Originally a word for union or fusion, the term coalition came in the eighteenth century to mean a temporary alliance of political parties. In modern social science, the meaning has broadened to include any combination of two or more social actors formed for mutual advantage in contention with other actors in the same social system. In most contemporary theories of coalition formation, it is taken for granted that the principles governing coalition formation are not much affected by the size of the actors, who may be small children or large nations, but are significantly affected by the number of actors in the system. In the sociological and social-psychological literature, interest has focused on coalition formation in social systems containing three actors, commonly known as triads, and on the factors that influence the formation of coalitions in that configuration. Coalitions in triads have certain properties that are very useful...