Abstract: Some groups endure longer, are more stable, and are better able than other groups to incorporate new members or ideas without losing their distinctiveness. I present a simple model of individual behavior based on the thesis that interaction leads to shared knowledge and that relative shared knowlege leads to interaction. Using this model I examine the structural and cultural bases of group stability. Groups that are stable in the short run do not necessarily retain their distinctiveness in the long run as new members enter or new ideas are discovered. A Theory of Group Stability Consider two hypothetical high-tech consulting companies -Fairview and Taliesin -that specialize in designing medical information systems. Over the years, both companies have gained considerable expertise. Despite these similarities, however, the companies are quite different. Fairview was founded by six men, all graduates of BL Tech with degrees in business. The current members of the company get along well -they frequently hold Monday evening meetings and tend to have a unified perspective on how to develop systems. Taliesin resulted from a chance meeting in an airport between a computer science major and a business major interested in health care. Taliesin currently employs 12 men and women, who graduated from different universities and who represent a variety of disciplines. As at Fairview, the Taliesin employees get along well. Even so, they spend less time together than do Fairview employees, and often split into subgroups to handle multiple clients. Fairview and Taliesin thus represent very different sociocultural configurations: Fairview is small, socially undifferentiated, and culturally homogeneous; Taliesin is large, socially differentiated, and culturally heterogeneous. Because of increasing requests by clients, both companies are considering moving into the area of limited medical expert systems. Such a move may require hiring at least one new person. Will the addition of a new member or new information destabilize these groups? What are the structural and cultural bases for group stability? For example, what types of groups are the most stable? What types of groups are least affected by the addition of new members? What types of groups are least affected by expansion of the group's knowledge base? Various theories attempt to explain why some groups endure longer than others. These explanations usually suggest that favorable contexts are necessary for group stability, particularly when memberships change and new technologies and ideas emerge, and that highly differentiated contexts produce multiple groups. Such contexts frequently are characterized in terms of their environmental (Aldrich 1979; Hannan and Freeman 1977), institutional (Blau 1967; Collins 1975; Etzioni 1964; Sills 1957; Simmel  1955), ritual (Durkheim  1954; Goffman 1959; Mead 1962), or functional (Aberle, Cohen, Davis, Leng, and Sutton 1950; Mack 1967; Parsons 1949, 1951) characteristics, but rarely in terms as simple as "who knows what." Although these explanations tend to assume that groups members learn, interact, and communicate, the precise mechanisms underlying such processes are underspecified and the power of these fundamental "cognitive" mechanisms in producing and maintaining groups are ignored. In contrast to these context-dependent themes, I present a "constructural" perspective that is spare and highly general (see also Carley 1986a, 1986b, 1990, forthcoming a, forthcoming c) . According to this perspective, social change and stability result from changes in the distribution of knowledge as individuals interact and acquire and disseminate information. Constructuralism can be viewed as a modification of structural symbolic interactionism (Stryker 1980) in which knowledge mediates interaction and language, or it can be viewed as a modification of social differentiation theory (Blau 1977) in which knowledge mediates social dimensions (e.g., religion, sex, and age) and interaction. According to both theories, groups can be defined by shared social, demographic, or sociocultural features -e.g., Catholic boys age 13. According to the constructural perspective, each position on a social dimension is associated with a particular body of knowledge that is acquired by individuals with that characteristic -e.g., Catholics learn the tenets of Catholicism, the order of the mass, the holy days of obligation, and so forth. It is the wealth and uniqueness of the information associated with that dimension, not the dimension per se, that determines behavior. Constructural theory derives group characteristics and behavior from the characteristics and behaviors of individual group members that, in turn, are generated by processes relating individual knowledge to individual behavior. Three axioms capture this relationship: (1) individuals are continuously engaged in acquiring and communicating information; (2) what individuals know influences their choices of interaction partners; and (3) an individual's behavior is a function of his or her current knowledge. According to the constructural perspective, groups form and endure because of discrepancies in who knows what. Groups typically are in flux simply because members are continually acquiring new information and communicating it to each other. A group is perfectly stable only when no new information enters the group and everyone in the group knows everything that anyone else in the group knows. From this perspective, neither institutional nor motivational factors are necessary for group stability, nor is a differentiated environment or a differentiated set of institutional or motivational factors necessary for distinct groups. Rather, these factors may serve as secondary forces modifying the impact of the primary force -interaction and the exchange of information. To the extent that institutions are forms of knowledge (Berger and Luckman 1966), this perspective suggests that the distribution of knowledge across the population corresponds to the distribution of institutions and that perfect stability signals the effective demise of institutions because individuals, by knowing everything, are effectively members of all institutions. Institutions can maintain their identity, stability, and cultural distinctiveness by preventing the flow of information. Differences in the information possessed by individuals may arise for many reasons, e.g., because they were born at different places or at different times. Demography, geography, and innovation permit information to be distributed unequally across the population. Regardless of the sources of these discrepancies, at any point societies can be characterized in terms of their social structure, culture (distribution of information), population, number of groups, size of groups, and total amount of information. According to the constructural perspective, this sociocultural configuration changes as individuals interact, communicate, and adapt to new information. The initial sociocultural configuration and the processes of information exchange will determine whether groups endure and whether these groups, when confronted with new members or new ideas, will be able to reconstruct, i.e., adopt new members or ideas without losing their uniqueness as a group. I develop a simple dynamic simulation model of the interaction shared knowledge cycle in which individuals interact, communicate, and adapt to new information. (The Appendix presents an outline of the simulation program.) A more detailed technical description of the model is presented in Carley (1990).) Despite its simplicity, important and complex social behaviors emerge, many of which are consistent with existent empirical data. I use the model to explore group stability and endurance in one-group and two-group societies in which there is no change in group membership and no new ideas. I then examine the ability of these groups to assimilate a new member or idea without losing their uniqueness as a group. Finally, I discuss the model's scope, some important extensions to the model, and the role of simulation in this type of analysis.