. When someone does something for us orgives us something, we feel obligated to return the favor in kind.
In the article, The Power of Persuasion, it mentions that, "Several years ago, a university professor sent Christmas cards to a sample of strangers. The response was amazing – holiday cards came pouring back from people he never met. Most never inquired as to his identity. They received his holiday card, and they automatically sent cards in return." It shows how powerful the reciprocity rule is, even though these people did not know who sent them a Christmas card they still felt the need to send one back . Cialdini, Robert B. “The Power of Persuasion (SSIR).” Stanford Social Innovation Review: Informing and Inspiring Leaders of Social Change, 2003.