Psychologists Angela Duckworth and Martin Seligman studied 140 eighth graders and published this study in Psychological Science.

Self-discipline, measured months before the school year ended through self-report, parent report, teacher report, and a task in which students chose between a smaller reward now or a larger one later, predicted final grades, school attendance, and selection into a competitive high school programme the following spring. A replication with 164 more students added a behavioural delay-of-gratification test and a group IQ test. Self-discipline accounted for more than twice the variance in final grades that IQ did, and the effect held even after controlling for the students’ grades from earlier in the year.

It’s a very interesting read.