A new study shows that if a person's friends become obese, that person is at a great risk of obesity themselves. For close mutual friends, the risk factor for transmitted obesity increased by 171%.
Update: Dr. Jonathan Robison calls the above study "junk science". "How does one conclude a direct causal relationship from an observational study? Bald men are more likely than men with a full head of hair to have a heart attack. Can we conclude from this that they should buy a toupee or begin using Rogaine lotion to lower their risk?" (thx, robby)