A person on the street asks you for a dollar. A fellow traveler on an airplane loses his wallet and asks you to loan him cab fare. Do you say yes? It doesn’t seem to depend on whether you’re rich or poor. Research shows that being richer doesn’t make people more generous. It also shows that we’re more likely to help people we perceive as being similar to us. What causes compassion and generosity? What causes selfishness? In a world with more and more people, fewer resources for us to divide up, and shared catastrophes, the answers to these questions are urgent.