Interdependence theory is a social exchange theory that states that interpersonal relationships are defined through interpersonal interdependence, which is "the process by which interacting people influence one another's experiences"(Van Lange & Balliet, 2014, p. 65). The most basic principle of the theory is encapsulated in the equation I = ƒ[A, B, S], which says that all interpersonal interactions (I) are a function (ƒ) of the given situation (S), plus the actions and characteristics of the individuals (A & B) in the interaction. The theory's four basic assumptions are 1) The Principle of Structure, 2) The Principle of Transformation, 3) The Principle of Interaction, and 4) The Principle of Adaption. Authors Interdependence theory was first introduced by Harold Kelley and John Thibaut in 1959 in their book, The Social Psychology of Groups. This book drew inspiration from social exchange theory and game theory, and provided key definitions and concepts instrumental to the development of the interdependence framework. In their second book, Interpersonal Relations: A Theory of Interdependence, the theory was completely formalized in 1978. Harold Kelley continued the development of Interdependence theory in 2003, with the book An Atlas of Interpersonal Situations. This book expanded on the previous work by adding two additional dimensions to the dimensions of interdependence, as well as by analyzing 21 specific situation types. In addition, the work of Kelly and Thibaut built on the work of Kurt Lewin, who first defined interdependence, and stated that ""The essence of a group is not the similarity or dissimilarity of its members, but their interdependence . . . A change in the state of any subpart changes the state of any other subpart . . . Every move of one member will, relatively speaking, deeply affect the other members, and the state of the group" (pp. 84–88). Four basic assumptions of interdependence theory Principle of structure (the situation) All interactions are set within the context of their given situation (known in Interdependence theory as structure). In order to best analyze this factor, Interdependence theory presents a taxonomy of situations that includes the six dimensions listed below. A key concept with the Principle of Structure is Affordance, or what the situation affords (makes possible) for the individuals within the interaction. Six dimensions of structure: Degree/level of dependence. This dimension focuses on the level of reliance that one member of an interaction has on another member of the interaction. This dimension