The empirics of prison growth: a critical review and path forward.

• Author: Pfaff, John F.

1. INTRODUCTION

   The single most striking statistic in the American criminal justice system is its thirty-year expansion in prison population. From 300,000 prisoners in 1977, the prison population has risen steadily to over 1.5 million as of June 30, 2005, a 400% increase. By 2005, states were collectively spending over $43 billion per year on corrections. That this followed nearly fifty years of relative stability makes the growth all the more remarkable.

   This growth touches on myriad major social issues: how governments have responded to declining faith in the welfare state; how the American legal system--and American society more generally--treats its minority and poorer citizens; how states and counties allocate their scarce financial resources, and how the federal government influences theses decisions; how governments respond to shifting patterns in crime and social disorder. Not surprisingly, given its far-reaching effects and implications, the prison system's growth has spawned a large literature examining the causes, implications, and justifications (or lack thereof) for this profound shift in the use of incarceration.

   I have two goals in this Article" to provide a systematic analysis of what we know about the forces that have driven this growth, and to set forth a framework for producing more reliable and accurate results in the future. In particular, I evaluate an empirical literature that spans criminology, economics, law, political science, and sociology. While hundreds of articles and dozens of books have been written about recent trends in U.S. prison populations, most have been predominantly normative or historical in their approaches. A small core of articles, however, has attempted to evaluate the factors influencing these trends using sophisticated empirical models, and it is on these that I focus. That is not to say at all that the other approaches are unimportant, but they require a different sort of evaluation.

   An accurate empirical understanding of prison growth is vital for policy planners of all stripes. Reformers concerned with the magnitude of the prison population may not know whether to focus on the adoption of sentencing guidelines, more expansive representation for the indigent, or the use of commission-drafted fiscal impact statements unless they know whether the growth of the prison population has been driven by racial inequalities, the poverty of defendants, or legislative budgetary concerns. Conversely, those who support incarceration for utilitarian or other normative (e.g., retributive) reasons need to understand the factors driving its expansion to assess its appeal. A clearer appreciation of how we got here and where we are heading benefits everyone--from fiscal conservatives to social liberals.

   I find, however, that in general the current empirical literature answers these questions poorly. Those who study prison populations have settled on a well-defined set of widely accepted explanatory theories. But the efforts to test these theories suffer from numerous important limitations that inhibit their ability to identify causal effects accurately. That is not to say that we know nothing at this point, but our current knowledge is quite rough.

   In this Article, I set out to accomplish two specific tasks. First, I flesh out what it is that we know, and (of equal importance) what we do not know. Though limited, the current findings do provide some useful insights. In particular, the studies point to seven broad claims, four with respect to prison admissions and three with respect to total population. First, the studies suggest that the admissions rate increases with the crime rate, the percent of the population between the ages of twenty-four and thirty-five, and the percent of the population that is black; and falls with personal income per capita. The studies provide fewer insights for the models looking at total prison population, due at least in part to complex (and unaddressed) dynamic features of those models. Nonetheless, the results imply that total prison population rises with the crime rate, the percent of the population between eighteen and thirty-five, and the percent of the population that is black.

   Second, as part of a larger empirical project looking at prison growth, (1) I discuss numerous methodological limitations with the current studies and improvements that will lead to more accurate and stable results. I focus on four concerns in particular: that models (1) suffer from uncorrected endogenous relationships; (2) exhibit little stability within and across articles, either due to problems of omitted variable bias or deeper structural limitations; (3) rely on poorly chosen variables (both dependent and independent); and (4) consistently use the same data rather than looking for alternative sources of information. Of course, in a world of limited and sometimes flawed data, it may prove impossible to satisfy a theoretical gold standard, and it is important not to cross the thin line between idealism ("we can do it better") and nihilism ("we will never be able to do it well enough")--or at least not without caution (and evidence). Some of the problems with the current studies can be overcome to produce increasingly reliable results, and I explore several such possibilities in this Article. But it also important to identify those issues that are, at least for now, intractable, and to consider the implications of such blind spots.

   This Article proceeds as follows. Part II lays out the broad trends in prison population and discusses the various theories developed over the years to explain them. Part Ill then critically reviews the current empirical literature, identifying the insights we can draw from it and highlighting the open questions. Part IV focuses on several significant improvements for future efforts that lead to more reliable results.

2. A BRIEF HISTORY OF PRISON GROWTH

   1. TRENDS IN THE USE OF PRISON

      Figure 1 is by now well known. The U.S. prison population fluctuated gently from the 1920s to the 1970s, but by the late 1970s it had started a slow and steady expansion that has continued unabated ever since. State expenditures on corrections grew rapidly during this period as well: data from the Census suggest that the share of state expenditures dedicated to corrections grew by approximately 4% per year during the 1980s and 1990s, reaching nearly $43 billion in 2005. And these numbers ignore the collateral implications of prison growth, such as its effects on inmates (both while in prison and after), their families, and their communities.

      [FIGURE 1 OMITTED]

      Showing that prison populations have soared is easy; explaining why they have is more difficult. Figure 2 suggests one reason why simple solutions may be hard to find. This Figure plots the variation in annual state incarceration rates between 1977 and 2004. It is clear that growth in the U.S. prison population has been accompanied by a divergence in state behavior. The rate of growth has varied widely across the states: between 1977 and 2004, prison population grew by only 59% in North Carolina, but by 678% in North Dakota and 538% in Mississippi. (2)

      [FIGURE 2 OMITTED]

      The trends in expenditures appear to be less divergent. Figure 3 plots the variation in the percent of state budgets dedicated to corrections. As in Figure 2, the average rises slowly and steadily, but in Figure 3 the range between the upper and lower bounds appears relatively stable. No state appears to dedicate more than 5.8% of its budget to corrections in a given year. Nonetheless, in 1998 over $30 billion was spent on corrections; in 2005, nearly $43 billion. Regardless of whether this is too much or too little--a challenging normative and empirical question beyond the scope of this Article--the total amount, along with the number of people incarcerated, indicates the critical importance of understanding the dynamics of prison growth.

      [FIGURE 3 OMITTED]

   2. THE GENERAL THEORIES OF PRISON GROWTH

      Though the issue of relatively short-term prison population trends has only recently attracted significant scholarly attention, the prison as an institution has long been studied. Among the most prominent to explore its role in society are Emile Durkheim, Max Weber, Michel Foucault, and Georg Rusche and Otto Kirchheimer. (3) Durkheim considers the historical rise of the prison, arguing that it becomes the primary form of punishment as a society matures. (4) His concern, however, is not with the trends in prison populations over several years within a society, but instead with the evolution of prison's use as the society itself develops. Weber focuses more on the rationalization of punishment, examining its centralization and use of hierarchical systems, uniformity, specialization, penal infrastructure, and professionalism. (5) And Foucault, like Durkheim, explores how the prison developed at a macro level, tying its birth to the growing efforts by the state to control its citizens. (6)

      Despite their importance to our understanding of the prison as an institution, these canonical sociological works provide little insight into the question of recent prison population trends. The one notable exception is the work of Rusche and Kirchheimer, which in fact explicitly considers prison population change during a given historical era. Rusche and Kirchheimer adopt a political-economic perspective, arguing that prison policies adapt to reflect changes in the labor market: the tighter the labor market, the less incarceration is used. While admitting that non-economic factors matter, they assert the centrality of economic forces. (7)

      Given the general absence of scholarship looking at prison population trends--and one could never expect scholars writing before the 1970s and 1980s to predict the subsequent explosion--recent researchers have developed new theories, and built on the old, to explain prison growth. Their efforts fall into four broad...