In this article, the authors used a statewide survey to investigate the extent to which beliefs regarding the age at which youth reach maturity, the role of peer influences, and other factors, such as abuse during childhood, are associated with measures of how harshly juveniles should be treated by the justice system. The results of this study provide strong support for the hypothesis that assessments of adolescent development are important predictors of attitudes about how juveniles should be treated in the justice system. In all cases, variables measuring attitudes surrounding adolescent development explained substantially more of the variance in attitudes toward punishment than did demographic and socioeconomic variables. (Contains 1 note and 4 tables.)