An economic approach to the evolution of male-female exchange.

Males and females of a number of animal species divide labor and provide jointly for offspring. Males may provide food, for example, while females protect defenseless young. This exchange is unlikely, however, unless a prior partnership has been established in which a female practices fidelity in exchange for a male's provisioning activity. The formation of the trading partnership is itself an exchange, and economic theory can help explain when and why there are mutual gains from trading fidelity for resources. Environmental factors determine the potential gains from trade while evolved psychological mechanisms influence the extent to which gains are realized.