<p>This study investigates the reciprocal, within-person associations between legal attitudes, specifically perceived legitimacy and legal cynicism, and correctional confinement during the critical developmental transition from adolescence to early adulthood. Drawing on data from the Pathways to Desistance project (ages 15–22), we employ a Random-Intercept Cross-Lagged Panel Model to separate trait-like stability from time-varying fluctuations. Results reveal a robust, bidirectional link between perceived legitimacy and correctional time. Specifically, lower legitimacy predicts greater correctional time in the following year, and more correctional time, in turn, is negatively associated with perceived legitimacy. In contrast, although cynicism increased in response to more correctional time, it did not predict later confinement when legitimacy was included. This finding runs counter to prior findings suggesting that cynicism is the more direct driver of rule-breaking and justice involvement. Instead, legitimacy, rather than cynicism, emerges as the belief system with forward-looking consequences. These patterns persist across ages 15–22. Methodologically, the study illustrates the utility of RI-CLPM in disentangling developmental trends from experiential effects on attitudes and behavior. Substantively, the results underscore legitimacy as a developmentally consequential belief system that predicts future confinement and highlight legitimacy-building practices as promising intervention targets to disrupt cycles of justice system entrenchment.</p>