The norm of reciprocity requires that we repay in kind what another has done for us.[1] 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**.
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.
A **positive norm** of reciprocity is "the embedded obligations created by exchanges of benefits or favours among individuals. The recipient feels indebted to the favour or benefit giver until he/she repays" .
"A **negative norm** of reciprocity represents the means by which individuals act against unfavourable treatments, and functions to keep balance in social systems" (Chen, 2009). In contrast to the positive reciprocity norm', the negative reciprocity norm emphasizes the return of unfavourable treatment as an appropriate response to a misdeed. The principle of this norm serves as a powerful deterrent for violent or symbolic mistreatment in society. Harming others invites anger and revenge,
https://en.m.wikipedia.org/wiki/Norm_of_reciprocity
HEIGHT 500
Wikipedia 
Reciprocity, as a fundamental principle in social psychology, revolves around the concept that individuals tend to respond to the actions of others in a manner that mirrors the positive or negative nature of those actions.
https://en.m.wikipedia.org/wiki/Reciprocity_(social_psychology)
HEIGHT 400
Wikipedia