Altruism inspires more altruism, according to many studies. In one experiment, an initial act of kindness prompted others to donate, but in progressively smaller amounts. Yet the total quantity donated was triple the initial gift (only a small portion of the experiment, which involved 24 people, is illustrated here). This cascade, identified by James Fowler, professor of medical genetics and political science at the University of California, San Diego, and Nicholas Christakis, director of the Human Nature Lab at Harvard University, represents one way to map contagious altruism.