The answer, the researchers report, was that people were most likely to become obese when a friend became obese. That increased one'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 of an influence than friends. It did not even matter if the friend was hundreds of miles away - the influence remained. And the greatest influence of all was between mutual close friends. There, if one became obese, the other had a 171 percent increased chance of becoming obese too.