Diet and exercise play their roles, but research shows that your friendships and marital status may have an impact on how much weight you gain — or lose. In the first study of its kind, scientists from Harvard Medical School and the University of California at San Diego have shown that obesity is socially contagious. Published in the New England Journal of Medicine, the 32-year study of 12,067 interconnected adults found that your risk of developing obesity increases 57% if someone you consider a friend (never mind whether the feeling is reciprocated) gains weight. Between mutual friends, the rate is an even higher 171%. But friends aren't the only ones expanding your waistline. Another large study released in October found that newly married men and women, ages 18 to 28, gain an average of 6 lbs. (2.7 kg) to 9 lbs. (4 kg) more than their single peers. Both marriage and friendships, it appears, really are for better or worse.