The Science
The "Coca-cola" experiment is probably the most well known study on reciprocity. It was published in 1971 by Dennis Regan. In the study, participants believed they were there to evaluate paintings. Also in the room was a fellow participant named Joe, but Joe was really Regan's assistant.
First, Regan manipulated the degree to which a participant liked Joe, by having the subject overhear Joe being either polite or rude to a person during a phone conversation. In one condition, Joe would leave and bring back two soft drinks, giving one as a gift to the participant. In another condition, Joe would leave and return empty handed. After evaluating some art, Joe was then left alone with the participant at which point he would hand them a note, asking them to buy some raffle tickets.
The results of the study showed the power of giving. In the condition where Joe did not give the participant a soda, the degree to which they liked Joe influenced how many raffle tickets they would buy. But, in the condition where Joe did give a soda, participants bought twice as many tickets as the no soda condition, regardless of the degree to which they liked Joe. So not only did people that did not like Joe buy tickets at twice the rate, they buy many more raffle tickets than the cost of the soda.