The principle of social proof says so: The greater the number of people who find any idea correct, the more the idea will be correct. - Robert B. Cialdini

Psychologists have long understood the power of the consistency principle to direct human action.

They behave in accordance with what the contrast principle would suggest: Sell the suit first, because when it comes time to look at sweaters, even expensive ones, their prices will not seem as high in comparison.

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.