We consider the possibility that cooperation in a prisoner's dilemma is fostered by people's voluntarily enhancement of their own vulnerability. The vulnerability of a player determines the effectiveness of possible punishment by the other. In the Gradual mechanism, players may condition their...

We consider the possibility that cooperation in a prisoner's dilemma is fostered by people's voluntarily enhancement of their own vulnerability. The vulnerability of a player determines the effectiveness of possible punishment by the other. In the "Gradual" mechanism, players may condition their...