With Friends Like These

The list of reasons a person might pack on too many pounds is already plenty long: genes, hormone disorders, a couch-potato lifestyle, love of cheeseburgers. Thanks to a new study in the New England Journal of Medicine, you can add another culprit to the list: friends.

Obesity spreads through social networks, according to the study, so if your friends put on weight, you’re more likely to put on the pounds, too. Your family members or spouse can also influence you; as they get heavier, you’re more likely to gain along with them. But, your friends—even if they don’t live anywhere near you—have the most sway. A close friend’s weight gain can even be downright dangerous.

“If your close friend becomes obese in a given time interval, there’s triple the risk that you will follow suit,” says Nicholas Christakis, a coauthor of the study, which was published Wednesday and a professor of medical sociology at Harvard Medical School. “Before you know it you have an obesity epidemic, where we're all kind of gaining weight together, like a fashion spreading through society, rising in lockstep.”

The research—which Richard Suzman, director of the National Institute on Aging’s Behavioral and Social Research Program, calls “one of the most exciting studies in medical sociology that I have seen in decades”—focuses on 12,067 participants in the Framingham Heart Study, a multidecade government health-research project. Each participant was asked to name a list of friends and family members when he or she joined the program in 1971. Then the participants and their friends and family were tracked over the years.

When one person in the study became obese, his siblings’ risk of also becoming obese jumped by 40 percent, while his spouse’s risk jumped by 37 percent. More strikingly, if that person had been named as a “friend” by another participant, the second participant’s risk of becoming obese shot up by 57 percent. If the friends were of the same gender, the risk was even higher, at 71 percent. (The study found a man’s weight gain would have no significant effect on his female friend’s weight, and vice versa, but the study did not have many male-female friendships to examine.) If the friends were particularly close—judged in the study by the fact that they both named each other on their lists of loved ones—the risk that one’s weight would follow the others’ increased by a whopping 171 percent.

Even people who’d never met each other were affecting each other in a six-degrees-of-separation way. If your friend’s friend’s friend, or your friend’s sibling’s friend, gains weight, “that will have a subtle effect on you over the course of two to four years,” says James Fowler, an associate professor of political science at the University of California, San Diego, and the other coauthor of the study. “When we change our own lifestyle and become heavier or thinner, that has a ripple effect through the whole population.”