Every day, people perform astonishing acts of generosity, sometimes at great risk to themselves. On a smaller scale, small acts of selflessness occur almost everywhere you look. Stephanie Preston is a professor of psychology at the University of Michigan, and an expert on why humans developed a powerful drive to help each other. She speaks to The World’s host Marco Werman about her book, “The Altruistic Urge: Why We’re Driven to Help Others.”