Epistemic virtues are character traits conducive to principled ways of thinking, leading to a life of flourishing. Recent years have witnessed an emergence of theoretical accounts describing how they develop. However, few if any studies have conducted rigorous empirical investigation into the mechanisms of intellectual virtue development. In this study, we review several significant frameworks before utilizing a randomized, pretest/posttest control trial design to understand the impacts of a novel thinking disposition intervention on intellectual virtue growth. We ascertain the direct and indirect effects of the intervention on four key intellectual virtues: curiosity, humility, integrity, and tenacity. Additionally, we test theoretical mediators of virtue learning. Clear evidence favoring a theory-inspired mediator is observed, though we observe weaker signals of direct effects, with nuances across the virtues. For instance, tenacity and curiosity variables appear to respond more to the intervention than do humility and integrity. Findings are discussed in light of contemporary theoretical perspectives.