A model is presentedto account for the natural selectionof what is termedrecipro- cally altruistic behavior. The model shows how selection can operate -against the cheater (non-reciprocator)in the system. Three instances of altruistic behavior are discussed, the evolution of which the model can explain: (1) behavior involved in cleaning symbioses; (2) warning cries in birds: and (3) human reciprocal altruism.Regarding human reciprocal altruism, it is shown that the details of the psycho- logical system that regulates this altruism can be explained by the model. Spe- cifically,friendship,dislike, moralisticaggression,gratitude,sympathy,trust,suspicion, trustworthinessa,spects of guilt, and some formsof dishonestyand hypocrisycan be explained as importantadaptations to regulate the altruisticsystem. Each individual human is seen as possessing altruistic and cheating tendencies, the expression of which is sensitiveto developmentalvariables that were selected to set the tendencies at a balance appropriate to the local social and ecological environment.