Throughout elementary, middle, and high school, girls earn higher grades than boys in all major subjects. Girls, however, do not out perform boys on achievement or IQ tests. To date, explanations for the underprediction of girls' GPAs by standardized tests have focused on gender differences favoring boys on such tests. The authors' investigation suggests an additional explanation: Girls are more self-disciplined, and this advantage is more relevant to report card grades than to achievement or aptitude tests. Eighth-grade girls at an urban magnet school were more self-disciplined than their male counterparts according to delay of gratification measures and self-report, teacher, and parent ratings. Whereas girls earned higher grades in all courses, they did only marginally better on an achievement test and worse on an IQ test. Mediation analyses suggested girls earned higher GPAs at least in part because they were more self-disciplined.