Work stamina seems to be one of the biggest predictors of long-term success.

Because the claim is stated as an empirical finding, judge it against research on predictors of long-term success. True: peer-reviewed research shows that work stamina, or a close construct such as sustained effort, persistence or grit, is among the strongest predictors of long-term success. Mostly true: research finds it a meaningful but moderate predictor, or the evidence concerns a closely related construct. Mixed: the findings are inconsistent. Mostly false: no rigorous study establishes it as one of the biggest predictors, and the claim generalizes from anecdote or weak associations; it also covers cases where studies such as grit meta-analyses find small effects compared with ability or conscientiousness. False: research shows work stamina has little or no predictive value for long-term success.

Set before the research began.

1. E1 Grit: perseverance and passion for long-term goals - PubMed PubMed (National Library of Medicine), reporting Duckworth AL et al., Journal of Personality and Social Psychology · June 2007

   “Defined as perseverance and passion for long-term goals, grit accounted for an average of 4% of the variance in success outcomes”

2. E2 Grit: perseverance and passion for long-term goals - PubMed PubMed (National Library of Medicine), reporting Duckworth AL et al., Journal of Personality and Social Psychology · June 2007

   “Grit did not relate positively to IQ but was highly correlated with Big Five Conscientiousness. Grit nonetheless demonstrated incremental predictive validity of success measures over and beyond IQ and conscientiousness.”

3. E3 A century of research on conscientiousness at work PNAS (via PubMed Central) · 30 October 2019

   “Evidence from more than 100 y of research indicates that conscientiousness (C) is the most potent noncognitive construct for occupational performance.”

4. E4 A century of research on conscientiousness at work PNAS (via PubMed Central) · 30 October 2019

   “Meta-analyses show C is a potent predictor of consequential life outcomes, including academic performance (10), physical health and mortality (11), work performance (12), marital stability (11), and subjective well-being (13).”

The closest construct in the evidence is grit, defined as perseverance and passion for long-term goals. In the original grit studies it explained an average of 4% of the variance in success outcomes (E1), a small effect. It added some predictive power beyond IQ and conscientiousness and was highly correlated with conscientiousness (E2). Research reviews describe conscientiousness, a broader trait, as the most potent noncognitive construct for occupational performance [E3] and a potent predictor of several life outcomes [E4]; they do not single out stamina or grit. Under the criteria, a small grit effect next to broader traits like conscientiousness fits "mostly false" rather than "one of the biggest predictors".