Why is it difficult to measure whether a punishment deters crime?

Measuring the effectiveness of punishment as a deterrent is complex due to numerous intervening variables that make it difficult to isolate the impact of a specific penalty on criminal behavior.1

For general deterrence, the theory assumes individuals are rational actors who calculate risks; however, proving that a specific sanction prevented a crime is challenging because many factors influence human behavior beyond the threat of punishment. Research often fails to find a systematic relationship between crime rates and imprisonment rates, suggesting that severe punishments are not necessarily effective at reducing crime.12

Individual deterrence is similarly difficult to measure. Studies often rely on reconviction rates to determine if an offender has repeated a crime, which is a misleading metric because many crimes do not result in convictions. Furthermore, the natural tendency for criminal behavior to decline as individuals age—a process known as "aging out"—complicates efforts to attribute changes in behavior to the deterrent effect of a specific punishment. Consequently, criminologists continue to analyze these policies empirically to better understand their actual impact on future criminality.34