Measuring Ideological Polarization on the United States Supreme Court
Clark, Tom S., Political Research Quarterly
The study of ideological polarization is an important topic in research ranging from behavioral-level to institutional studies of politics. Polarization, however, has received little attention in the context of the Supreme Court, even while popular press and legal commentary suggest ideological heterogeneity on the Court is consequential for the Court's policy outputs. In this article, I apply an axiomatic measure of polarization developed by Esteban and Ray (1994) to study ideological heterogeneity on the Court to develop a "polarization statistic." I compare this method with other common polarization measures and provide evidence for the reliability of the measure.
Keywords: polarization; U.S. Supreme Court; judicial decision making
An issue of central importance in the study of politics is the extent to which ideological polarization within an institution affects its policy outputs. Public debate and the popular press often focus on political polarization, while there has been rising scholarly and popular interest in the so-called "culture war" in American politics (Greenberg 2004; Fiorina, Abrams, and Pope 2006). Recent scholarship has examined the existence and consequences of ideological polarization among the public and elected elites (Bond and Fleisher 2000; Fiorina, Abrams, and Pope 2006; McCarty, Poole, and Rosenthal 2006). Furthermore, journalists and legal academics often talk of polarization on the Supreme Court as though a polarized Supreme Court is somehow more volatile than an ideologically homogenous one or as though the policy outputs from a polarized Supreme Court are qualitatively different than those of a unified bench (see, for example, Rosen 2007; Lazarus 1998). Indeed, in a recent documentary for PBS, legal commentators commented on the significance of ideological polarization on the Court and Chief Justice John Roberts' concern about the need for consensus among the justices.1
Despite an apparent interest in polarization within the judiciary, scholarship on the courts has not been able to directly address the consequences of ideological polarization on the bench. To be sure, there has been considerable interest in the formation of coalition voting blocs, and some scholarship has sought to determine the consequences of ideologically heterogeneous voting blocs. However, mese studies have generally suffered from a lack of a measure of ideological heterogeneity that is exogenous to the justices' voting blocs. In this article, I propose a measure of ideological polarization that has the benefit of being exogenous to judicial voting blocs and may be used to investigate the consequences of ideological heterogeneity on the Court's decision making. I consider two widely used measures of political polarization as well as a method developed by Esteban and Ray (1994) to measure ideological polarization on the Court. I men present the first systematic analysis of polarization on the Court as well as an initial empirical examination of the consequences of ideological polarization for the Court's policy outputs.
The article proceeds as follows: section 1 briefly reviews the current focus of scholarship on ideological polarization; section 2 presents a theoretical description of polarization and an empirical metiiod for estimating polarization on the Supreme Court; section 3 contrasts the empirical method against other possible measures of ideological polarization that may be advanced; section 4 presents an initial empirical analysis of the consequences of polarization for opinion- writing; section 5 suggests avenues for future research on ideological polarization on the Court and offers some concluding remarks.
1. Ideological Polarization and the Supreme Court
Polarization has long received significant attention from economists. Beginning in the 1980s, students of Congress began to express an interest in polarization (Schlesinger 1985), and more recently, students of mass behavior have begun to study the existence, causes, and consequences of polarization. …