Waiters who gave customers extra mints scored higher tips, thanks to the rule of reciprocity.

Asking someone to help you out can be stressful.

What if they feel like you’re bothering them? Or using them? What if they say no, or worse, don’t even bother responding?

But according to psychologist Robert Cialdini, the author of “Influence: The Psychology of Persuasion,” there’s an easy way to get someone to give you what you need: Do something useful for them first.

He calls it the rule of reciprocity. “People will help if they owe you for something you did in the past to advance their goals,” he told The Harvard Business Review.

For example, let’s say you want a coworker to proofread a project report before you submit it to management. A few days before, consider asking that coworker if you can pick up dinner for her when you’re both staying late at the office.

The key part is reminding your coworker that she can return the favour. Instead of saying “no big deal” when she thanks you for grabbing an extra sandwich, Cialdini recommends saying something like, “Of course; it’s what partners do for each other.”

Cialdini says this strategy works because people across cultures learn the reciprocity norm from a young age. And it affects all kinds of interactions.

In an interview with NPR, Cialdini cited a study that highlighted the role of reciprocity in tipping behaviour. In one experiment, waiters gave customers a piece of candy and then spontaneously allowed them to select a second one before leaving the bill on the table. Tips increased 21%.

The researchers say the reciprocity rule explains their findings — people felt obligated to return the act of generosity, even though they didn’t specifically request that the waiter bring them candy.

This strategy takes a lot of the pressure out of asking for favours. Presumably, once you help someone out, you’ll feel like you deserve their help and won’t worry so much about annoying them. Meanwhile, they will probably feel like they owe you one and won’t think twice about giving you what you need.

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