Explored the effect of imprisonment on violent crime rates prior to 1991. Previous research focused exclusively on rates of imprisonment, rather than using a measure that combines institutionalization in both prisons and mental hospitals. Using state-level panel-data regressions over the 68-year period from 1934 to 2001 and controlling for economic conditions, youth population rates, criminal justice enforcement, and demographic factors, this study found a large, robust, and statistically significant relationship between aggregated institutionalization (in mental hospitals and prisons) and homicide rates. This finding provided strong evidence of what should now be called an institutionalization effect -- rather than an imprisonment or incapacitation effect. Demographic information collected include national unemployment rates and institutional race and age composition.