Reciprocity

Cialdini recounts a study (since replicated) showing that waiters increase their tips when they give customers small gifts. In the study, a single mint increased tips by 3 percent on average, two mints increased tips by 14 percent, and two mints accompanied by a "For you nice people, here's an extra mint" increased tips by 23 percent. The mental model this study illustrates is called reciprocity, whereby you tend to feel an obligation to return (or reciprocate) a favor, whether that favor was invited or not.