Around 1970, psychologist Walter Mischel launched a classic experiment. He left a succession of 4-year-olds in a room with a bell and a marshmallow. If they rang the bell, he would come back and they could eat the marshmallow. If, however, they didn't ring the bell and waited for him to come back on his own, they could then have two marshmallows.

In videos of the experiment, you can see the children squirming, kicking, hiding their eyes -- desperately trying to exercise self-control so they can wait and get two marshmallows. Their performance varied widely. Some broke down and rang the bell within a minute. Others lasted 15 minutes.

The children who waited longer went on to get higher SAT scores. They got into better colleges and had, on average, better adult outcomes. The children who rang the bell quickest were more likely to become bullies. They received worse teacher and parental evaluations 10 years later and were more likely to have drug problems at age 32.

The ability to delay gratification, like most skills, correlates with socioeconomic status and parenting styles. Children from poorer homes do much worse on delayed gratification tests than children from middle-class homes. That's probably because children from poorer homes are more likely to have their lives disrupted by marital breakdown, violence, moving, etc. They think in the short term because there is no predictable long term.