Comparative Analysis of Social Reintegration and Criminal Recidivism in the Spanish Civilian and Military Prison Systems: A Quantitative Study with Sensitivity Analysis

This study comparatively analyzes the effectiveness of social reintegration in the Spanish civilian and military prison systems, using re-imprisonment recidivism as an inverse proxy indicator. Official administrative data for the 2009–2019 period were analyzed for the civilian system (n = 19,909) and the military system (n = 644), the latter comprising 539 released individuals with complete outcome data and 105 transferred to the civilian system whose post-release outcomes were unavailable. To address this missing information, a sensitivity analysis was implemented under three transparent, a priori scenarios (conservative, intermediate, and optimal). The civilian reintegration rate was 80.02%, whereas the military rate ranged from 95.19% to 98.14% across scenarios. The military advantage was substantial and consistent (absolute risk difference +15.17 to +18.13 percentage points; all p < 0.001; large effect sizes). However, the two populations differ markedly in prior criminal history, offense type, and risk/protective factor profiles; the observed gap therefore reflects the combined effect of population selection and institutional factors and should be interpreted as descriptive rather than causal. These findings establish a robust empirical phenomenon and generate testable hypotheses for future comparative and quasi-experimental research on penitentiary effectiveness.