Find Yourself Packing It On? Blame Friends Obesity can spread from person to person, much like a virus, researchers are reporting today. When one person gains weight, close friends tend to gain weight, too. Their study, published in The New England Journal of Medicine, involved a detailed analysis of a large social network of 12,067 people who had been closely followed for 32 years, from 1971 to 2003. The investigators knew who was friends with whom as well as who was a spouse or sibling or neighbor, and they knew how much each person weighed at various times over three decades. That let them reconstruct what happened over the years as individuals became obese. Did their friends also become obese? Did family members? Or neighbors? The answer, the researchers report, was that people were most likely to become obese when a friend became obese. That increased a person's chances of becoming obese by 57 percent. There was no effect when a neighbor gained or lost weight, however, and family members had less influence than friends. It did not even matter if the friend was hundreds of miles away, the influence remained. And the greatest influence of all was between close mutual friends. There, if one became obese, the other had a 171 percent increased chance of becoming obese, too. The same effect seemed to occur for weight loss, the investigators say. But since most people were gaining, not losing, over the 32 years, the result was, on average, that people grew fatter. Dr. Nicholas A. Christakis, a physician and professor of medical sociology at Harvard Medical School and a principal investigator in the new study, said one explanation was that friends affected each others' perception of fatness. When a close friend becomes obese, obesity may not look so bad. 'You change your idea of what is an acceptable body type by looking at the people around you,' Dr. Christakis said. The investigators say their findings can help explain why Americans have become fatter in recent years--each person who became obese was likely to drag along some friends. Their analysis was unique, Dr. Christakis said, because it moved beyond a simple analysis of one person and his or her social contacts and instead examined an entire social network at once, looking at how a person's friend's friends, or a spouse's sibling's friends, could have an influence on a person's weight. The effects, he said, 'highlight the importance of a spreading process, a kind of social contagion, that spreads through the network.' Of course, the investigators say, social networks are not the only factors that affect body weight. There is a strong genetic component at work, too. Science has shown that individuals have genetically determined ranges of weights, spanning perhaps 30 or so pounds for each person. But that leaves a large role for the environment in determining whether a person's weight is near the top of his or her range or near the bottom. As people have gotten fatter, it appears that many are edging toward the top of their ranges. The question has been why. If the new research is correct, it may say that something in the environment seeded what some call an obesity epidemic, making a few people gain weight. Then social networks let the obesity spread rapidly. Who had the greatest influence on people who became obese?