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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.

Click for full graphic

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.


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 stu­dy’s au­thors. “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 in­fluence seems strong­er am­ong friends than among family mem­bers, the re­search­ers added. The study ap­pears in the July 26 is­sue of the New Eng­land Jour­nal of Med­i­cine.

Fowler and Chris­ta­kis scoured 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 reveal not only family mem­bers of the par­ti­ci­pants, but al­so friends, whose names they wrote down so that re­search­ers could find them if they moved.

Fowl­er and Chris­takis used this data for a new purpose: draw­ing up a gi­ant map of the par­ti­ci­pants’ so­cial net­works. The map 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 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.”


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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.”