Recent work on adult thought has focused on the emergence of relativistic, dialectic, or integrative assumptions about knowledge. In 1981, Kitchener and King developed a model of reflective judgment that describes seven sets of assumptions about reality and knowledge and corresponding concepts of intellectual justification. This article reports a 2-year longitudinal study of three groups of adolescents and young adults (n = 59) who were tested on the Reflective Judgment Interview and on the Concept Mastery Test, a measure of verbal aptitude. Significant group (p < 0.01) and time (p < 0.01) main effects were found on both measures, with older subjects scoring higher than younger subjects, and all groups showing increases in scores over the 2-year period. Observed differences in reflective judgment persisted when verbal ability was statistically controlled. A significant (p < 0.05) sex effect favoring males was found when the effects of verbal aptitude were statistically removed. Thus, the sequentiality of the reflective judgment stages was empirically tested, with results indicating support for the seven hypothesized shifts in epistemic assumptions over time.