Reciprocal behavior is an integral part of social interactions that can promote gains to cooperating parties. We think of reciprocity as giving back in response to the action of another, such as returning a beneﬁt when one was conferred. Even when no legal obligation exists, the social norm of reciprocity tells us what should be done. It is argued that this social norm contributes to the stability of societies (Gouldner, 1960). Numerous experiments in economics have reported greater cooperative behavior than would be predicted by game theory (Fehr & Schmidt, 1999). Biologists, economists, psychologists, and sociologists recognize that reciprocal behavior has evolved over time in human and non-human populations (Cosmides & Tooby, 1989; Gouldner, 1960; Hoﬀman et al., 1998; Trivers, 1971). Reciprocal behavior persists in repeated games and is even observed in ﬁnite games, despite the absence of potential sanctions, because people are hardwired to behave cooperatively (Cox, 2009).