Key terms used in the study of altruism. Altruism: cooperative behavior that lowers the donor"s reproductive success while increasing the reproductive success of the recipient of the altruistic act. Kin selection: a type of natural selection that favors the reproductive success of the relatives an organism helps, even at a cost to the organism"s own fitness. Direct fitness: a measure of the reproductive (genetic) success of an individual based on the number of its offspring that live to reproduce. Indirect fitness: a measure of the genetic success of an altruistic individual based on the number of relatives (or genetically similar individuals) that the altruist helps reproduce that would not otherwise have survived to do so. Inclusive fitness: a total measure of an individual"s contribution of genes to the next generation generated by both the direct fitness (derived from reproduction) and the indirect fitness (which depends on social interactions with relatives).