The reciprocity rule is so powerful, that I wish more people realized the implications of it.
In short, this rule says that if someone does something for you, an intangible obligation is created that you do something for them.
This is not a 100% clear cut rule, of course, but it will increase the probability of the outcome you seek.
What do I mean?
Giving your boss a gift does not guarantee employment and goodwill – but I would for sure still get your boss, and your boss’s boss, an annual Christmas present.
Buying your in-laws dinner does not guarantee their hospitality – but it will sure make you the most liked son-in-law/daughter-in-law, and who knows what else.
And getting a co-worker Starbucks for no reason at all does not ensure an easier workday – but I would still be willing to pony up for the chance at easy compliance.
Understand: Gifts are persuasive.
The rule says that we should try to repay, in kind, what another person has provided us. If a woman does us a favor, we should do her one in return; if a man sends us a birthday present, we should remember his birthday with a gift of our own; if a couple invites us to a party, we should be sure to invite them to one of ours. By virtue of the reciprocity rule, then, we are obligated to the future repayment of favors, gifts, invitations, and the like. So typical is it for indebtedness to accompany the receipt of such things that a term like “much obliged” has become a synonym for “thank you,” not only in the English language but in others as well.
Robert Cialdini, Influence