In findings sure to gladden the heart of anyone who's ever wondered whether tiny acts of kindness have larger consequences, researchers have shown that generosity is contagious.

Goodness spurs goodness, they found: A single act can influence dozens more.

In a game where selfishness made more sense than cooperation, acts of giving were "tripled over the course of the experiment by other subjects who are directly or indirectly influenced to contribute more," wrote political scientist James Fowler of the University of California, San Diego, and medical sociologist Nicholas Christakis of Harvard University.

Their findings, published March 8 in the Proceedings of the National Academy of Sciences, are the latest in a series of studies the pair have conducted on the spread of behaviors through social networks.

In other papers, they've described the spread of obesity, loneliness, happiness and smoking. But there was no way to know whether those apparent behavioral contagions were actually just correlations. People who are overweight, for example, might simply tend to befriend other overweight people, or live in an area where high-fat, low-nutrient diets are the norm.

The latest research was designed to identify cause-and-effect links. In it, Fowler and Christakis analyze the results of a so-called public-goods game, in which people were divided into groups of four, given 20 credits each, and asked to secretly decide what to keep for themselves and what to contribute to a common fund. That fund would be multiplied by two-fifths, then divided equally among the group. The best payoff would come if everyone gave all their money — but without knowing what others were doing, it always made sense to keep one's money and skim from the generosity of others.

Only at the end of each game did players find out what the rest of their group had done. The game was run again and again, each time mixing group members and keeping their identities anonymous, so that decisions were never personal.

When one person gave, others in their group tended to be generous during the next two rounds of play. Recipients of their largess became more generous in turn, and so on down the chain. When a punishment round was added — players could spend their own money to reduce the rewards of selfish players — generosity lasted even longer.

"It is often supposed that individuals in experiments like the one described here selfishly seek to maximize their own payoffs," wrote Fowler and Christakis. "The equilibrium prediction is to contribute nothing and to pay nothing to punish noncontributors, but the subjects did not follow this pattern."

According to the the researchers, the explanation lies not in calculations of odds and rewards, but in simple behavioral mimicry: Monkey see, monkey do, human style. When people are irrationally generous, others follow suit.

The network described by Fowler and Christakis doesn't necessarily replicate natural group dynamics, but suggests a general model for how behaviors spread. They suggest that researchers of altruism and cultural evolution study how different group configurations promote or limit the spread of behaviors.

However, the findings aren't just a feel-good story. Selfish behavior spreads easily, too.

Images: 1) Heath Brandon/Flickr.

2) James Fowler.

See Also:

Citation: "Cooperative behavior cascades in human social networks." By James H. Fowler and Nicholas A. Christakis. Proceedings of the National Academy of Sciences, Vol. 107 No. 10, March 9, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.