Acts of kindness, generosity and cooperation spread just as easily as bad ones. And it takes only a handful to really make a difference.

Researchers from the University of California, San Diego (UCSD) and Harvard provide the first lab evidence that cooperative behaviour is contagious, spreading from person to person.

When people benefit from kindness, they 'pay it forward' by helping others who were not originally involved. This creates a cascade of cooperation that influences dozens more in a social network.

The research was conducted by James Fowler, associate professor at UCSD in political science, and Nicholas Christakis of Harvard, a sociology professor.

The duo co-authored a recently published book 'Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives'.

Fowler and Christakis show that when one person gives money to help others in a 'public-goods game', where people have the opportunity to cooperate with one another, the recipients are more likely to give their own money away to other people in future games.

These findings were published in the Monday online edition of the Proceedings of the National Academy of Sciences.