The impact of out-of-state prison population on state homicide rates: Displacement and free-rider effects

Abstract

Past studies of the impact of prison population on homicide rates have produced widely divergent results. Those using state-level data find small impacts, but those using national data find very large ones. We use displacement/free-rider theory to explore the difference between these results. Displacement, in the current context, refers to a criminal's movement away from states with higher imprisonment rates. Free riding occurs when a state benefits from criminals being incarcerated in other states. If the displacement effect holds, a state's prison population has a stronger impact on crime within the state than would be accomplished by deterrence and incapacitation alone. If the free-rider effect holds, higher prison populations outside the state reduce homicide in the state because criminals are incapacitated elsewhere. Using vital statistics data for 1929 to 1992, we conduct separate homicide regressions for each state using both in-state and out-of-state prison population as independent variables. We find that the out-of-state variable has a much larger (negative) association with homicide, indicating substantial free riding. We also find evidence of a small displacement impact.