After developing a Grit Scale the authors tested hypotheses in a number of settings. They found that grittier individuals had attained higher levels of education than less gritty individuals of the same age. Older individuals tended to be higher in grit than younger individuals, suggesting that the quality of grit, although a stable individual difference, may nevertheless increase over the life span.
Within a university setting, undergraduates who scored higher in grit also earned higher GPAs (Grade Point Averages) than their peers, despite having lower SAT scores. At a military training centre, grit was a better predictor of first summer retention than either self-control or ‘cadet quality’ as measured by the admissions committee. However, among the cadets who persisted to the fall semester, self-control was a better predictor of academic performance. Finally, grittier competitors in a national (US) Spelling Bee outranked less gritty competitors of the same age, at least in part because of more accumulated practice.
In conclusion, achievement is the product of talent and effort, the latter a function of the intensity, direction, and duration of exertions toward a goal.