Theories of commitment, altruism, and reciprocity have been invoked to explain and describe behavior in public goods and social dilemma situations. Commitment has been used to explain behaviors like water conservation and voting. Altruism has been applied to explain contributions to charities and intergenerational transfers and bequests. Reciprocity has been invoked to explain gift exchange and labor market decisions. This paper describes a set of experiments, which distinguish between these competing theories by testing their comparative statics predictions in a linear public goods setting. Results provide strong support for reciprocity theories over either theories of commitment or of altruism. (JEL C9, D64, H41, C72).