of Medicine and Division of Social Sciences, and co-author of the 2007 The New England Journal of Medicine paper that described the influence of social ties on obesity rates. The finding that even 20 percent of weight status can be attributed to social norms suggests that “at least some of what is spreading are ideas about body size.” And that, he says, is “incredibly exciting,” as it hints at some methods to start stemming the social spread of obesity.