The answer is hope.
That's right: a study led by Psychologist Alex Wood of University of Manchester has revealed that hope trumps general intelligence, personality and even previous academic achievement in predicting success:
Link to study [PDF] - via Barking up the wrong tree
A 3-year longitudinal study explored whether the two-dimensional model of trait hope predicted degree scores after considering intelligence, personality, and previous academic achievement.
A sample of 129 respondents (52 males, 77 females) completed measures of trait hope, general intelligence, the five factor model of personality, divergent thinking, as well as objective measures of their academic performance before university (‘A’ level grades) and final degree scores.
The findings suggest that hope uniquely predicts objective academic achievement above intelligence, personality, and previous academic achievement.