When paired with an overweight friend, kids who were overweight themselves consumed an average of 300 more calories than when they were with friends of normal weight.
Obesity certainly isn't contagious, but there are powerful social forces at work here. Women tend to eat less around men, for example, but the opposite isn't true. Men just don't seem to care.
The Time article makes one especially important point: "Overweight teens eat an average of 400 calories more when they're alone, compared with when they're in the company of friends of any weight group." I think that's why group weight loss and activity challenges have been so successful in the corporate world. We're social creatures, and it's easier to reach our goals when we have the support of others.