a theory that conceptualizes relationships as an evolving social network hierarchy of three concentric circles surrounding an individual, with each circle representing a relationship that has a different level of perceived intimacy or closeness to the individual at a given point in time. The theory attempts to account for how relationships of importance to the individual (i.e., that provide a secure base for individual functioning) change as she or he ages. [originally proposed by U.S. social psychologist Robert L. Kahn and U.S. developmental psychologist Toni C. Antonucci (1948– )]