Humans possess rich theories of groups and social association.  These theories – perhaps best described collectively as a folk sociology - guide human inferences about events related to groups.  The full content, structure, and function of this folk sociology are not yet clear.  However, it seems likely that people are attuned to two dimensions of social association and that beliefs about these two aspects of social association are a key element of the folk sociology.   One of these dimensions – essentiality – relates to people’s beliefs about the inherence of qualities within members of a social group.  The other dimension – interpersonal interdependence – relates to people’s beliefs about how members of groups coordinate actions and mutually influence each other.  People believe that different kinds of social groups generally possess differing degrees of essentiality and interdependence.  For example, people generally believe that members of kin groups are defined both by a common essence (the same inherent qualities) and by a high degree of interpersonal interdependence.  However, other groups that may be perceived to be defined by a particular essence (like “Blacks”) are not generally perceived to be bound together by high levels of interpersonal interdependence.  To provide some evidence of the value of this framework, I will present data demonstrating that people’s perceptions of the essentiality and interdependence of different group memberships influence their emotional and behavioral reactions to the acts of fellow group members.  These data suggest that perceptions of essentiality and interdependence have strikingly different effects upon how people appraise and react to the actions of fellow group members.