Their analysis was unique, Dr. Christakis said, because it moved beyond a simple analysis of one person and his or her social contacts, and instead examined an entire social network at once, looking at how a person’s friend’s friend’s friends, or spouse’s sibling’s friends, could have an influence on a person’s weight. The effects, Dr. Christakis said, “highlight the importance of a spreading process, a kind of social contagion, that spreads through the network.”

Of course, the investigators say, social networks are not the only factors that affect body weight. There is a strong genetic component at work as well.

Science has shown that individuals have genetically determined ranges of weights, spanning perhaps 30 or so pounds for each person. But that leaves a large role for the environment in determining whether a person’s weight is near the top of his or her range or near the bottom. As people have gotten fatter, it appears that many are edging toward the top of their ranges. The question has been why.

If the new research is correct, it may mean that something in the environment seeded what many call an obesity epidemic, leading a few people to gain weight. Then social networks let the obesity spread rapidly.

It also may mean that the way to avoid becoming fat is to avoid having fat friends.

That is not the message they meant to convey, say the study investigators, Dr. Christakis and his colleague, James Fowler, an associate professor of political science at the University of California in San Diego.

You don’t want to lose a friend who becomes obese, Dr. Christakis said. Friends are good for your overall health, he explains. So why not make friends with a thin person, he suggests, and let the thin person’s behavior influence you and your obese friend?