© 2008 Elsevier B.V. In behavioral ecology, altruistic behaviors are those that reduce the lifetime reproduction of an actor and benefit another individual. These behaviors are found in a diverse set of social species, ranging from microbes to higher vertebrates and humans. Altruism presents a conundrum for evolutionary thinking because natural selection is expected to favor selfish and competitive strategies. Gene-level thinking, however, solves this conundrum: an actor can be selected to help another individual when they are genetically related and share copies of the same genes. This inclusive fitness or kin selection explanation for altruism is well supported by empirical data and can explain many examples where organisms act to reduce their own reproduction. Other partially overlapping explanations include group selection, reciprocation, and enforcement but the importance of these processes is dependent upon the definition of altruism that is used. By most behavioral definitions, however, the archetypal example of altruism is worker behavior in the social insects, whereby individuals spend their entire life building, guarding, and foraging to raise their relatives’ offspring.