"While it may seem obvious that locking up more people would lower the crime rate, the reality is much more complicated. Sentencing and release policies, not crime rates, determine the numbers of persons in prison. This point is illustrated by examining what happened to incarceration rates and crime rates nationally in the period from 1991-1998. This was a period in which crime rates fell but rates of incarceration continued to increase. During that time, the states that experienced below-average increases in their rate of incarceration actually experienced above-average decreases in crime. The three largest states offer useful examples: Texas experienced a 144% increase in incarceration with a 35% drop in crime rates, and California had a 44% rise in its incarceration rate with a 36% drop in crime rates. In contrast, New York saw its incarceration rate increase by only 24%, yet nonetheless experienced a drop in crime rates of 43%."
Alexander, Elizabeth. Michigan Breaks the Political Logjam: A New Model for Reducing Prison Populations. American Civil Liberties Union (November 2009.