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Obesity found to spread socially

July 25, 2007
Courtesy University of California - San Diego
and World Science staff

Are your friends mak­ing you fat? Or keep­ing you slim? 

The an­swer may be yes, to both. Obes­ity spreads among friends and family mem­bers in a sort of so­cial con­ta­gion, a study has found—so your chances of be­com­ing obese may al­most tri­ple if a close friend is that way.

A ti­ny part of a map of the so­cial net­work of 2,200 peo­ple—the larg­est group of con­nect­ed peo­ple in the study—in the year 2000. Click he­re for the full map. Each ball rep­re­sents one per­son. The size of each one is pro­por­tion­al to the per­son's bod­y-mass in­dex, a meas­ure of body fat. Yel­low balls stand for med­i­cally obese peo­ple; green, for the non-o­bese. Glob­ules with red cir­cles around them de­note wom­en; blue cir­cles de­note men. Lines stand for so­cial con­nec­tions: pur­ple for friend­ship or mar­i­tal ties, or­ange for fa­mil­ial ties.

Are your friends making you fat? Or keeping you slim? The answer may be yes to both. Obesity is socially contagious, a study has found, spreading among friends and family members—so your chances of becoming obese may almost triple if a close friend is that way. Part of the reason seems to be that each person influences the “social norm” for his or her circle, researchers theorized. That is, “people come to think that it is okay to be bigger since those around them are bigger,” said Nicholas Christakis of Harvard Medical School in Boston, one of the authors of the study, published in the July 26 issue of the New England Journal of Medicine. “Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much,” added co-author James Fowler of the University of California San Diego. Surprisingly, the effect seems stronger for friends than family members, the researchers added. The researchers analyzed data covering 32 years for over 12,000 adults who underwent repeated medical tests as part of the Framingham Heart Study, a long-term project administered by the U.S. National Heart, Lung and Blood Institute. Archived records from this study list not only family members of the participants, but also friends, whose names they wrote down so that researchers could find them if they moved. Using this information, Fowler and Christakis drew up a giant map of the particpants’ social networks. It also includes information on the participants’ body-mass index, a commonly accepted measure of body fat. Among the first things the researchers noticed was that—consistent with other studies finding an obesity epidemic in the U.S.—the whole network grew heavier over time. Also obvious were distinct clusters of thin and heavy individuals, Fowler and Christakis said. Statistical analysis found that these clusters couldn’t be attributed only to people making friends with others of comparable weight: rather, they gain or lose weight under friends’ influences. There’s “a direct, causal relationship,” said Christakis. “It’s not that obese or non-obese people simply find other similar people to hang out with.” Nor could the effect be chalked up only to similarities in lifestyle and environment, such as people eating the same foods or living in the same area, the researchers added. “Your friend who’s 500 miles away has just as much impact on your obesity as [one] next door,” said Fowler, a political scientist and expert in social networks. If a person that a participant listed as a friend was obese, the researchers found, the participant’s own chances of becoming obese rose 57 percent. If two people listed each other as friends, the effect multiplied in strength: increase in obesity risk shot up 171 percent. Among siblings, they found, if one becomes obese, the likelihood for the other to do so rises 40 percent; among spouses, 37 percent. No effect was found among neighbors, unless they were friends too. Fowler and Christakis said they believe people affect not only only each other’s behaviors but also, more subtly, social norms. They came to this conclusion partly because the study also identified a larger effect among people of the same sex. The study suggests that in addition to looking for genes and physical processes behind obesity, researchers “should spend time looking at the social side,” said Fowler. There are profound policy implications, he added. The social effects extend three degrees of separation—to your friends’ friends’ friends—so “when we help one person lose weight, we’re not just helping one person, we’re helping many,” he said. “That needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier.” But “It’s important to remember,” Fowler said, “that we’ve not only shown that obesity is contagious but that thinness is contagious.”