Colds are not the only things that spread. Scientists are finding obesity, workplace blame, smiling, loneliness and now cooperation are also contagious. In fact, cooperation and "paying it forward" by one person can infect dozens if not hundreds of people, a new study of social networks finds.

Scientists have known that if one person is kind to another, that person will likely return the favor, a phenomenon called reciprocity. But the new research shows this contagion extends beyond the initial pair of giving individuals, reaching others as far as three degrees of separation.

A paper explaining the study is published in the March 8 early online edition of the journal Proceedings of the National Academy of Sciences.

"For me this is a very romantic paper, because I like the idea that the consequences of my generosity extend far beyond what I can see," said study author James Fowler of the University of California, San Diego. "One act of kindness can spread to dozens and in some cases hundreds of people we don't know and have never met."

The work adds to past research revealing loneliness spreads like a virus; obesity is contagious and the blame-game is alive and well in the workplace.

What's good for the group…

Fowler and Nicholas Christakis of Harvard University analyzed data from previous experiments that had used public-goods games. In these games, participants were placed into groups of four. Each person received 20 money units, and they had to decide how many units to keep or contribute to a group project.

While it's costly to the individual to give up money, it is beneficial to the group; each money unit donated yielded 0.4 units for each individual. If every group member contributed the maximum of 20 units, each player would earn 32 units. If everyone was selfish, they'd each take home just 20 money units.

For each round, participants played with different individuals. Since the players were strangers to one another and never interacted with the same person twice, Fowler said the results would not be due to direct reciprocity. An analog in the real world might be deciding how much to tip a waitress at a truck stop, he said.

Every dollar the first person gave in the first round resulted in the second person (someone on the receiving end of that first act of generosity) giving about 20 cents more in the next round. That in turn caused the next person to give 8 cents more and the next person to give 6 cents more in following rounds.

"Since each person was connected to three others in the network, this means giving spread first to three people, then to nine people, then to everyone else in the experiment," Fowler told LiveScience. "These cascades of altruism triple the amount the first person gives – If I give an extra dollar, it causes everyone in the network to give a total of three extra dollars."

Uncooperative behavior also spread in a similar way, Fowler said.

Evolution of altruism

The findings suggest this process of contagion might have contributed to the evolution of cooperation, as groups with altruistic individuals would be more altruistic as a whole and more likely to survive than selfish groups.  

The team's research suggests "there is a deep and fundamental connection between social networks and goodness," Christakis said. "The flow of good and desirable properties like ideas, love and kindness is required for human social networks to endure, and, in turn, networks are required for such properties to spread. Humans form social networks because the benefits of a connected life outweigh the costs."

The pair has also looked at how behaviors and emotions spread between Facebook friends online. Preliminary results suggest these connections don't tend to affect our behaviors on average. But when they studied a person's closer Facebook friends, those he or she is most likely to have a real-world connection with, the researchers did find things like a person's weight and whether they smiled in a profile picture did spread – the contagion extended three degrees of separation.

Christakis and Fowler are co-authors of "Connected" (Little, Brown and Company, Sept. 28, 2009), a book about how people influence others' tastes, health, wealth, happiness, beliefs, and even obesity, through social networks.

Their recent research was funded by the National Institute on Aging, the John Templeton Foundation, and a Pioneer Grant from the Robert Wood Johnson Foundation.