Consider the results of a study conducted at Northwestern University. Researchers gave online participants information about a pair of sofas we’ll ca… - Robert B. Cialdini

This parallel form of human automatic action is aptly demonstrated in an experiment by Harvard social psychologist Ellen Langer. A well-known principle of human behavior says that when we ask someone to do us a favor we will be more successful if we provide a reason. People simply like to have reasons for what they do. Langer demonstrated this unsurprising fact by asking a small favor of people waiting in line to use a library copying machine: Excuse me, I have five pages. May I use the Xerox machine because I’m in a rush? The effectiveness of this request-plus-reason was nearly total: Ninety-four percent of those asked let her skip ahead of them in line. Compare this success rate to the results when she made the request only: Excuse me, I have five pages. May I use the Xerox machine? Under those circumstances, only 60 percent of those asked complied.

With this single maneuver, Vincent engaged several important principles of influence. First, even those who did not take his suggestions felt Vincent had done them a favor by offering valuable information to help them order. Everyone felt grateful, and consequently, the rule of of reciprocation worked in his favor when it came time to decide on his gratuity. Besides hiking up the percentage of his tip, Vincent’s ploy also placed him in a position to increase the size of the party’s order. It established him as an authority on the current stores of the house: he clearly knew what was and wasn’t good that night. Moreover — and here is where seeming to argue against his own interests comes in — it proved him to be a trustworthy informant because he recommended dishes slightly less expensive than the one originally ordered. Rather than having appeared to try to line his own pockets, he seemed to have the customers’ best interests at heart.

Once we have made a choice or taken a stand, we will encounter personal and interpersonal pressures to behave consistently with that commitment. Those pressures will cause us to respond in ways that justify our earlier decision.