An Institutionalization Effect: The Impact of Mental Hospitalization and Imprisonment on Homicide in the United States, 1934-2001

Previous research suggests that mass incarceration in the United States may have contributed to lower rates of violent crime since the 1990s but, surprisingly, finds no evidence of an effect of imprisonment on violent crime prior to 1991. This raises what Steven Levitt has called “a real puzzle.” This study offers the solution to the puzzle: the error in all prior studies is that they focus 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 finds a large, robust, and statistically significant relationship between aggregated institutionalization (in mental hospitals and prisons) and homicide rates, providing strong evidence of what should now be called an institutionalization effect – rather than, more simply but inaccurately, an imprisonment or incapacitation effect