This chapter presents the collected data among tribal people living in the Ecuadorian Amazon and argues that reciprocal altruism plays an important role in the motivation of warriors to undertake the risks involved in war and feud. It analyses reciprocal altruism with regard to tribal warfare provides an important tool in the study of tribal social structures. Warfare is an aggressive act, but it also entails cooperation and alliance. The application of the theory of reciprocal altruism in the context of human warfare can be approached from multiple perspectives. The mathematical model for reciprocal altruism is structurally similar to the model for kin selection. The most conclusive ethnographic data indicating that reciprocal altruism may play an important role in motivating warriors is Napoleon A. Chagnon's reporting of a significant correlation between reproductive success and attaining the status of unokai among the Yanomamo.