Abstract
We live in a society. Cooperation makes societies work. Every day, we choose to help others at personal cost and forgo chances to cheat others for personal gain–even when the “others” in question are unrelated strangers. Reputations help explain this puzzling behavior; we seek the approval of our peers and fear the sting of shame. If my behavior yields a good reputation, others may be more likely to cooperate with me. Thus, reputation-based cooperation is called "indirect reciprocity".

But under what circumstances can indirect reciprocity facilitate a cooperative status quo? What happens when members of a society have divergent opinions about each other’s moral standing? And what kinds of rules for assigning reputations tend to emerge in the first place?

I leverage the machinery of evolutionary game theory, theoretical population genetics, and adaptive dynamics to answer these critical questions. I demonstrate the importance of consensus-building processes, including gossip and institutions, for the sustenance of cooperation in complex societies. I show that social norms which mandate the punishment of bad actors are especially likely to evolve–a double-edged sword, because such norms are vulnerable to death spirals of disagreement. Finally, I furnish implications for philosophy, politics, social justice, network science, and the broader discourse, all of which will shape future research into this exciting mechanism.