Hazing - the abuse of new or prospective group members - is a widespread and puzzling feature of human social behavior, occurring in divergent cultures and across levels of technological complexity. Some past research has examined the effect of hazing on hazees, but no experimental work has been performed to examine the motivational causes of hazing. This paper has two primary objectives. First, it synthesizes a century of theory on severe initiations and extracts three primary explanatory themes. Second, it examines the dynamics of enduring human coalitions to generate an evolutionary theory of hazing. Two laboratory experiments suggest that one potential function of hazing is to reduce newcomers’ ability to free ride around group entry. These results are discussed in light of two common but largely untested explanations of hazing.