A simple “thank-you” can go a long way. It is more than a polite gesture; it makes the other person feel more valued and competent.

We tend to think of gratitude as an emotion that affects the receiver: The person’s face lights up because you were pleased with what she did for you. And it does more than make her feel good; it motivates her to do more. Social psychologists call it increasing pro-social behavior. Thanking someone for helping you can make the helper feel better about herself, and it can improve the chance that she will help you again.

But little research has been done into why it makes that person feel good, so Adam Grant, a psychologist and professor of management at the University of Pennsylvania’s Wharton School, conducted four experiments to determine the effect gratitude has on people who do favors, give gifts, and demonstrate kindness. The answer is that it affects their self image, Grant wrote in the Journal of Personality and Social Psychology.

In two experiments, people who received a written thank-you note were motivated to help that same person again, as well as another person. In a third experiment, after a supervisor had expressed gratitude to a group of fundraisers in a call center, they made more phone calls. And in a fourth experiment, people who were thanked felt more socially valued.

Increasing the sense of self-worth is where gratitude has its greatest impact, Grant said, because people form a self-view according to their feeling of competence and their sense of connectedness to people around them.

Psychologists have found that the sense of being valued by others is a fundamental human motivation. People who need help sometimes refuse it because getting help will make them feel needy and incompetent. Consequently, people who want to lend a hand will withhold the help, thinking that their gesture won’t be valued. But if the person who needs assistance says he is grateful for the help, that reduces the helper’s uncertainty about whether his kindness is valued and appreciated.

“Expressions of gratitude signify that a beneficiary values, needs, appreciates, and accepts one’s assistance, rather than rejecting or devaluing it,” Grant said.

Making a habit of saying thank you in the workplace can make you more persuasive by increasing your likability, which can make you more credible.

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Ken O’Quinn teaches communications workshops in corporations. A former Associated Press writer, he teaches professional business writing, managerial communication, and journalistic writing for communicators. He is the author of “Perfect Phrases for Business,” and is a contributing author of “Focus on Them,” about leadership.