chrb writes "The NY Times Magazine has an interesting article about research, based on the long-running Framingham Heart Study, modeling real world social networks. It seems that tendencies to be happy, not to smoke, and not to become obese are passed between nodes in a directed graph in a way that suggests such concepts are 'contagious.' Well-connected nodes in the graph (i.e., people with more friends) are more likely to be happier than less-connected nodes, even when the edges represent more distant friendships. Individuals quitting smoking, or becoming obese, influence not only their immediately connected friends but also friends of friends, with the effect sometimes skipping the intermediary node. The contagion effect is most noticeable when a tendency is passed from one person to another of the same sex — friends of the opposite sex, including spouses, are not as influential."