Evolution of cooperation with asymmetric social interactions

TL;DR

This paper extends the theory of cooperation to include asymmetric, uni-directional social interactions, showing that cooperation can still be promoted in directed networks and identifying structural motifs that enhance cooperation.

Contribution

It introduces a novel framework for analyzing cooperation with asymmetric interactions and demonstrates how directed social networks can favor cooperation, which was not previously well-understood.

Findings

Cooperation can be favored in directed social networks.

Maximum cooperation occurs at an intermediate proportion of directed interactions.

Structural motifs can significantly enhance cooperation.

Abstract

How cooperation emerges in human societies is both an evolutionary enigma, and a practical problem with tangible implications for societal health. Population structure has long been recognized as a catalyst for cooperation because local interactions enable reciprocity. Analysis of this phenomenon typically assumes bi-directional social interactions, even though real-world interactions are often uni-directional. Uni-directional interactions -- where one individual has the opportunity to contribute altruistically to another, but not conversely -- arise in real-world populations as the result of organizational hierarchies, social stratification, popularity effects, and endogenous mechanisms of network growth. Here we expand the theory of cooperation in structured populations to account for both uni- and bi-directional social interactions. Even though directed interactions remove the opportunity for reciprocity, we find that cooperation can nonetheless be favored in directed social networks and that cooperation is provably maximized for networks with an intermediate proportion of directed interactions, as observed in many empirical settings. We also identify two simple structural motifs that allow efficient modification of interaction directionality to promote cooperation by orders of magnitude. We discuss how our results relate to the concepts of generalized and indirect reciprocity.