People gain more willpower to do good after performing a good deed, according to a recent Harvard study.

Psychology graduate student Kurt J. Gray found that people who chose to donate a dollar rather than keep it had greater physical endurance.

“To be good and feel the positive effects, you just have to do some good,” he said. “You don’t have to donate a whole kidney. You can just donate a dollar.”

Gray stopped random passersby in Harvard Square and put a weight in their hands, asking them to hold it for as long as they could. He then handed them a dollar and told them they could either keep it or donate it to charity. After he gave them the weights again, those who donated the dollar were able to hold the weight longer than those who did not donate the dollar.

In a variation of the first study, Gray gave a weight to passersby and told them to hold it for as long as they could in one hand while writing a story with their other hand about themselves doing something heroic, neutral, or evil.

The subjects who wrote about themselves doing good held the weight much longer than those who wrote about themselves doing something neutral. But the subjects who wrote about themselves doing something evil were able to hold the weight longest of all.

“It takes willpower to do something good,” Gray said. “It takes even more to do something evil, for then you have to crush something in your conscience.”

“If you make the decision, you start seeing yourself as good, and then you start becoming your own mold that you just set for yourself,” he added.

Psychology Professor Joshua D. Greene ’97 said that he found the study’s results to be interesting.

“How you behave affects how you see yourself, which then influences your subsequent behavior,” he explained. “It really shows how conception informs reality.”