After completing his doctorate, Bainbridge served as an assistant (1975-1980) and associate professor (1980-1982) of sociology at the University of Washington. During this period, he worked with departmental colleague Rodney Stark on the Stark-Bainbridge theory of religion. Upon returning to Harvard as a visiting associate professor of sociology (1982-1987), he co-wrote the books "The Future of Religion" (1985) and "A Theory of Religion" (1987) with Stark. As of 2013, their theory, which aims to explain religious involvement in terms of rewards and compensators, is seen as a precursor of the more explicit recourse to economic principles in the study of religion later developed by Laurence Iannaccone and others.