Results do not support the contention that neoclassical changes to the Nation’s sentencing policies account for the rapid increase in the State prison population between the early 1970s and late 1990s. Findings show that on the aggregate, sentencing reforms are not directly related to changes in State prison populations; however, abolition of parole is negatively associated with State prison population rates. Two types of sentencing reforms, the voluntary sentencing guidelines and the three-strike laws are indirectly related to changes in prison population and have opposite influences on rates of new court commitments. While the study largely focused on testing the formal rationality hypotheses rising out of the sentencing reform movement as a determinant of prison growth, the analyses also controlled for several substantive factors that might influence changes in the Nation’s prison population. These results indicate that later factors may still have greater effects on aggregate incarceration outcomes than those from the formal rational perspective. Besides the three substantive factors included, there remains other similar variables not included in the current analysis that may also account for variations in the Nation’s prison populations, such as political parties controlling State legislatures and governors’ offices, the stability of a State’s manufacturing base, or the degree to which States elect judges or appoint them through a political process. Future research should focus on adding these contextual factors to see if there can be an improved fit of prison population models. Tables, figures, appendix, notes, and references