The norm of reciprocity requires that we repay in kind what another has done for us. It can be understood as the expectation that people will respond favorably to each other by returning benefits for benefits, and responding with either indifference or hostility to harms. The social norm of reciprocity often takes different forms in different areas of social life, or in different societies. All of them, however, are 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 the revitalisation of reciprocal altruism underneath the new social psychological concept, norm of reciprocity. Reciprocal altruism has been applied to various species, including humans, while mainstream psychologists use the norm of reciprocity to only explain humans.

An underlying norm of reciprocity is by itself a powerful engine for motivating, creating, sustaining, and regulating the cooperative behavior required for self-sustaining social organizations, controlling the damage done by the unscrupulous, and contributing to social system stability. See the discussions in tit for tat and reciprocity (social psychology). The power and ubiquity of the norm of reciprocity can be used against the unwary, however, and is the basis for the success of many malicious confidence games. Minor, usually less malicious examples are techniques used in advertising and other propaganda whereby a small gift of some kind is proffered with the expectation of producing a desire on the part of the recipient to reciprocate in some way, for example by purchasing a product, making a donation, or becoming more receptive to a line of argument.

For some legal scholars, reciprocity underpins in general the international law "and the law of war specifically". Until well after World War II ended in 1945, the norm of reciprocity provided a justification for conduct in armed conflict. British jurist Hersch Lauterpacht noted in 1953 that "it is impossible to visualize the conduct of hostilities in which one side would be bound by rules of warfare without benefiting from them and the other side would benefit from rules of warfare without being bound by them."