This report "examines one of the nation’s least understood recent phenomena – the dramatic decline in crime nationwide over the past two decades – and analyzes various theories for why it occurred, by reviewing more than 40 years of data from all 50 states and the 50 largest cities. It concludes that over-harsh criminal justice policies, particularly increased incarceration, which rose even more dramatically over the same period, were not the main drivers of the crime decline. In fact, the report finds that increased incarceration has been declining in its effectiveness as a crime control tactic for more than 30 years. Its effect on crime rates since 1990 has been limited, and has been non-existent since 2000. More important were various social, economic, and environmental factors, such as growth in income and an aging population. The introduction of CompStat, a data-driven policing technique, also played a significant role in reducing crime in cities that introduced it" (website). This report is divided into two parts following an executive summary. Part I—State-Level Analysis of Crime: criminal justice policies—increased incarceration, increased police numbers, use of the death penalty, and enactment of right-to-carry gun laws; economic factors—unemployment, growth in income, inflation, and consumer confidence; and social and environmental factors—decreased alcohol consumption, aging population, decreased crack use, legalization of abortion, and decreased lead in gasoline. Part II—City-Level Analysis of Crime: policing—introduction of CompStat.