When Don Corleone agrees to grant a favor to the neighborhood funeral director in the first Godfather movie, the Don is creating a future obligation on the part of the undertaker (and certainly one he won’t be able to refuse). It’s not only the power of the Don that exerts the influence, though – it’s the power of reciprocity.

Wanting to reciprocate for something we’re given is a deeply rooted cultural norm; when we get something, we feel it’s only proper to give something back. This overarching worldview, says psychologist Robert Cialdini, is common to every culture, in every part of the globe. The rule, he says, is drilled into us as children, “…essentially thou shall not take without giving in return.”

Cialdini is a professor emeritus at Arizona State University and widely considered the world authority on the topic of influence. For more than three decades he’s been studying how our behavior is affected by social rules, rules that we’re often unaware of but which have incredible power over how we act.

Not many of us have the clout of Don Corleone, but we can all make use of the power of reciprocity in lots of ways. Consider this: in a well-known social science experiment one group of diners were presented with the check alone and another group with the check plus a mint. Tip revenues from the “mint” group were 3.3% higher – a definite but modest boost.

Now, when the server brought the check and mint and then, with a friendly smile, told the diners they were a great group and added an extra mint for everyone, the increase was a whopping 20%. Our desire to reciprocate is even stronger when the initial gesture is unexpected and personal.

Marketers use the rule of reciprocity in many ways: including a “gift” of a crisp dollar bill accompanying a seven page research survey gets more people to respond (even though the dollar is pretty small compensation for their time); sending personalized address labels along with donation requests boosts donations big time; giving handsome datebooks to clients at the New Year builds customer loyalty.

So take the initiative: invite clients to lunch, send that bottle of scotch, and be generous every day. You can give savvy advice, good counsel, and show appreciation often. You can offer your knowledge and insight in the form of white papers, blog posts, no-charge consultations. You can offer your time and energy and take the trouble to go the extra mile. And not just for your prospects and clients, but co-workers, venders and everyone else.

Just beware. The power of reciprocity lies, perhaps ironically, in not expecting anything from it. Attempts to evoke reciprocity can backfire if people sense they’re being manipulated or guilt tripped. But as long as the gesture is genuine, unexpected and personal, statistics show your good will is highly likely to be reciprocated. Like the law of gravity, the social law of reciprocity simply takes care of itself. And the more we give, the more we’ll get back in return.