Walter Mischel’s marshmallow test is one of the best-known studies in the history of psychology. In the 1960s, Mischel, then a professor at Stanford, took nursery-school students, put them in a room one-by-one, and gave them a treat (they could choose a cookie, a pretzel stick, or a marshmallow) and the following deal: They could eat the treat right away, or wait 15 minutes until the experimenter returned. If they waited, they would get an extra treat. Tracking the kids over time, Mischel found that the ability to hold out in this seemingly trivial exercise had real and profound consequences. As they matured and became adults, the kids who had shown the ability to wait got better grades, were healthier, enjoyed greater professional success, and proved better at staying in relationships—even decades after they took the test. They were, in short, better at life.