Analysis of a Public Good Game Permitted New Entries: a Role of Defectors to Maintain Cooperation

We studied a public good game, in which metanorms work, permitted new entries. Although whoever wants to entry the game, either cooperators or noncooperators, can try to participate in the game, a manager of the game can control a policy to permit who are ok. By changing the level of control, we investigated what type of policies is effective to maintain cooperation. Especially, we compared a strict policy of population management that only cooperative participants are permitted to entry it, and a simple policy of population management that non-cooperative participants are permitted. According to our simulation of the model, it is revealed surprisingly that a level of cooperation of the game collapses when a strict policy was adopted. On the other hand, cooperation level maintains high robustly if a tolerant policy that some invaders who are perfect defectors are permitted. We conclude that an existence of a few amount of defection has an effect on cooperation of the society. We call this effect a social vaccine effect.