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Can hanging out with fat people make you fat? Could you catch obesity by friending the wrong people on your Facebook page? James Fowler and Nicholas Christakis think so - obesity can spread like a virus person-to-person through social networks.

Obesity Can "Go Viral" In Both Senses of the Word

James Fowler, a social scientist at the University of California at San Diego, and Nicholas Christakis, a scholar at Harvard, believe that overeating, like smoking, drinking,  happiness, loneliness, and drug addiction, can spread like a virus person-to-person through social networks. They also believe that focusing on just a few people in a social network can help everyone in their networks make changes for the better.

Dr. Christakis and Dr. Fowler used data collected from 12,067 people who had participated in the famous Framingham Heart Study, an ongoing research study into the interconnections of lifestyle and disease. The data included 32 years of weight measurements and medical diagnoses. Because participants in the study listed a contact for emergency communications, Christakis and Fowler were able to identified which participants in the study knew each other, and whether they had similar experiences of obesity, loneliness, happiness, or substance abuse.

The researchers found that obese people tended to list other obese people as their friends to call in case of emergency. Why should overweight people have overweight friends?

  • One possibility was that overweight people simply preferred to associate with other overweight people.
  • Another possibility was that friends lived in similar circumstances or environments and that environmental factors caused overweight.
  • A third possibility was that a person's idea of what an acceptable weight is, or what an acceptable serving size is, changes when he or she gets to know a heavier person or sees a friend overeating. In other words, obesity, Fowler and Christakis theorized, obesity might be socially contagious.

These explanations have important social implications. If people simply tend to associate with people who look like themselves, or if they gain weight because of the same environmental influences, then there is no danger in a thin person's association with an overweight person. But if people gain weight after they see other people gain weight, then it might be important for thin people to stay away from fat people to keep from getting fat.

Christakis and Fowler found that if person A names person B as a friend, but person B does not name person A as a friend, and person B gets fat, then person A has a 57% chance of getting fat. At first Christakis and Fowler through the effect was one-way, that is, if person A got fat, then person B would not. Then other researchers critiqued the study and revealed that the effect really was two-way, but if the person named as a friend (B) was originally thin, he or she only had a 13% chance of gaining weight if the friend gained weight.

The study is now interpreted as finding that fat role models probably have influence over other people's weight, but other factors may also be involved. However, there is no incontrovertible scientific proof that associating with overweight people causes overweight.

And since the study's data were collected, overall rates of obesity have passed 57%. Perhaps obesity is a pandemic, affecting everyone, but the idea that people can blame obesity on their choices of friends is questionable at best.

  • The Spread of Obesity in a Large Social Network over 32 Years. Nicholas A. Christakis, M.D., Ph.D., M.P.H., and James H. Fowler, Ph.D. N Engl J Med 2007, 357:370-379. July 26, 2007.