How did human cooperation evolve? In review of both the theoretical and empirical research, Rand and Nowak set out to understand the theoretical frames of cooperation alongside the behavioral evidence. Theory suggests that human cooperation developed via five different mechanisms: reciprocity, reputation, group interaction, competition, and kin recognition. These mechanisms are significant, the theory posits, because within these behavioral patterns cooperation was favored from a reproduction and selection standpoint. Drawing on research in game theory, the authors present the existing behavioral evidence for each of the five identified mechanisms, in order to describe why and how, in a seemingly competitive world, cooperative behavior developed. The authors also examine one-off instances of cooperation where no mechanism was involved. They conclude that regularly used cooperative strategies often “spill over” to affect anonymous, one time interactions. Lastly, the authors suggest avenues for future research and ways in which the fields of Evolutionary Dynamics and Psychology can work together to better understand human cooperation.