At some point during their education, biology students are told about a conversation in a pub that took place over 50 years ago. J.B.S. Haldane, a British geneticist, was asked whether he would lay down his life for his country. After doing a quick calculation on the back of a napkin, he said he would do so for two brothers or eight cousins. In other words, he would die to protect the equivalent of his genetic contribution to the next generation. The theory of kin selection—the idea that animals can pass on their genes by helping their close relatives—is biology's explanation for seemingly altruistic acts. An individual carrying genes that promote altruism might be expected to die younger than one with 'selfish' genes, and thus to have a reduced contribution to the next generation's genetic pool. But if the same individual acts altruistically to protect its relatives, genes for altruistic behavior. might nevertheless propagate. Acts of apparent altruism to non-relatives can also be explained away, in what has become a cottage industry within biology. An animal might care for the offspring of another that it is unrelated to because it hopes to obtain the same benefits for itself later on (a phenomenon known as reciprocal altruism). The hunter who generously shares his spoils with others may be doing so in order to signal his superior status to females, and ultimately boost his breeding success. These apparently selfless acts are therefore disguised acts of self-interest. All of these examples fit economists' arguments that Homo sapiens is also Homo economicus—maximizing something that economists call utility, and biologists fitness. But there is a residuum of human activity that defies such explanations: people contribute to charities for the homeless, return lost wallets, do voluntary work and tip waiters in restaurants to which they do not plan to return. Both economic rationalism and natural selection offer few explanations for such random acts of kindness. Nor can they easily explain the opposite: spiteful behavior, when someone harms his own interest in order to damage that of another. But people are now trying to find answers. When a new phenomenon is recognized by science, a name always helps. In a paper in Human Nature, Dr. Fehr and his colleagues argue for a behavioral propensity they call 'strong reciprocity'. This name is intended to distinguish it from reciprocal altruism. According to Dr. Fehr, a person is a strong reciprocator if he is willing to sacrifice resources to be kind to those who are being kind, and to punish those who are being unkind. Significandy, strong reciprocators will behave this way even if doing so provides no prospect of material rewards in the future. The story of J.B.S. Haldane is mentioned in the text