Studies of the impact of prison population on homicide rates have produced widely divergent rates. Those using State-level data find small impacts, those using national data find very large ones. This study used displacement/free-rider theory to examine the difference. Displacement refers to a criminal's movement away from a State 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, this study conducted separate homicide regressions for each State using both in-state and out-of-state prison populations as independent variables. The out-of-state variable had a much larger (negative) association with homicide, indicating substantial free riding. There was evidence of a small displacement impact. Notes, tables, references