a form of helping behavior that is sustained when one individual (A) helps another (B) and at some future time B helps A or A’s offspring. The requirements of reciprocal altruism are (a) that the participants are able to identify each other individually, (b) that they are able to remember past actions and who helped whom, (c) that the cost to the helper is less than the gain to the recipient, and (d) that there is a mechanism to protect against cheating. Game theory provides a theoretical system for understanding reciprocal altruism. See also altruism.