The concept of “pay it forward”—doing a favor someone and asking that person to do the same in the future—can promote prosocial behavior. social scientists refer to this concept as

The correct answer is upstream reciprocity.

Upstream reciprocity refers to an indirect process of reciprocity wherein individuals who are helped by someone, reciprocate and "pay it forward" by helping or showing altruism towards an unrelated third party or person. An instance of upstream reciprocity is holding the door open for a stranger after feeling good about someone else helping you pick up a heavy grocery bag. 

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