People vote in presidential elections. They give anonymously to public television stations and private charities. They donate bone marrow to strangers with leukemia. They endure great trouble and expense to see justice done, even when it will not undo the original injury. At great risk to themselves, they pull people from burning buildings and jump into icy rivers to rescue people who are about to drown.
Soldiers throw their bodies atop live grenades to save their comrades.
Seen through the lens of the self-interest theory emphasized in current microeconomics textbooks, such behavior is the human equivalent of planets traveling in square orbits. Microeconomics and Behavior freely concedes the importance of the self-interest motive in many contexts, but also contains a supplementary chapter on the role of unselfish motives in social and economic transactions. Using elementary game theory, this chapter identifies circumstances in which people who hold such motives have a competitive advantage over pure opportunists. For example, the chapter shows that people known to have cooperative predispositions can solve prisoner's dilemmas and other commitment problems in a way that purely self- interested persons cannot. Our theoretical models of human nature are important, not least because they mold our expectations about how others will behave.