Reciprocity is a social norm that maintains that people should return favors and other acts of kindness. For example, most of us have no problem picking up the tab for a co-worker or friend who picked did the same last time. However, this bias can also be exploited.
Many studies show that manipulating reciprocation can increase compliance. Requestors can ignite this norm by performing a small (often unsolicited) favor. The result is that the recipient of the favor is more likely to agree to a request from the original favor giver (even when that request is larger then the original favor!).
But why? Why do people adhere to an unwritten social rule that tells them to return favors? Researchers have identified two possible explanations. First, we return favors out of a concern for what the other person will think of us. In short, we're worried that others might see us as ‘free-loaders.' I suspect our distant ancestors had a way of dealing with free-loaders that helped natural selection. The second explanation points to internal standards of behavior—we feel good about ‘doing the right thing.' In order to do the right thing we need to return favors because we don't want to be in debt to anyone. Of course, these possible explanations are not mutually exclusive: We can return favors for both of these reasons.
Recent research asked a particularly interesting question on “whether people would reciprocate a favor even in a situation in which no one would know whether the favor was returned.” The answer was yes.
As the authors conclude: “our findings also are consistent with an evolutionary interpretation of the reciprocity norm. That is, an inherited tendency to return favors rather than act solely on self-interest would likely help groups of people and societies survive. Like internalized social norms, this kind of inherited characteristic could operate in the absence of self-presentation concerns. At any rate, our findings demonstrate that people sometimes choose to do the right thing—in this case, doing a favor for someone who did a favor for them—without worrying about the approval or disapproval of others”
Source:The norm of reciprocity as an internalized social norm: Returning favors even when no one finds out (PDF)