Public health officials have been working hard to account for the dramatic rise in U.S. obesity rates. Many obvious factors, such as poor diet and a sedentary lifestyle, certainly contribute to the swelling statistics. However, these and other explanations tend to focus exclusively on how individuals’ choices and behaviors affect their own weight.

Now, researchers from Harvard Medical School and the University of California, San Diego have found that obesity is hardly a private matter. Reporting in the July 26 edition of the New England Journal of Medicine, the researchers found that obesity spreads through social ties. When an individual gains weight, it dramatically increases the chances that their friends, siblings, and spouses will likewise gain weight. The closer two people are in a social network, the stronger the effect. Interestingly, geographical distance between persons in a social network appears to have no effect.

“What we see here is that one person’s obesity can influence numerous others to whom he or she is connected both directly and indirectly,” says Nicholas Christakis, MD, PhD, a professor in Harvard Medical School’s Department of Health Care Policy. “In other words, it’s not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship.”

Over the last 25 years, the incidence of obesity among U.S. adults has more than doubled, shooting from 15 to 32 percent. In addition, roughly 66 percent of U.S. adults are considered overweight. Christakis and U.C. San Diego researcher James Fowler, PhD, decided to analyze data from the Framingham Heart Study (an ongoing cardiovascular study begun in 1948) to see if any social patterns might elucidate these alarming rates.

Christakis and Fowler derived information from archived, handwritten administrative tracking sheets dating back to 1971. All family changes for each study participant, such as birth, marriage, death, and divorce, were recorded. In addition, participants had also listed contact information for their closest friends. Coincidentally, many of these friends were also study participants. Focusing on 12,067 individuals, Christakis and Fowler observed a total of 38,611 social and family ties. As they analyzed the data, the researchers also looked closely at the influence of gender, smoking, socioeconomic status, and geographic distance.

The study found that when an individual becomes obese, the chances that a friend of theirs will become obese increase by 57 percent. Their siblings have a 40 percent increased risk of obesity, and their spouse a 37 percent increased risk. However, that person’s neighbor, if not a part of their social network, has no effect.

Gender played an important role in how these statistics broke down. In same-sex friendships, individuals experienced a 71 percent increased risk if a friend of theirs became obese. This pattern was also observed in siblings. Here, if a man’s brother became obese, his chances of becoming obese increased by 44 percent. Among sisters, the risk was 67 percent. Friends and siblings of opposite genders showed no increased risk. While the researchers note that correlations among siblings provide evidence for a biological, and possibly even a genetic, component to obesity, patterns seen among friends indicate that there’s more than biology at work.

Social connections seem to be key. Moreover, as Christakis notes, “The fact that neighbors don’t affect each other and that geographic separation doesn’t influence the risk among siblings or friends tells us that environmental factors are not essential here,” says Christakis. “Most likely, the interpersonal, social network effects we observe arise not because friends and siblings adopt each other’s lifestyles. It’s more subtle that that. What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads.”

Obesity, the authors conclude, needs to be seen not simply as a clinical issue but as a public health problem.

“We need to understand that a significant part of an individual’s health is embedded in their network,” says Fowler. “In fact, we really need to revisit our whole notion of cost-effectiveness. The fact that certain healthcare approaches won’t just affect the individual but will also cascade through their social ties means that healthcare interventions are far more cost-effective than previously thought.”

“The rising rate of obesity threatens to reverse the decline in disability in the older population, with major implications for the health care system,” says Richard Suzman, Ph.D., director of the NIA’s Behavioral and Social Research Program. “This seminal study breaks important new ground in showing how social networks may amplify other factors and help account for the dramatic increase in obesity across the population.”

This research was funded by the National Institute on Aging.