The best gift of all can be as simple as an act of kindness, generosity and cooperation. Even better, we hope that our good acts may spread. Now, a study published in the online edition of Proceedings of the National Academy of Sciences provides the first laboratory evidence that kind behavior is indeed contagious and that it spreads from person to person. In short, the researchers found that when just a few people benefit from kindness, they pay it forward by helping others who were not originally involved. This cascade of cooperation can influence many known and unknown participants in a broadening social network.

Here’s how researchers at the University of California, San Diego (UCSD), and Harvard University illustrated the principle: When one person gave money to help others in a public-goods game where people had the opportunity to cooperate, the recipients were more likely to give away their own money to other people in subsequent games.

This type of behavior created a domino effect in which one person’s generosity spread first to three people, then to nine, and then to others in subsequent waves of the experiment. Better yet, the effect of being part of such a kindness circle persists, observes James Fowler, associate professor at UCSD, who co-led the study. “You don’t go back to being your old selfish self.”