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.

Cooperation is key to the survival of all biological systems. The spatial structure of a system constrains who interacts with whom (interaction partner) and who acquires new traits from whom (role model). Understanding when and to what degree a spatial structure affects the evolution of cooperation is an important and challenging topic. Here, we provide an analytical formula to predict when natural selection favours cooperation where the effects of a spatial structure are described by a single parameter. We find that a spatial structure promotes cooperation (spatial reciprocity) when interaction partners overlap role models. When they do not, spatial structure inhibits cooperation even without cooperation dilemmas. Furthermore, a spatial structure in which individuals interact with their role models more often shows stronger reciprocity. Thus, imitating individuals with frequent interactions facilitates cooperation. Our findings are applicable to both pairwise and group interactions and show that strong social ties might hinder, while asymmetric spatial structures for interaction and trait dispersal could promote cooperation.