Reciprocal cooperation can be stable when the presence of defectors selects for discrimination

Human cooperation among non-kin has long been accepted as being based on reciprocity - the idea that we return help from individuals that we re-meet (Trivers, 1971). However, this paradigm has been challenged by Efferson et al. who make the bold claim that repeated interaction is insufficient to support cooperation (Efferson et al., 2024). The authors base their conclusion on a model in which cooperative partners use a novel function to determine their response. In their system, cooperation through repeated interaction breaks down. We point out here that the collapse of cooperation is not the result of the novel strategy set. Instead, it is a standard modelling result in which a cooperative environment allows drift towards unstable, undiscriminating strategies. Efferson et al.s simulations are unusual in not allowing mutation from always cooperating to always defecting. We show how allowing full defectors into a cooperative environment provides selection for the maintenance of discrimination. Discriminating strategies resist invasion by strategies that exploit undiscriminating cooperation. In consequence, cooperation is stabilized. We conclude that Efferson et al.s results can readily be explained by existing theory and that repeated interaction can support cooperation without additional processes such as between group competition.