The norm of reciprocity requires that people repay in kind what others have done for them. It can be understood as the expectation that people will respond to each other by returning benefits for benefits, and with either indifference or hostility to harms. The social norm of reciprocity may take different forms in different areas of social life, or in different societies. This is distinct from related ideas such as gratitude, the Golden Rule, or mutual goodwill. See reciprocity (social and political philosophy) for an analysis of the concepts involved.
The norm of reciprocity mirrors the concept of reciprocal altruism in evolutionary biology. However, evolutionary theory and therefore sociobiology was not well-received by mainstream psychologists. This led to reciprocal altruism being studied instead under a new social-psychological concept: the norm of reciprocity. Reciprocal altruism has been applied to various species, including humans, while mainstream psychologists use the norm of reciprocity only to explain humans.