It has convincingly been shown that punishments provide an effective way to encourage cooperation in social dilemmas. In practice, the possibility to punish may not automatically exist, or the extent to which free riders can be punished may be limited by property rights. In this paper, we endogenize punishments in social dilemmas. In particular, we consider the possibility that cooperation in a prisoners’ dilemma is fostered by people’s voluntarily enhancement of their own vulnerability. By enhancing the own vulnerability, a player increases the effectiveness of a possible punishment by the other player. Thus, players have a means to signal their intention to (conditionally) cooperate. Provided that the other player punishes opportunistic behavior with sufficiently high probability, players have the means to make a binding commitment of conditionally cooperative play. We investigate this possibility in a harsh environment where initial attempts at cooperation quickly unravel if no punishments are allowed. We consider two versions of the mechanism. In the gradual mechanism, players may start small and may condition their incremental enhancements of their own vulnerability on the extent to which the other player enhances the own vulnerability. In the sealed mechanism, they simultaneously submit one (final) vulnerability level. Theoretically, we show that both mechanisms allow for multiple equilibria; both mutual defection and mutual cooperation can be supported in equilibrium when one of the two mechanisms is employed. In an experiment, we show that after some time subjects start to voluntarily enhance their own vulnerability. This process provides a substantial boost to the extent in which mutual cooperation is observed. Although the gradual mechanism outperforms the sealed mechanism in some aspects (for instance, the outcome where one player cooperates and the other defects is observed less often), even the sealed mechanism raises the contribution level substantially compared to the baseline where no punishments are possible.