A fundamental principle of human communication is that the exchange of ideas occurs most frequently between individuals who are alike, or homophilious. Homophily is the degree to which a pair of individuals who communicate are similar. Such similarity may be in certain attributes, such as beliefs, education, socioeconomic status, and the like. The conceptual label of “homophily” was given to this phenomenon several decades ago by Paul F. Lazarsfeld and Robert K. Merton (1964, p.23), but the general idea of homophilous behavior was noted a century ago by the French sociologist Gabriel Tarde (1903, p.64): “Social relations, I repeat, are much closer between individuals who resemble each other in occupation and education.”