Comparison of social exclusion and punishment in promoting cooperation: Who should play the leading role?

Abstract Pro-social exclusion and punishment are effective strategies in preventing free-riding. Recent literature has demonstrated that when exclusion and punishment coexist, the former is more effective to promote cooperation. However, when exclusion and punishment coexist, it is inefficient from the perspective of social welfare for them to take effect at the same time. Furthermore, it is also unrealistic for ousted defectors to pay a fine as punishment. Inspired by this observation, we propose a new concept of playing the leading role in this study. Specifically, when punishment plays the leading role—punishers dominate, excluders in the same group act as pure cooperators; and vice versa. We compare the two cases in which exclusion or punishment plays the leading role in turn in sanctioning defectors. The evolutionary stable state of the system under different parameter spaces and the phase transition processes of the system between different states are analyzed. We find that in the case where exclusion plays the leading role, all parameter regions for the system to reach a fully cooperative state are significantly expanded. Moreover, under the same parameter values, the ratio of defectors when exclusion plays the leading role is always lower than that when exclusion plays the leading role. Thus, we can conclude that exclusion playing the leading role is more conducive to the evolution of cooperation. These results allow us to compare the effects of exclusion and punishment in promoting cooperation from another perspective.