With Friends Like These

The study’s effects don’t just come down to the idea that thin people seek out other thin people as friends, while heavy people seek out other heavy people. In fact, what’s going on is much more interesting, according to the researchers: heavy and thin people are causing their friends to become more like them. The reason people have such a powerful effect on each other’s weight is hinted at by one of the study’s most intriguing findings, says Fowler: “Friends who are hundreds of miles away from you have as much of an effect as friends who are [geographically] close.”

Obesity, then, doesn’t spread among friends simply because they're hanging out together, “going out to eat at the same places or going to the bar or going to the park and running together,” he says. “It’s spreading through ideas about what appropriate behaviors are, or what an appropriate body image might be.” In other words, if you admire your friend and she happens to get heavier, you’ll be comfortable with the idea of getting heavier yourself. “If I see you gaining weight, and I respect you, and want to emulate you in other ways, that changes my ideas about what is an acceptable body size. I think, 'All my buddies are getting obese, so it's OK for me to be obese too',” says Christakis. “And even if you’re 1,000 miles away, or I only see you once a year, that’s enough to transmit the norm.”

The study suggests a new explanation for the obesity epidemic, says Matthew Gillman, director of the Obesity Prevention Program at Harvard Medical School. “Genes can certainly affect whether one individual is obese rather than the other, but they can’t really explain the obesity epidemic, because they haven’t really changed in the last 30 years,” he says. True, plenty of changes in American society have contributed to the epidemic: most obviously, an increase in fatty, carb-heavy processed foods and a decrease in built-in daily exercise. But social networks have changed, too. Compared to the years before the epidemic started, Americans also now have more ways to keep in contact with their loved ones, such as e-mail, instant-messaging and videoconferencing. The study suggests that the obesity norms could indeed be transmitted via those technologies; a friend 1,000 miles away can still send an e-mail bemoaning his recent weight gain.

There’s still a lot left to figure out about these new dynamics of obesity. One question the research brings up, but fails to completely answer, is where neighbors fit into the picture. They appear to have no influence: if your neighbor becomes obese, your risk of doing likewise doesn’t change. It’s unclear why neighbors aren’t playing a larger role, although Christakis notes that if you don’t particularly admire or even know your neighbors, you're not likely to base your ideas about body size on theirs.

The study also brings up several other questions: Why are same-sex friendships and relationships so much more influential over weight than male-female friendships are? Where does the ripple effect stop? Does the same dynamic apply to other behavior-related health problems, such as drinking, smoking and risky sexual behavior? It may be some time before researchers fully know the answers.

It’s not too early, however, for public-health officials to start thinking about the study’s implications. Over the last 25 years, obesity in the United States has doubled; 66 percent of Americans are overweight and 32 percent of those are on the next level, classified as obese, according to the National Center for Health Statistics. And measures to combat the problem aren’t bringing those numbers down. “We see no evidence that the obesity epidemic has peaked,” says Christakis. And it’s possible the epidemic won’t peak until weight-loss groups and health advocates start taking social ties into account. But in a way, that's good news, says Fowler: “The flip side of this is that thinness is contagious, too." If you really want to lose weight, he adds, maybe you should encourage some of your buddies to trim down as well.

© 2007