The people we associate with can have a powerful effect on our behavior—for better or for worse. This holds true for human health and body mass, too. The heavier our close friends and family, the heavier we are likely to be.

This correlation, described in 2007 by a team that analyzed data from the longitudinal Framingham Heart Study, is well established. But just how this transpires—whether via shared norms, common behavior or just similar environments—has been the subject of much debate.

The authors of the 2007 study proposed that social norms shared among friends and relatives might be a strong determinant of body mass index (BMI). And a new study, published online May 5 in the American Journal of Public Health, drills down to see just how these social forces might be at work. The study of more than 100 women—and hundreds of their friends and family members—however, suggests that social attitudes might not be key in determining obesity clusters after all.

"Going in as anthropologists we assumed that the norms would have a strong influence" on BMI, says Alexandra Brewis, executive director of the School of Human Evolution and Social Change at Arizona State University in Tempe. She and her colleagues found themselves surprised how small an effect the norms had on a person's BMI. Just one type of social dynamic seemed to play a statistically significant role—and that was only about 20 percent.

But a small effect is not no effect, points out James Fowler, a professor at the University of California, San Diego School of Medicine and Division of Social Sciences, and co-author of the 2007 The New England Journal of Medicine paper that described the influence of social ties on obesity rates. The finding that even 20 percent of weight status can be attributed to social norms suggests that "at least some of what is spreading are ideas about body size." And that, he says, is "incredibly exciting," as it hints at some methods to start stemming the social spread of obesity.

The influence of friends
To try to better understand the ways in which social norms might be influencing social clusters of obesity, Brewis and her colleagues interviewed 101 Arizona women aged 18 to 45—and 812 of their close social ties, including friends, spouses, family members, coworkers and others—about their attitudes toward body size. All subjects were questioned about their ideal size, their preference for being non-obese (relative to other conditions, such as being an alcoholic or blind), and how much stigma they associated with a person's being overweight or obese. 

The subjects' answers correlated with their BMI (by design, half of the women were overweight or obese) and with the BMI and answers given by their social ties. Brewis and her team then tested the data against a framework of three possible pathways that might help explain why people and those they are close to have similar body sizes. The frameworks held up in the analysis: The first describes the case in which social norms between people converge "and that makes you both behave the same way," Brewis explains; the second, occurs when two people might have different social norms, but a friend's social norms "shape [the other's] behavior even if they don't internalize them"; and the third possibility—"it's not really due to the relationship at all," but rather something else that is related to body size, such as simply frequently seeing friends who are overweight.

Given the way the study was designed, however, it is quite possible that the effect of social norms is "even higher" than 20 percent, Fowler says.

The findings are by no means the final word on the influence of social norms, says Ross Hammond, director of the Center on Social Dynamics and Policy at The Brookings Institution. "It examines one kind of norm," he says. "I think this study is a good start," but, he notes, "even for these kinds of norms, there are other pathways through which this might matter." He also notes that although the study focused on a certain age range of women, it did not pull out differences that might be at work in subsets of age, BMI or demographics. "Some of the results they find may mask quite strong effects in subgroups," he says. Brewis says that her group hopes to start studying the role of social norms in different countries as well, such as in India and Paraguay.

The new study underscores just "how difficult social norms are to measure," says Hammond, who wrote a review paper on social influence and obesity that was published last year. "What we'd like to do is uncover the mechanisms that might explain the social clustering of obesity."

Would you like to see the dessert menu?
If socially gleaned attitudes are not the driving force spreading obesity among social circles, what is? "The other part could explain the spread of behavior," says Nicholas Christakis, a professor of medical sociology at Harvard Medical School and co-author of the 2007 Framingham paper.

And that is exactly where Brewis is hoping to look next. "What we really need to do next is get the behaviors down as well—are they eating together; are they exercising together?" Brewis says. "I think it might be what people do together" and the behavioral cues they follow from those around them—even if they are not related to deep-seated beliefs. "We'll call it the 'Would you like to see the dessert menu?' phenomenon," she says, offering as an example the moment when a waiter arrives to ask if anyone would like to see the dessert menu. In this situation, people often pause to glance around the table and see if anyone is going to ask for it; if no one else asks for the menu, an individual will often pass as well, whereas if one person does ask for the menu, others will be more likely to order dessert.

Fowler and his colleagues are looking into this behavioral side, he says, trying to figure out, "If I start eating more fatty foods, do you start eating more fatty foods?" And Christakis emphasizes that ideas and actions are all part of the complex web of forces that shape us—"Both are part of the story," he says.

In addition to social attitudes and socially determined behaviors, other factors might be at work influencing people in the same social networks.

The environment—from the prevalence of fast food restaurants to the availability of good walking trails—has been shown to play a large role in people's health and BMI. But in their analysis of the Framingham data, Fowler says, his team found that "it's true that your environment has an impact on you, but it doesn't seem to be the reason" for your body weight. Having an obese neighbor did not make a person more likely to be obese, but having a friend or relative who was—even if that person lived hundreds of miles away—did.

Because a large portion of the social relations in the study were listed as family members, genetics could be playing a role, too. And Brewis notes that, "in terms of moving this research forward, that's certainly something to think about."

Winning hearts and minds?
How this and other studies are interpreted could have a large impact on public health messages going forward, determining whether campaigns focus on changing attitudes surrounding healthy body size, zeroing in on directly changing what people eat, or a combination of these.

"We assume that changing the way people think changes how people behave—that's really a core message of public health," Brewis says. "Maybe what people think doesn't really matter that much."

Public health messages have long relied on the notion that by changing attitudes, you can shift behavior. But this and other recent research has shown how this simple notion can be dismantled by a host of complex and still poorly understood factors. People understand that improving their diet and getting more exercise are key to losing weight. "They know, and they want to do it," Brewis says. But that "is really different from being able to do it." Even study-based weight-loss interventions fail to help most people keep weight off—"and we really don't understand why they fail," she says.

And that key disjunction is what Brewis and a host of other researchers are targeting. "If it's not social norms, it's a clue we can start looking for something else," she says.

Fowler says that even if social norms are exerting a minority of influence on weight status, they could still play a powerful role in curbing the rise in obesity. "It still tells you that networks have got to be a key to understanding—and potentially fighting—the obesity epidemic."

Social norms are, after all, an appealing target for intervention. As Fowler points out, they are "very easy to act on. You don't have to change eating or exercise behavior," which are hard habits to alter directly. Instead, "all you have to do is operate on what they think is an appropriate weight" and, at least theoretically, people's weight will follow suit.