According to this study, "Both IQ and self-discipline are correlated with GPA, but self-discipline is a much more important contributor: those with low self-discipline have substantially lower grades than those with low IQs, and high-discipline students have much better grades than high-IQ students." But what is self-discipline? The examples are not compelling - "Students were given an envelope containing $1, and were told they could spend it immediately or bring it back in a week for a $2 reward." But what if that amount of money isn't important enough to spend a week sitting on? Discipline is related to value, which makes measurements dubious. Maybe there's something to this study, but my cynical self leans toward believing that people who have internalized their teachers' values do better, and people who have higher IQs express their values in other ways.