Marie, an employee of a software company, had an epiphany one day at her company’s vending machine.  She decided that every time she came for her afternoon Coke, she’d leave money in the machine for the next person, with a note and a card: Your can of Coke has been paid for.  Take this Smile card and pay it forward.
From the moment Marie began her campaign, frantic emails began circulating around the office in an attempt to pinpoint the identity of the company’s secret Santa. A Neighborhood Watch scheme was set up with two or three employees on constant lookout.  At this point, Marie decided that it was time to escalate operations.  She moved to another floor, where she surreptitiously left a daily supply of donuts. For months everyone was talking about it.  It completely changed the conversation. More important, though, it entirely changed the atmosphere of her office.
“When generosity is the basic social capital, you see things from a broader perspective,” says Nipun Mehta, who runs CharityFocus and distributes Smile cards. “You come from a different place of openness. You’re more likely to see multiple views. It deepens trust. The cup of gratitude overflows, and turns into action in so many ways.”
Social contagion
Nicholas Christakis, a sociologist at Harvard who specializes in networks, recently discovered a pay-it-forward phenomenon in communities.  The participants were randomly assigned to a sequence of different groups in order to play a series of one-shot games with strangers in which people could decide how much to put into a public ‘pool’ of money. 
This enabled Christakis and his partner, James Fowler, to draw up networks of interactions, so that they could explore exactly how the behavior spreads from person to person along the chain.  They discovered a scientific demonstration of what Marie carried out:  giving creates a contagion of giving, a network of “pay-it-forward” altruism. The actions of participants affected the future interactions of other people along the network. 
“If Tom is kind to Harry, Harry will be kind to Susan, Susan will be kind to Jane, and Jane will be kind to Peter,” writes Christakis. “So, Tom's kindness to Harry is seen in Jane's kindness to Peter, even though Jane and Peter had nothing to do with Tom and Harry and never interacted with them.”
Three degrees
All it took was one act of kindness and generosity to spread through multiple periods of play and up to three degrees along the network. “Each additional contribution a person made to the public pot in the first period of play is tripled over the course of the experiment by other people who are directly or indirectly influenced to contribute more as a consequence,” Christakis and Fowler write.
So, for every act of kindness or generosity you do for a friend, he or she pays it forward to their friends and their friends’ friends and their friends’ friends’ friends.
Christakis has proven that which Marie had instinctively figured out: kindness and generosity create a cascade of cooperative behavior, even in the most hardened of hearts.
All in the small
As Mehta says, don’t think in terms of big donations, but just the smallest things that you can do in the here and now.  May you pay it forward in tiny acts of kindness this holiday season and watch them spread throughout the world.