An often-replicated result in the experimental literature on social dilemmas is that a large share of subjects choose conditionally cooperative strategies. Cooperation generated by such choices is notoriously unstable, as individuals reduce their contributions to the public good in reaction to other subjects' free-riding. This has led to the widely-held conclusion that cooperation observed in experiments (and its decline) is mostly driven by imperfect reciprocity. In this study, we explore the possibility that the type of reciprocally cooperative choices observed in experiments may themselves evolve over time. We do so by observing the evolution of subjects' choices in an anonymously repeated social dilemma. Our results show that a significant fraction of reciprocally cooperative subjects become unconditional defectors in the course of the experiment, while the reverse is rarely observed.