Norm of reciprocity

The norm of reciprocity is invoked in 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. Typical examples of this technique include gifts of stickers and pens distributed by charities and flowers handed out by members of the Hare Krishna group.

Norm of Reciprocity in Evolutionary Psychology
The Norm of Reciprocity is an attempt by evolutionary psychologists to explain altruism by emphasizing our expectations that “helping others will increase the likelihood that they will help us in the future.” The underlying justification lies in the human desire to reciprocate kindness and cooperate for survival value has enabled our continued existence in a hostile world. Thus, the norm of reciprocity ultimately has survival value. Furthermore, being as this sentiment is intrinsic to our evolutionary history and existence, adherence to the norm would constitute “natural” behavior whose neglect might necessarily cause a degree of dissonance in an individual who, among many other self-concepts, consciously labels himself a human being, perhaps leading to a reduction in self-esteem.