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.