Experimenting with buddies

Go to any social gathering in your neighborhood and you will notice that people interact mostly with others who are similar in terms of age, gender, race, attributes, and behaviors. This tendency of people to have similar friends—known as homophily—is one of the most pervasive features of social networks (1). A key question is how much of the homophily in behavior can be attributed to social diffusion, that is, direct causal influence of one person on another through social ties (2, 3). Results from two clever Internet experiments reported by Centola last year (4) and on page 1269 of this issue (5) shed light on how the particular arrangement of social ties promotes social diffusion.