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Steven Raphael and Michael A. Stoll Why Are So Many 2 Americans in Prison? The United States currently incarcerates its residents at a rate that is greater than any other country in the world. 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 compares with a world average of 166 per 100,000 and an average among European Community member states of 135. Of the approximately 2.1 million U.S. residents incarcerated in 2005, roughly 65 percent were inmates in state and federal prisons, while the remaining 35 percent resided in local jails. Moreover, current U.S. incarceration rates are unusually high relative to historical U.S. figures. For the fifty-year period spanning the 1920s through the mid-1970s, the number of state and federal prisoners per 100,000 varied within a ten- to twenty-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. Why are so many Americans incarcerated? Why did the incarceration rate increase so much in such a short time period? A nation’s incarceration rate is determined both by the criminal behavior of its residents as well as by policy choices made by the electorate, elected officials, and representatives of the criminal-justice system. The relationship between criminal behavior and incarceration is simple and mechanical: the more people that engage in criminal activity, the greater the proportion of the population at risk of doing time. The determinants of criminal behavior, however, are complex and multifaceted. Public policies defining which offenses are punishable by incarceration along with the pronounced severity of the punishment also play a key role in determining the overall incarceration rate. Clearly, the greater the scope of activities deemed deserving of a prison spell are the higher the fraction of the population that will be incarcerated. Moreover, holding offense type constant, longer sentences will result in more prisoners. The past twenty-five years have witnessed several shocks to the likely behavioral determinants of incarceration as well as many drastic policy changes pertaining to the scope and severity of punishment. Changes in illicit drug markets, the deinstitutionalization of the mentally ill, the declining labor-market opportunities for low-skilled men, changes in sentencing policy, and a more punitive community corrections system are all commonly offered as explanations of recent trends. This chapter seeks to sort out these competing hypotheses and to offer a comprehensive evaluation of the sources of the increase in U.S. incarceration rates. We focus primarily on the growth in state-prison incarceration, though we often analyze variation in the overall incarceration rate inclusive of federal prisons and jails. Over the last two and a half decades, we observe two principal changes that bear the lion’s share of responsibility for growth in the nation’s incarceration rate. First, conditional on the violation that led to the prison sentence, average time served has increased considerably. Second, the likelihood of being sent to prison conditional on committing a crime has increased substantially. These facts suggest that changes in sentencing policy along the extensive margin (defining the difference between offenses meriting incarceration and those that do not), as well as along the intensive margin (determining average time served), explain most of the increase in U.S. incarceration rates. A relatively small proportion of the overall increase in incarceration is attributable to increases in criminal behavior (at most, 17 percent of overall growth). We begin by presenting a simple model of the steady-state incarceration rate. The model is used to outline an empirical decomposition that permits attributing relative importance of changes in sentencing policy and changes in criminal behavior in understanding the increase in incarceration rates. We then present estimates of the key component statistics (time served by offense, admissions rates, crime rates, and so on) needed to perform the decomposition. Next, we analyze the possible effects of several potential shocks to criminal behavior. In particular, we explore and quantify the potential contribution of changing demographics, the deinstitutionalization of the mentally ill, the declining value of real wages earned by less-skilled men, and the influence of recent drug epidemics. In all, the collective influence of these factors is minor relative to the impact of...

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