Reciprocity. It is a basic human desire, even an expectation, to be treated fairly and with respect. There are norms in every society that influence a person's behavior, many unspoken, expectations for the return of a kindness done to another. Sometimes the kindness done is motivated by a selfish desire to receive some favor back, the "I'll scratch your back, you scratch mine" philosophy. Sometimes. less obviously or even hidden from our own conscious recognition, a kindness is done in the hopes of fulfilling a deep need, such as love or friendship, the "if I do this thing, what will I gain and what will it cost?" type of mathematical…