CIRJE-J-172. Okuno-Fujiwara, Masahiro, Takako Fujiwara-Greve and Nobue Suzuki, "Social Norms an Voluntary Cooperations", January 2007.

Unlike the ordinary repeated games, in the real world, people can run away after cheating. In this paper we construct a social game, in which players can repeat Prisoners' Dilemma only if both players agree to continue the partnership. We investigate how a social sanction prevents moral hazard in such a voluntary relationship. We have three conclusions. First, it is possible to enforce voluntary long-term cooperation by trust-building. Second, the trust-building periods can be shortened under diverse strategy distributions. Third, if there is a reference letter system which conveys information that a partnership ended by an unavoidable cause, then the trust-building periods can be shortened as well.