The first universal Principle of Influence is Reciprocity.

Simply put, people are obliged to give back to others the form of a behavior, gift, or service that they have received first.

If a friend invites you to their party, there’s an obligation for you to invite them to a future party you are hosting. If a colleague does you a favor, then you owe that colleague a favor. And in the context of a social obligation people are more likely to say yes to those who they owe.

One of the best demonstrations of the Principle of Reciprocity comes from a series of studies conducted in restaurants. So the last time you visited a restaurant, there’s a good chance that the waiter or waitress will have given you a gift. Probably about the same time that they bring your bill. A liqueur, perhaps, or a fortune cookie, or perhaps a simple mint.

So here’s the question. Does the giving of a mint have any influence over how much tip you’re going to leave them? Most people will say no. But that mint can make a surprising difference. In the study, giving diners a single mint at the end of their meal typically increased tips by around 3%.

Interestingly, if the gift is doubled and two mints are provided, tips don’t double. They quadruple—a 14% increase in tips. But perhaps most interesting of all is the fact that if the waiter provides one mint, starts to walk away from the table, but pauses, turns back and says, “For you nice people, here’s an extra mint,” tips go through the roof. A 23% increase, influenced not by what was given, but how it was given.

So the key to using the Principle of Reciprocity is to be the first to give and to ensure that what you give is personalized and unexpected.

courtesy: 6 Principles of Persuasion in his classic book Influence, Dr. Robert Cialdini