Obesity rates are at an all-time high in the U.S.—one third of the population is obese and another third is overweight. But why is obesity spreading? Some say poor nutrition, others genetics and still others blame inactive lifestyles. This study suggests another culprit: friends.

This 32-year longitudinal study found that obesity literally spreads from person to person within a social network. Researchers looked at networks of family and friends surrounding 12,067 people and saw a strange effect: One obese person had a domino effect on the other people around them.

The effect varied depending on the relationship: When a spouse became obese, a person’s risk of also becoming obese rose 37 percent, while an obese sibling increased the risk by 40 percent. Friends had the biggest impact of all. An obese friend upped a person’s chances of becoming obese by 57 percent, and when a BFF became obese, the risk rose to an astounding 171 percent. Yikes!