"It is very plain to those of us who work in community settings that health behaviors occur in the context of a social network," said Dr. Katherine Kaufer Christoffel, a childhood obesity expert at the Northwestern University Feinberg School of Medicine who was not connected with the research.
Researchers said the methodology could also be used to devise ways to break the social connections that feed smoking and drug addiction.
"People are interconnected, and their health is interconnected," Christakis said.
In the latest study, researchers used data from the decades-old Framingham Heart Study, which has collected information on health, diet, exercise, family ties and, to a limited extent, friendships among the residents of Framingham, Mass.
They used a standard measure, body mass index, to determine whether a subject was obese. BMI is a ratio of height to weight; a person with a score of 30 is considered obese. For example, a man 6 feet tall and 225 pounds has a BMI of 30.5.
Researchers looked closely at the influence of gender, smoking, socioeconomic class and geographic distance among participants.
They found that the influence of friends on weight gain was as powerful as the effect of genetics found in other studies.
Neighbors who weren't friends had no influence on each other, suggesting community characteristics often linked to obesity -- such as a lack of parks or a dependence on cars -- weren't as important as previously thought.
Overall, researchers found that if a person becomes obese, the chances that a friend will become obese rises 57%. Among siblings, the risk goes up 40%. Between spouses, the odds rise 37%.
Mutual friends -- study participants who identified each other as friends -- had the greatest influence. If one became obese, the risk skyrocketed 171%.
The gender mix in friendships played an important role. In same-sex friendships, the chance that a friend will become obese increases 71%. Friends and siblings of the opposite sex had no influence on weight gain, researchers said.
Researchers constructed diagrams of social networks and plotted the spread of obesity through chains of friends.
They found that a person who becomes obese increases the odds of obesity in about 100 people connected to one another though family or friendship.
"It is not only friends, but friends of friends' friends who are affected by this," said Fowler, a political science professor.
