Explaining the rise in U.S. incarceration rates *

By all measures, the United States currently incarcerates its residents at an exceptionally high rate. Aggregating the state and federal prison populations as well as inmates in local jails, there were 737 inmates per 100,000 U.S. residents in 2005 (International Centre for Prison Studies, 2007). This number compares with a world average of 166 per 100,000 and with an average among European Union member states of 135. Of the approximately 2.1 million U.S. residents incarcerated in 2005, roughly 65% were inmates in state and federal prisons, whereas the remaining 35% resided in local jails. Moreover, current U.S. incarceration rates are unusually high relative to historical figures for the United States itself. For the 50-year period spanning the 1920s through the mid-1970s, the number of state and federal prisoners per 100,000 varied within a 10-unit to 20-unit band around a rate of approximately 110. Beginning in the mid-1970s, however, state prison populations grew at an unprecedented rate, nearly quadrupling between the mid-1970s and the present. Concurrently, the rate of incarceration in local jails more than tripled. In this excellent article, Spelman (2009, this issue) provides an econometric analysis of the growth in the U.S. prison population from 1977 through 2005. The article first presents a model of the optimal prison population. The author hypothesizes the existence of a marginal benefit function that declines with the incarceration rate. This inverse relationship is driven by the highly plausible proposition that as the incarceration rate increases, the criminality of the marginal inmate likely declines leading to declining incapacitation and deterrence effects and, in turn, to marginal benefits. Marginal costs are assumed to increase in the incarceration rate because as incarceration expands, resources less suited for corrections are