Is obesity contagious? SEP 10 2007
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)
Update: Clive Thompson's NY Times Magazine article (Sept 2009) covers this study in more detail. In addition to obesity, the study indicates that smoking, happiness, and drinking may be contagious.