One of the big health news stories of 2007 was a study showing that your friends influence the size of your waist (and the rest of your body). Like any study, it raised as many questions as it answered, including why this happens. A new study from Arizona State University looked into that question by testing three pathways by which friends might influence one another’s body size:

1. Collaboration. Over time, you might start to share the ideas of the people close to you after talking with them about what the proper body size is. Then you might choose your food and exercise habits in order to reach that body size, whether that means eating more food to look like your plus-sized friends, or less food to look like your thin ones.

2. Peer pressure. You feel bullied into trying to look like your friends and family members. They make you feel bad about your body, so you go about eating and exercising to look like them.

3. Monkey see, monkey do. You change your habits to mirror those of your friends without necessarily thinking or talking about an ideal body weight. Alexandra Brewis Slade, PhD, one of the Arizona State researchers, gave an example of this pathway that most of us can relate to: You’re at a restaurant with friends and the waiter brings over the dessert menu. Everyone else decides not to order anything, so you pass, too, even though you were dying for a piece of chocolate mousse cake.

Read more:

How your friends make you fat—the social network of weight – Harvard Health Publications.