Kindness is contagious, up to three people away

If you give a little to someone you will never see again, does it stop there? A 2010 study by James Fowler of UC San Diego and Nicholas Christakis, then at Harvard, suggests it doesn't. Generosity in their study rippled outward to people who were never part of the original exchange, and by the end of the game each extra contribution had roughly tripled.

A game among strangers

The researchers didn't run new experiments. They went back to data from a well-known set of economics games and looked at it in a new way. In the paper, published in the Proceedings of the National Academy of Sciences, they analyzed 240 people who played "a series of single-shot public goods games with strangers." In each round, players in groups of four got a small stake of money units and decided how much to keep and how much to put into a shared pot that benefited everyone. Then the groups were reshuffled, and nobody played with the same person twice.

That reshuffling is what made the study clever. In real life, people who are kind tend to make friends with other kind people, so it's hard to tell whether behavior spreads or just clusters. Here, partners were assigned at random, and there was no chance of being paid back by the same person or building a reputation, as a University of California, San Diego summary of the study explains.

What spread, and how far

People who had been in a group with generous players went on to give more in later rounds, to entirely new partners. The effect lasted for several rounds and, in the authors' words, "spreads up to three degrees of separation (from person to person to person to person)." Put together, they estimated that "each additional contribution a subject makes to the public good in the first period is tripled over the course of the experiment" by others who were influenced, directly or indirectly, to give more.

"The network functions like a matching grant," Christakis told UC San Diego. Fowler described how long the effect lasted: "You don't go back to being your 'old selfish self.'"

There is a catch, and the paper is upfront about it. The results "summarize the spread of both cooperative and uncooperative behavior." When someone gave less, that stinginess cascaded too. As the science writer Ed Yong noted in National Geographic, the study really showed "that social contagion can be just as harmful as it is helpful." He also pointed out that the original players were Swiss university students, who probably knew they were playing with classmates, and people cooperate more readily within their own group.

The authors are careful too. They say the tripling held "at least in a network of this particular, experimentally controlled structure," and Fowler said "the multiplier in the real world may be higher or lower than what we've found in the lab." So treat "kindness triples" as a striking lab result, not a law of nature. For more on what giving does for the giver, and which famous findings have and haven't held up, see our science of giving guide.

Why it still matters

Even with the caveats, the study points to something useful. What you do around other people sets a norm for what comes next, in both directions. A kind act seen and felt by others makes the next one more likely. A stingy one does the opposite. You can't control the whole chain, but you can choose which kind of link you are.

Today's act

Sources

1. Fowler, J.H. & Christakis, N.A. — Cooperative behavior cascades in human social networks, PNAS 107(12):5334–5338 (March 8, 2010)
2. ScienceDaily (from University of California, San Diego) — Acts of kindness spread surprisingly easily: just a few people can make a difference (March 10, 2010)
3. National Geographic (Ed Yong) — Pay it forward? Cooperative behaviour spreads through a group, but so does cheating (March 8, 2010)