Résumé : In sequential social dilemmas with stranger matching, initiating cooperation is inherently risky for the first mover. The disclosure of the second mover’s past actions may be necessary to instigate cooperation. We experimentally compare the effect of mandatory and voluntary disclosure with non disclosure in a sequential prisoner’s dilemma situation. Our results confirm the positive effects of disclosure on cooperation. We also find that voluntary disclosure is as effective as mandatory one, which is surprising given the results of existing literature on this topic. With voluntarydisclosure, second movers with a good track record decided to disclose because they expect that not disclosing signals non-cooperativeness. First movers interpret nondisclosure correctly as a signal of non-cooperativeness. Therefore, they cooperate less than half as often when the second mover does not disclose.