How Altruism Can Prevail Under Natural Selection

Introduction Why have we economists been convinced for so long that our old friend, homo eco-nomicus must be selfish? No doubt we find considerable support for this hypothesis in the behavior of our colleagues. We might also expect that evolutionary pressures tend to produce selfish behavior–with the notable exception of the relation between parents and offspring. But can we expect natural selection to act in favor of altruistic behavior in other relationships? Evolutionary biologists have created a theory that predicts altruistic behavior, not only between parents and children, but also among siblings and other close relatives. 1 Richard Dawkins' expression of this view in The Selfish Gene, is that the replicating agent in evolution is the gene rather than the animal. If a gene carried by one animal is likely to appear in its relatives, then a gene for helping one's relatives, at least when it is cheap to do so, will prosper relative to genes for totally selfish behavior. This paper presents a series of examples in which natural selection sustains cooperative behavior in single-shot prisoners' dilemma games. In prisoners' dilemma, cooperation always gets a lower payoff for oneself and a higher payoff for one's opponent than defection. Therefore it seems appropriate in this simple case to identify altruism with playing cooperate in prisoners' dilemma. 2 The reason that cooperative behavior toward siblings can be sustained even where defection is a dominant strategy, is that an individual who has a gene for cooperating with its siblings has a good chance of benefiting from the presence 2 More subtle questions about the nature of altruistic preferences are left for other investigations. Each of us has done some work of this kind. 1 of the same gene in its siblings. Similar reasoning applies to behavior that is imitative rather than genetically inherited if those who share common role models are more likely to interact with each other than with randomly selected members of the population. Individuals will be assumed to play one-period, two-person games of prisoners' dilemma with their siblings or neighbors. In each game that it plays, an individual can choose one of two strategies, cooperate or defect. The payoffs from this game are listed in the matrix below. If the parameters satisfy the restriction S < P < R < T , then defect will be a dominant strategy for each game. For the game to be called a …