The evolution of reciprocity: social types or social incentives?

The vast majority of human beings regularly engage in reciprocal cooperation with nonrelated conspecifics, and yet the current evolutionary understanding of these behaviors is insufficient. Intuitively, reciprocity should evolve if past behavior conveys information about future behavior. But it is not straightforward to understand why this should be an outcome of evolution. Most evolutionary models assume that individuals' past behavior informs others about their stable social type (defector, cooperator, reciprocator, etc.), which makes it sensible to reciprocate. In this article, after describing the central source of difficulty in the evolutionary understanding of reciprocity, I put forward an alternative explanation based on a work by O. Leimar. It consists of taking into account the fact that the payoffs to individuals in social interactions can change through time. This offers a solution because individuals' past behavior then signals their payoffs, which also makes it sensible to reciprocate. Even though the overwhelming majority of evolutionary models implicitly endorse the social types mechanism, I argue that the social incentives mechanism may underlie reciprocity in humans.