Sociobiologist E. O. Wilson called altruism the “central theoretical problem” of the sciences. Humans exhibit a level and diversity of altruism that is unknown among other species. Thus, the study of human altruism has attracted attention from researchers in all the biological and social sciences. Growing empirical evidence increasingly points to the varieties of human altruism, leading to decreasing emphasis on once-standard models that assume narrow self-interest. Such models are now being replaced with new explanations for the emergence and existence of human altruism.