It took me a long time to understand fairness because, like many people who study morality, I had thought of fairness as a form of enlightened self-interest, based on Trivers’s theory of reciprocal altruism. Genes for fairness evolved, said Trivers, because people who had those genes outcompeted people who didn’t. We don’t have to abandon the idea of Homo economicus; we just have to give him emotional reactions that compel him to play tit for tat.
