Although a variety of personal traits influence weight gain, obesity is socially contagious, moving from person to person through networks of friends and relatives, a new investigation finds.

HEFTY TIES. New data from a 32-year investigation indicate that obesity spreads contagiously among networks of friends and relatives. iStockphoto

The study, the first to examine how social ties influence the development of obesity over time, finds that if one person becomes obese, others who know that person well have an increased risk of also becoming obese within the next 4 years. This effect occurs especially strongly among people identifying each other as friends.

The proliferation of permissive attitudes about weight gain and large body sizes among social groups has contributed to soaring U.S. obesity rates, propose medical sociologist Nicholas A. Christakis of Harvard Medical School in Boston and political scientist James H. Fowler of the University of California, San Diego.

"Obesity is not just an individual problem, it's a collective problem," Christakis says.

The new findings appear in the July 26 New England Journal of Medicine.

Christakis and Fowler tapped into previously unexamined data on 12,067 adults who underwent health assessments every 2 to 4 years, from 1971 to 2003, as part of the Framingham Heart Study. The researchers traced social networks for study participants by consulting records of contact information for each volunteer's close friends and relatives, many of whom also participated in the Framingham study and whose weights could also be tracked.

In this largely white, middle class sample, roughly one in three individuals displayed a body mass index that qualified him or her as obese by the end of the study.

The scientists found that when an individual becomes obese, the likelihood that a person who regards that individual as a friend will also become obese increases by 57 percent. This obesity risk increases far more, by 171 percent, when one of two people who regard each other as friends becomes obese.

Friends' impact on obesity appears equally strong whether they live next door to each other or 500 miles apart. Smaller but significant influences on obesity risk extend to friends of friends of people who become obese as well as to people with even less-direct ties to obese individuals. If one sibling becomes obese, the likelihood of the other following suit increases by 40 percent. A comparable effect occurs between spouses.

The sex of social partners also sways obesity's spread. In same-sex friendships, an individual's obesity risk increases by 71 percent if a friend becomes obese. Same-sex siblings display a comparable pattern. Friends and siblings of opposite sexes showed no such liability.

Obesity doesn't spread among neighbors, unless they're also friends. Nor does the risk of obesity rise in an individual dubbed a friend by someone who becomes obese but who doesn't consider that person a friend in return.

Obese people didn't simply seek out similar-looking friends but actually influenced others, Christakis contends. He says that in many cases, people overweight to begin with were encouraged to eat even more, sending them over the line into obesity.

The new findings suggest that obesity treatment should target groups of people who belong to the same social networks, remarks Harvard Medical School psychologist Matthew Gillman, who heads an obesity-prevention program.