A student having partly adopted the methodology used by Steven D. Levitt attempts to estimate the impact of prison population (PP), on violent crime rate (UCK) across 20 contiguous states in the U.S with the highest crime rates. He uses the number of ongoing trials (Trials) as an instrument to remove simultaneity between prison population and violent crime rate. Verdicts in ongoing trials affect the prison population but are unlikely to be related to the violent crime rate, except through thei effect on prison population. PP measures the per capita prisoners held in state prisons as on December 31, 2015. UCR is measured in violent and property crimes indexes; i.e., per capita rate of violent and property crimes reported to the police, as compiled in the FBI's Uniform Crime Reports. Trials is measured by the number of ongoing trials where the jury had not reached a verdict as on December 31, 2015. The student estimates the following second stage regression equation: UCR=0.0032-0.72PP. (0.0003) (0.42)