My longitudinal studies show these three virtue clusters predict different outcomes. For academic achievement, including stellar report card grades, the cluster containing grit is the most predictive. But for positive social functioning, including how many friends you have, interpersonal character is more important. And for a positive, independent posture toward learning, intellectual virtue trumps the others.

Of course, your opportunities — for example, having a great coach or teacher — matter tremendously, too, and maybe more than anything about the individual. My theory doesn't address these outside forces, nor does it include luck. It's about the psychology of achievement, but because psychology isn't all that matters, it's incomplete. Still, I think it's useful. What this theory says is that when you consider individuals in identical circumstances, what each achieves depends on just two things, talent and effort. Talent — how fast we improve in skill — absolutely matters. But effort factors into the calculations twice, not once. Effort builds skill. At the very same time, effort makes skill productive. Let me give you a few examples.

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One biographer describes Darwin as someone who kept thinking about the same questions long after others would move on to different — and no doubt easier — problems: The normal response to being puzzled about something is to say,"I'll think about this later," and then, in effect, forget about it. With Darwin, one feels that he deliberately did not engage in this kind of semi-willful forgetting. He kept all the questions alive at the back of his mind, ready to be retrieved when a relevant bit of data presented itself.