Study, published in the New England Journal of Medicine, involved a detailed analysis of a large social network of 12,067 people who had been closely followed for 32 years, from 1971 until 2003. The investigators knew who was friends with whom, as well as who was a spouse or sibling or neighbor, and they knew how much each person weighed at various times over three decades. That let them examine what happened over the years as some individuals became obese. Did their friends also become obese? Did family members or neighbors?
The answer, the researchers report, was that people were most likely to become obese when a friend became obese. That increased a person’s chances of becoming obese by 57 percent.
There was no effect when a neighbor gained or lost weight, however, and family members had less influence than friends.
Proximity did not seem to matter: the influence of the friend remained even if the friend was hundreds of miles away. And the greatest influence of all was between mutual close friends. There, if one became obese, the odds of the other becoming obese were nearly tripled.
The same effect seemed to occur for weight loss, the investigators say. But since most people were gaining, not losing, over the 32 years of the study, the result was an obesity epidemic.