Rethinking reciprocity.

Reciprocity theory holds a hallowed status in evolutionary social sciences because it provides a way to explain cooperation among unrelated individuals. Theorists have developed a large body of mathematical work based on the iterated prisoner's dilemma game that explores which strategies are evolutionarily stable in a range of conditions, including scenarios involving behavioral errors. However, there is a mismatch between the vigorous pace of theoretical research and the trickle of empirical studies that have tested predictions generated by the theoretical work. Furthermore, findings from the empirical work on dyadic cooperation in humans diverge from theoretical predictions in significant ways. This discrepancy indicates that the standard theoretical framework is poorly equipped to account for key aspects of real-world cooperation. To better align with real-world observations, theoretical explorations of reciprocity need to shift their focus toward understanding how actors choose partners and maintain relationships.