Apparent health encourages reciprocity

Reciprocity evolves only when social partners reliably repay, with interest, the investments of others. However, not all individuals are equally able-or motivated-to recompense others satisfactorily. As such, reciprocity relies greatly on the capacities and motives of partners. Apparent health may provide a cue to the value of potential exchange partners in this regard: healthier individuals will tend to live longer and accrue more, higher quality resources, thus increasing the incentives for mutual cooperation. In a monetary exchange task, we show that the apparent health of partners' faces affects human reciprocity. Specifically, participants were more willing to return a profitable amount to, but not more willing to invest in, apparently healthy than unhealthy partners. This effect appears to be a function of the attractiveness of apparent health, suggesting a preference for repayment of attractive partners. Furthermore, the effect of apparent health on reciprocal exchange is qualified by the sex of the partners, implicating a history of sexual selection in the evolution of human social exchange.