Integrating upstream and downstream reciprocity stabilizes cooperator-defector coexistence in others-only public goods games
Yutaka Nakai
IntroductionHuman cooperation persists among strangers in large, well-mixed populations despite theoretical predictions of difficulties, leaving a fundamental evolutionary puzzle. While upstream (pay it forward: helping others because you were helped) and downstream (rewarding reputation: helping those with good reputations) indirect reciprocity have been independently considered as solutions, their joint dynamics in multiplayer contexts remain unexplored.MethodsWe study public goods games (PGGs