Perhaps you're familiar with the rule of reciprocity eloquently described by Robert Cialdini is his book Influence.1 He defines a persuasive rule that transcends borders and culture.
The rule of reciprocity asserts that people are hardwired to respond to a favor or gift by returning one of their own. Cialdini argues that this internal obligation to reciprocate is nearly beyond our control.
Here are two examples of how the rule works, from Cialdini's book.
Two students were taking part in a study focused on art appreciation—or so it seemed. Their task was to rate the quality of some paintings.
In the middle of the study, one of the students (called Joe, by Cialdini) left the room and returned with two cans of Coke. As Joe entered the room, he said, “I asked him [the experimenter] if I could get myself a Coke, and he said it was okay, so I bought one for you, too.”
Joe was not an unknowing subject, however. In reality, he was a critical part of the experiment.
Later, Joe asked a favor of the person who received the Coke. Joe explained he needed to sell raffle tickets, and asked the subject to help him by purchasing some tickets.
The true purpose of the experiment was to measure whether an unspoken obligation existed between the giver (Joe) and the receiver. The research was repeated over and over again with different subjects.
The results were undeniable: Those people who received a free Coke purchased twice as many raffle tickets as those who never received ...