Explaining animal and human social behavior has long been recognized as an important question in evolutionary theory. Over the years, two theoretical approaches came to dominate the field: kin selection and evolutionary game theory (EGT) models. Kin selection models have explained interactions between related individuals successfully, while EGT models, notably those based on the prisoner's dilemma setup, have been less successful. Recently, several arguments for new theoretical and conceptual approaches have been advanced that share a common idea. This is the idea that selection acts not directly on a specific behavior, but rather, on the decision making mechanisms of individuals that interact with the environment and other individuals to determine the actual behavior. Therefore, these approaches all suggest that the evolution of social behavior needs to be addressed using a two-tiered theory. The purpose of this symposium is to discuss the ways in which this idea can be incorporated into new modeling approaches. This will facilitate the development and dissemination of a framework in which both behavioral/developmental and evolutionary processes are modeled in their own right, and integrated with each other. The aim of the symposium is not to advocate a certain modeling approach, but to highlight similarities and understand differences between the various approaches to a two-tier theory, and to facilitate the early development of the theory. The symposium will bring together researchers working in areas ranging from interspecific mutualisms to human economic behavior. Talks will be focused on developmental models of behavior, the population dynamic consequences of behavior, and models of the evolution of cognitive and psychological mechanisms. A brief summary and some concluding remarks will be given at the end of the session by the session organizer.