It is influenced by our social networks of relationships, our friends and family, and the cues we pick up from a shared culture. Research from the decades-long Framingham study is uncovering surprising relationships between a person’s level of weight and the weight of friends: If a male in the Framingham study had a male friend who became fat, his chances for becoming fat himself doubled. Conversely, if his wife became fat, his chance for significant weight gain increased 37 percent.

It makes sense when you stop and think about it. We tend to compare ourselves – our bodies, our behaviors – with members of our own sex. We can “catch” obesity when we hang out with friends who have gained weight. Their behavior and notions of acceptability subtly influence our own. Hang out at the mall sometime and look around you: Birds of a feather flock together.

To what extent can we choose and influence our social networks? Could we in fact “design” social networks to promote health? And what about individual responsibility? Do we have free will, or is it a comforting illusion in a socially deterministic world?

Big questions. Meanwhile, if you’re overweight, adopt a thin friend. If they’re willing to hang out with you, that is.