Why do people vary in how well they discriminate musical sounds? The present research explored personality traits as predictors of auditory discrimination ability, a cornerstone of many popular musical aptitude tests. According to investment-theory approaches, personality traits can shape the growth of cognitive abilities by affecting the kinds of activities and experiences people select. It thus seems likely that Openness to Experience – a broad trait associated with aesthetic and creative interests – would predict variation in auditory abilities because it is associated with greater engagement with music. A sample of 183 young adults completed an auditory discrimination task (the Musical Ear Test), the HEXACO personality inventory, and items measuring past music training. As expected, Openness to Experience significantly predicted auditory ability (β = .28 [.14, .42]). Mediation models indicated that this effect was fully mediated by music training: people high in Openness had significantly more formal training in music, and music training in turn significantly predicted auditory ability. The findings thus strongly support an investment-theory approach to understanding the role of personality in musical auditory abilities.