Altruism is sometimes defined very broadly so that it refers to all human behavior not motivated by the self-interest of the agent. In this use of the term, human actions are either egoistic or altruistic—there is no third alternative. However, such a broad definition may not be very useful. One reason is that many human actions have mixed motives—one acts in a way that benefits other people, but does so partly because one expects benefits in return, if not immediately, then at some time in the future. Such behavior is sometimes described as reciprocal altruism: It is not motivated just by self-interest, but neither is it pure altruism whereby the only concern is the interests or well-being of the recipient.

Another reason for narrowing the definition of altruism is that one may want to exclude actions that are motivated by respect for agreements, rules, social expectations, and so forth, even when their motivation is...