The role of strategy-affected emotions in the evolution of cooperation

Emotion plays a critical role in cooperation evolution, yet its mechanisms remain underexplored. In this paper, we propose a new mechanism for the cooperation evolution from an emotional perspective: strategies affect emotions, which in turn influence strategies. We establish an index called friendliness to characterize the influence of strategies on emotions, which is determined by both current and expected strategies. In this mechanism, cooperation increases the intensity of friendly emotions and decreases the intensity of unfriendly emotions, while defection has the opposite effect. Simulation reveals that friendliness facilitates cooperation by reshaping emotion characteristics in subsequent game rounds: individuals exposed to defection develop indifference, while those experiencing cooperation amplify their friendly emotions and cooperation. Friendliness mainly plays a role during the unstable stage of game evolution, driving the formation of extensive and cohesive cooperation within populations, along with widespread friendly emotions. As the intensity of friendliness increases, the evolutionary advantages of friendly groups and cooperative strategies are greatly amplified. Furthermore, cooperation exhibits complex behavior with memory length: under low friendliness conditions, memory length amplifies cooperation. Conversely, under high friendliness conditions, memory length attenuates cooperation. In summary, the mechanism by which strategies influence emotions reveal the formation of stable cooperation in terms of emotion. This mechanism improves the fitness of the friendly population and increases the proportion of cooperation without the need for additional rewards or punishments.