利他・協力のメカニズムと社会の進化

Abstract

How did humans evolve to such an altruistic and cooperative animal? This review paper discusses the primate origin of altruism and cooperation from the viewpoints of cognitive mechanisms and adaptation to social systems. Our previous studies have revealed three characteristics of chimpanzees' altruistic helping behavior: 1) helping upon request, but seldom voluntarily; 2) understanding others' goals by visually assessing the situations; and 3) understanding of others' goals does not automatically lead to voluntary helping. It is suggested that the mechanism in chimpanzees' helping is different from that in human helping, which is often solicited by only witnessing others in trouble. This difference in spontaneity in helping might be a result of their different social systems. In human societies, where indirect reciprocity works, individuals who behave altruistically can gain good reputations. In such societies, voluntary helping is favored and rewarded. Meanwhile, institutions and social sanctions exist in human societies: selfish individuals can be punished by third-party group members. This system also maintains altruism and cooperation. In contrast, there has been no empirical evidence for existence of reputation and social sanction in chimpanzees, which might explain their lack of voluntary helping. Instead of indirect reciprocity, fission-fusion dynamics might be an alternative system for maintaining altruism and cooperation in chimpanzee societies. It is possible that an ecological environment influences a social system, which in turn determines behavior and its mechanism. This emphasizes the importance of empirical studies with broad perspectives. Comparative studies with humans, chimpanzees and bonobos both in the wild and under experimental conditions are expected to deepen our understanding of the evolution of altruism and cooperation, and accordingly to reveal multiple dimensions of human evolution from the viewpoints of cognition, behavior, society, and ecology.