In 1970, Walter Mischel conducted a now classic experiment. He left four-year-olds in a room with a bell and a marshmallow. If the child rang the bell, he would come back and let her eat the marshmallow. But if she resisted ringing the bell, and waited for him to return on his own, the child would be given two marshmallows. The choice was between instant gratification and delayed-but-increased gratification.

Mischel videotaped the kids desperately trying to exercise self-control. Some broke down and rang the bell in less than a minute. Others lasted up to 15 minutes. Mischel continued to track the children through their teen years and into adulthood. He concluded that the children who exhibited the least self-control at four were more likely to become bullies, receive poor grades in school, and have substance abuse problems by age 32. The preschoolers who waited, on the other hand, tended to outperform their peers in school, went on to college, and became healthier adults.

I have ...