Evolution of cooperation with asymmetric social interactions

Significance Humans have a remarkable, but not ubiquitous, tendency toward altruism. This behavior reflects a classic enigma in evolutionary theory: When and why would individuals forgo selfish interests to help strangers? Population structure is known to catalyze cooperation because it enables local reciprocity—I help you, and you help me. But this explanation assumes bidirectional social interactions, whereas human interactions are often unidirectional: One individual has the opportunity to contribute altruistically to another, but not conversely. Here we uncover a surprising result, that directionality can actually facilitate cooperation. We study this effect theoretically and also in empirical social networks. We suggest several practical implications, including how to modify the directions of social interactions to promote cooperation.