Ideological Polarization on the Supreme Court

Abstract

Judicial polarization is an important but underexplored aspect of judicial behavior. This analysis uses a gamut of measures to assess polarization on the Supreme Court across chief justice and jurisprudential regimes. I examine individual justice polarization and ideological extremity over full tenures on the Court and also how Court polarization is responsive to polarization in coordinate institutions. I find mixed evidence of greater polarization in the abortion rights regime. I find strong evidence of increasing Court polarization concomitant with congressional and presidential polarization since the 1950s across chief justice regimes. Court polarization is responsive to polarization in coordinate institutions. I do not find that individual justices become more polarized over time. Justices shift ideologically over their careers, and this shift is on average to the Left. These findings are robust across multiple specifications of the models and multiple alternative measurements, controlling for other factors which might influence polarization.

Keywords

judicial politics polarization jurisprudential regimes U.S. Supreme Court separation-of-powers models

Political Polarization and Decision-Making on the U.S. Supreme Court

Beginning with Hunter’s (1991) classic study depicting an inevitable “culture war” in America, scholars have warned of a growing political schism in the American body politic. These concerns have centered on the potential for polarization to frustrate political accommodation and, in the extreme, spur violent social unrest. The literature has focused on whether these portents are an empirical reality or media myth (Abramowitz & Saunders, 2005; DiMaggio, Evans, & Bryson, 1996; Fiorina, Abrams, & Pope, 2004; Fiorina & Levendusky, 2006; Hetherington, 2009; Jacobson, 2003). Although the debate on mass polarization remains unresolved, there is general consensus that elites have polarized in the past decades. Furthermore, scholars agree that elites dominate the political discourse and serve as opinion leaders for the masses. Elites thus influence which issues drive mass political and partisan beliefs and preferences (Hetherington, 2009). Institutions, such as the courts, are potentially susceptible to elite, mass, contextual, and institutional polarization and thus are ripe for examination.

One of the major debates in the literature on judicial politics is whether justices primarily reference their own ideological beliefs, respond to institutional constraints and incentives, or adhere to legal norms encapsulated in precedents and principles such as stare decisis. Scholars have developed measures and models of behavior on the Court which can be harnessed to address polarization within the Court and responsiveness to polarization from without (Bailey, 2007; Bergara, Richman, & Spiller, 2003; Gillman & Clayton, 1999a; Hagle & Spaeth, 1991; O’Brien, 1999; Sala & Spriggs, 2004; Segal & Spaeth, 2002). Furthermore, the study of ideological disparity juxtaposed with consensus in the Court’s decisions has provided a great deal of insight into how and to what extent the justices agree or disagree with one another in decisions as a whole, within case issue typologies, and over time (Brenner & Arrington, 1987; Edelman, Klein, & Lindquist, 2012; Hensley & Johnson, 1998; Sheldon, 1999). Scholars have found that ideological disposition on the Court is not static over time, positing that justices “drift” to one extent or the other over the course of their careers (Epstein, Martin, Quinn, & Segal, 2007).

The research on polarization suggests that institutional elites in the elective branches are responsive to polarization entrepreneurs (Fiorina & Levendusky, 2006; Hetherington, 2009). The scholarship on the Court suggests that the Court is responsive to its own institutional environment, to the mass public, and that their ideological dispositions can fluctuate over time (Anderson & Tahk, 2007; Bailey, 2007; Baum, 1999; Epstein, Landes, & Posner, 2013; Epstein et al., 2007). Furthermore, some scholars have argued that ideological movement on the federal bench can be a consequence of the ideological disposition of their fellow court members—that ideological extremists can yield further polarization of a court (Sunstein, Schkade, Ellman, & Sawicki, 2006). Assessing polarization on the Court over time, assessing whether polarization on the Court is responsive to polarization of institutions within its strategic environment, and determining the causal mechanisms in ideological movement of justices and the extent to which it is responsive to internal and external factors is an important next step in the study of Court behavior and provides another avenue for examining the Court’s responsiveness to the institutional environment and mass political opinion.

The lay out for this article is as follows: I review the literature which examines the institutional constraints on the U.S. Supreme Court, decision-making on the Court, and judicial polarization. I specifically address the debates over ideological behavior on the Court and distinguish polarization from the related concepts of ideological disagreement and consensus. I then use three distinct empirical measures of ideological polarization on the U.S. Supreme Court, using the Martin–Quinn and Segal–Cover measures of justice ideology, to assess judicial polarization over time and across jurisprudential regimes. I begin by assessing judicial polarization in relation to the Court’s landmark “culture war” case on abortion rights, Roe v. Wade. I include an alternative specification using three periods that demarcates the Casey and Carhart jurisprudential regimes. Next, I examine ideological polarization on the court over the past 50+ years and assess the extent to which it is responsive to polarization in the elective branches. Finally, I assess whether justices become more polarized, whether they change ideologically, and in what direction ideological movement occurs the longer they are on the Court within the separation-of-powers (SOP) institutional context.¹

The analysis presented here suggests that Court polarization is a real and significant phenomenon. I find strong evidence of Court polarization concomitant with congressional and presidential polarization over time. Court polarization has increased over time, and it has tracked closely with the ideological distance between the President and the Senate floor median across chief justice regimes. Court polarization has become significantly more pronounced since the 1950s, the Court has become more polarized over successive chief justices, and Court polarization appears to be responsive to coordinate institutions and macro trends in American public opinion that drive polarization in the elective branches. Examining polarization using two conceptualizations of abortion rights jurisprudential regimes as a proxy for social issue polarization in the Court’s decisions; I find mixed evidence of polarization. Court ideology has become more bimodal in the more recent abortion rights regimes relative to that prior to Roe v. Wade. I do not find that individual justices become more polarized over time. However, justices do shift ideologically the longer they are on the Court, and this shift on the whole is in a liberal ideological direction (ID). These findings are robust across multiple specifications of the models and multiple alternative measurements.

Polarization Defined, Judicial Polarization Distinguished From Dissent, and Ideological Drift

Polarization has factored significantly in the study of income inequality in economics (Esteban & Ray, 1994). A substantial literature has developed on political polarization at the mass behavioral and elite levels in the American electorate (Abramowitz & Saunders, 2005; DiMaggio et al., 1996; Fiorina et al., 2004; Hetherington, 2009). Furthermore, polarization has been an ongoing topic of interest of scholars of the U.S. Congress (Poole & Rosenthal, 1984). For example, recent studies have examined the effect polarization has on the increasing use of the filibuster (Garand, 2010; Jones, 2001). There is significant disagreement over whether political polarization has occurred in the former and very little disagreement that polarization has occurred in the later. However, despite the consensus on institutional polarization and its responsiveness to polarization entrepreneurs, there has been little attention yet paid to other political institutions where polarization may occur outside of the U.S. Congress. Specifically, there is a dearth of literature examining polarization on the U.S. Supreme Court or at any other level of courts in the U.S. government. Court polarization is an important area of focus for research on polarization, as it provides an institutional contrast to the elective branches, a unique and uniquely small set of elites who have a unique relationship with mass public opinion and the institutional elites elsewhere in the federal government, and a complete set of explanatory voting data that rivals even that of the U.S. Congress as a basis for examining polarization and ideology.

Although it is true that ideological differences and similarities among justices on the Court have received a great deal of attention from judicial scholars, ideological disparity and consensus are not polarization per se. Polarization is conceptually distinct from ideological location and raw agreement versus raw disagreement. DiMaggio et al. (1996) defined polarization as a state where opinions on an issue are opposed in reference to a theoretical maximum, and argued that polarization as a process refers to the increase in that opposition over time. At first blush, one may be tempted conflate polarization with an agreement–disagreement difference engine. After all, agreement versus disagreement is essential to the DiMaggio et al. definitions of polarization. Although there is no question that polarization is a function of voting agreement versus voting disagreement, assessing polarization requires additional information. Polarization speaks to the distribution of agreement and disagreement within an institution, group, or cohort. It is not enough to aggregate differences or agreements. Rather, we must focus on the spread or dispersion of that agreement or disagreement on the Court relative to a maximum (or a past distribution), and just as important, we must account for the shape of that distribution—specifically the degree to which the distribution has become more bimodal, that is, polarized. Where individuals are located on the spectrum and how many other individuals are proximate to them on that distribution is essential to the concept of polarization and distinguishes it from mere ideological difference accounting. Different measures of polarization have been developed to capture these characteristics.²

The research paradigm studying ideological behavior on the Court is well-developed and well-explored, but this literature has not, for the most part, focused specifically on the concept of polarization as a separate characteristic of voting behavior on the Court. Scholars and Court observers have certainly discussed polarization on the Court as a real and important phenomenon, and many have pointed to polarization on the Court as a factor in the quality and nature of the Court’s judicial output (Clark, 2009a; Lazarus, 1998; Rosen, 2007). However, studies examining polarization on the Court or incorporating measures of judicial polarization are relatively few in number. Furthermore, not all of them have accounted for both spread and shape of opinion distributions. For example, Corley, Steigerwalt, and Ward’s (2013) study of consensus on the Court uses a measure of polarization defined as the absolute distance between the median ideological justice and the most extreme ideological justice. Although Corey’s measure importantly captures spread in the distribution, it does so without assessing either the shape of the distribution or the extent to which justices are alienated from or identify with one another as a whole. Tom Clark’s article on polarization is the best effort to date to examine judicial polarization incorporating identification and alienation, and it is an important first step toward developing conceptually appropriate measures of judicial polarization that can be used to study behavior on the courts. Clark (2009a) develops a measure of judicial polarization based on the Esteban and Ray polarization measure, in addition to assessing the traditional measures of polarization. Although primarily a methodological article, Clark (2009a) does examine aggregate polarization trends as well as assessing the effect of polarization on dissents.

Ideological movement is the empirical bridge between the study of polarization on the Court and the study of ideological behavior on the Court. It is an aspect of judicial behavior that has been mostly accounted for with anecdotes by scholars and journalists alike. Judicial scholars are just beginning to quantify and assess the heretofore well-known fact that justice ideology is not necessarily static over time. Although the ideology of judges tends to be relatively stable and predictable, judges can and do change their ideological disposition over their judicial careers. The most comprehensive accounting of ideological change on the Court to date can be found in Epstein et al.’s study of ideological drift on the U.S. Supreme Court (Epstein et al., 2007). Their analysis of judicial common space (JCS) scores finds that nearly every justice serving since the 1930s has moved either to the Left or the Right during their time on the Court. However, they characterize this movement as “ideological drift”—drift implies motion without intention or direction. They thus provide no theoretical mechanism by which to explain such movement comprehensively or systematically. Polarization provides just such a mechanism. Cass Sunstein et al.’s (2006) work on intra-court polarization and the affects ideologically extreme justices can have on their fellow members of the bench highlights the importance of polarization as a conceptual tool for understanding judicial behavior. Sunstein et al. found that polar justices tended to act as ideological gravity wells with respect to their ideological compatriots—drawing their more moderate brethren of the same ideological bent toward themselves, and thus the poles, on the federal appellate bench. Sunstein et al. illustrate the utility of polarization as a descriptive model of judicial ideological evolution and how the ideological distribution of courts changes over time.

The Supreme Court and Sources of Institutional Constraint

One of the enduring debates in the judicial politics literature is over whether and to what extent the U.S. Supreme Court operates in a constrained institutional environment. Attitudinalists argue that the Court exists in a relatively unconstrained environment, with the Court free to act as it will within its domain of authority (Segal & Spaeth, 2002). Justices vote their ideal points in conference and write opinions consistent with those ideal points. The Court’s decisions on the constitutionality of enacted laws and regulations, and its statutory interpretations, go largely unanswered by other political actors (Hagle & Spaeth, 1991).

However, other scholars, particularly those adhering to rational choice theory, have posited that the Court is bound by the limits of its institutional legitimacy, which in large part is shaped by public opinion and the democratically responsive institutions of U.S. government, such as the U.S. Congress (Bailey, 2007; Bergara et al., 2003; Gibson, Caldeira, & Spence, 2003; Harvey & Friedman, 2006; Mishler & Sheehan, 1996). There are opportunities for strategic actions and sophisticated votes both within the institution of the Court, in policy competition with fellow justices, and without, in policy competition with other actors in the U.S. political system (Clark, 2009b; Harvey & Friedman, 2006; Hoekstra & Johnson, 2003; Owens, 2010; Sala & Spriggs, 2004; Segal, Westerland, & Lindquist, 2011). Strategic concerns, that is, agenda setting, may constrain behavior in an environment where the Court is extreme relative to the congressional status quo (Harvey & Friedman, 2009; Hausegger & Baum, 1999; Hettinger & Zorn, 2005; Ignagni & Meernik, 1994; Martin, 2001). As competing institutions polarize, the Court may polarize as well. Court polarization may be a response to institutional signals, a shift in institutional norms, a strategic response to the changed SOP environment, or a function of the changing composition of the Court (Anderson & Tahk, 2007). Furthermore, polarization in the public may also lead to polarization on the Court, either directly through signaling or indirectly through democratic processes.

Judicial polarization is an important empirical phenomenon which may be shaped by the institutional constraints of the SOP system. Assessing polarization on the Court has the potential to not only contribute to a greater understanding of the behavior of judges and justices but also spur development of empirical models of court behavior within the environmental constraints of the SOP institutional context of the federal government. One primary avenue of inquiry is that of the relationship between the American electorate and courts as elite political institutions. Although the courts are the most insulated institutions in the American political system (life tenure, high impeachment bar, operates in relative obscurity), the degree to which the court is or is not responsive to mass political opinion is a topic of much interest and scholarly work among students of judicial politics (Gibson et al., 2003; Mishler & Sheehan, 1996). Furthermore, mass political polarization may have an indirect effect on the Court, through the more democratically responsive American political institutions (i.e., Congress) that compete with the Court over policy in the SOP context. Thus, political polarization at the mass level may have a direct or indirect influence on decision-making on the Court and within the federal and state court systems. This could manifest as Court polarization or depolarization concomitant with the polarization trends in the mass public or other institutions of American government.

The role of the other branches of the federal government is particularly relevant to the study of judicial polarization when one turns to potential causes and consequences of judicial polarization (Bailey, 2007). Polarization in one institution may beget polarization in another institution. This is particularly likely to be the case when the composition of one institution is dependent on that of the other institution, as is the case with the U.S. Supreme Court and the Congress and President (Bailey, 2007; Bergara et al., 2003; Gillman, 2002; Harvey & Friedman, 2006, 2009; Hausegger & Baum, 1999; Hettinger & Zorn, 2005; Segal, 1999). It is dependent on the President for nominations and on the Senate for confirmation. Thus, selection to the judicial branch occurs within a strategic context which may be influenced by polarization within and between these institutions (Primo, Binder, & Maltzman, 2008; Rohde & Shepsle, 2007). Indeed, to the degree that new justices taking seats on the Court leads to political polarization, nominations matter (Moraski & Shipan, 1999; Shipan & Shannon, 2003). There is little question that political polarization in Congress has had an impact on the increasingly caustic confirmation proceedings for the federal courts, and that may be a function of polarization trends on the Court.

Judicial polarization is substantially shaped by the Court’s institutional environment, and it thus substantially affects the judicial output of the Court. Polarization on the Court may lead to more decisions out of step with not only public opinion but also with the other branches of the federal government (Epp, 1999; Hansford, 2011; Mishler & Sheehan, 1996). This harms the Court’s legitimacy as a policymaker, inducing competing branches to use their prerogatives to reign in the Court. Although institutional responses may be mitigated by the brinkmanship and gridlock characteristic of elective branch polarization, those same factors may degrade traditional institutional barriers that have served to stymie Court curbing. The filibuster on judicial nominations, curtailed in the Senate in 2014, is just one example of institutional changes that have been adopted to make it easier for polarized majorities to impose their will (Kane, 2013). Judicial polarization can significantly disrupt the ability of the Court to be responsive to signals from the public, the Congress, and the President. Polarization of the Court could also seriously affect the Court’s internal workings. It may disrupt Court norms, such as stare decisis, as polarization in Congress has disrupted the institutional norms for using the filibuster. It could lead to anomalies in workload, agenda setting, overturn rates, dissent and concurrence rates, and the size of majority coalitions. Polarization serves as a key indicator of the state of the internal environment of the Court and it influences the Court’s interaction with its external environment.

Measuring Ideology of the President, on the U.S. Supreme Court, and in the U.S. Congress

Determining the ideology of political actors is no simple matter. Although “cheap talk” abounds in the polarized segments of political talk shows on news and radio, the actual behavior of political actors may diverge significantly from their rhetoric. Furthermore, behavior may often be inconsistent with stated ideological beliefs for a variety of prudential concerns. For decades, scholars have been forced to rely on imperfect measures such as interest group scores and partisan affiliation as a basis for distinguishing between political actors. Fortunately, advances in defining the political ideology of policy actors have led to comprehensive models of ideological behavior within discrete political institutions in the United States. Poole and Rosenthal developed D-W NOMINATE scores as measures of revealed ideological behavior in Congress, allowing for cross-sectional comparisons of congressional behavior and for comparisons of ideological dispositions over time and the ideological composition of Congress from congressional term to congressional term (Poole & Howard, 1997; Poole & Rosenthal, 2001). D-W NOMINATE scores recover an ideological space and locate congress members within that space through a multi-dimensional scaling method that takes into account all the votes cast during a particular congressional term (Poole, 1998).

For judicial ideology, I use the two conventional measures of revealed ideology on the Court based on merit votes in cases decided: Segal–Cover scores and JCS scores. The Segal–Cover scores are based on a content analysis of four newspaper editorial pages’ coverage of U.S. Supreme Court nominees prior to their confirmation (Segal & Cover, 1989; Segal, Epstein, Cameron, & Spaeth, 1995). Segal–Cover scores range from 0 (conservative) to 1 (liberal). Segal–Cover scores have the benefit of being exogenously determined—They are not a derivative of votes the justices have taken while on the Supreme Court. However, they have the disadvantage of being static—Segal–Cover scores do not vary once a justice is on the Court. The second measure is the Martin–Quinn JCS scores. JCS scores are based on a methodology similar to that used by Poole and his coauthors. JCS scores are endogenously determined—the JCS ideology measure is based on the votes that the measure is often used to predict—however, they are a dynamic measure of ideology. JCS scores for justices vary from term to term, allowing for an assessment of how justice ideology evolves over time on the Court, just as D-W NOMINATE does with Congress. Bailey (2007) has noted that using D-W NOMINATE as a proxy for Congressional preferences while using JCS scores as a measure of Court preferences for cross-institutional and cross-temporal comparisons is problematic. Although Bailey’s preference measures are an important methodological advancement in cross-institutional and cross-temporal comparisons of behavior, the focus of this analysis is not to assess whether the cross-institutional positions are comparable (i.e., whether similar scores mean the same thing in both institutional contexts) but rather to determine whether there is a causal mechanism that exists between the disposition of ideological behavior across institutions and across time. For that purpose, it is not strictly essential that D-W NOMINATE and the JCS scores are measuring similar attributes in the respective institutions. Furthermore, Bailey’s preference estimates use a bridging technique that limits its applicability to those issues which have been taken up by all of the included actors and institutions. This restricts the number of data points that can be analyzed, complicating efforts to use them to study polarization of preferences on the Court and between the Court and other institutions. Although it may be possible to include Bailey preference estimates in a future study of polarization on the Court and between the Court, Congress, and the President, this analysis is restricted to Segal–Cover and JCS scores as proxy for ideology on the Court and D-W NOMINATE as a proxy for ideology of the President and in Congress.

Measuring and Modeling Judicial Polarization

Key questions about the definition and measurement of political polarization persist. What are the relevant dimensions (e.g., ideology), and what empirical tests will identify polarization in the mass public, among elites, within institutions, and over time? What is the appropriate baseline for comparison? Measurement and conceptual challenges have frustrated scholarly efforts to assess the relationship between the mass public and elite political players within and without the institutional political environment. Recent advancements in our measurement and understanding of polarization have begun to be incorporated into the study of the judiciary (Bailey, 2007; Clark, 2009a; Corley et al., 2013; Duclos, Esteban, & Ray, 2004; Esteban & Ray, 1994). In addition, journalists, lawyers, and legal scholars routinely use polarization as an ad hoc explanation for any number of policy outputs from the Supreme Court (Rosen, 2007).

Thus, the first step in assessing political polarization on the Court is adopting a conceptual definition subject to empirical investigation. DiMaggio et al. (1996) developed empirical measures of polarization that account for inter-group distances (difference of means), dispersion in an opinion dimension (standard deviation), and the relative bimodality of that dimension (kurtosis) in their massive study on political polarization. Some of these are useful in the study of judicial polarization, whereas others are not. Inter-group means cannot be calculated between groups of justices on the U.S. Supreme Court, as there are not two well-defined groups on the court between which to calculate an absolute difference (Clark, 2009a). Party is an unreliable indicator of judicial ideology/philosophy. Some very liberal justices have been Republican identifiers or appointed by Republican presidents (Earl Warren, Harry Blackmun, John Paul Stevens, David Souter, etc.) and some conservative justices have been Democrat identifiers or appointed by Democratic presidents (Felix Frankfurter, Byron White, etc.). Two applicable measures used by DiMaggio et al. account for distributional characteristics such as kurtosis (shape) and standard deviation (spread). Kurtosis and the standard deviation of the distribution of ideology in a small-n setting, such as the Court, may be problematic as measures of polarization.³ Given this, one must be cautious in drawing strong conclusions using kurtosis as a measure of bimodality. That said, kurtosis does correlate with other measures of polarization, and the distribution of ideology on the Court is unlikely to fall into the set of distributions where kurtosis is more likely to be a poor proxy for bimodality (Balanda & MacGillivray, 1988; Clark, 2009a).

One theoretical drawback of the common polarization measures is the absence of a weighting function for the differing levels of importance that actors in the space place on a particular issue or set of issues or considerations (DiMaggio et al., 1996). An issue or policy-relevant dimension can exhibit polarization but have little real-world relevance if that dimension is collectively viewed as unimportant or not salient. This is a particular problem when assessing voting behavior on the courts. A number of studies have noted the importance that case salience and precedent play in establishing a hierarchy of cases within a jurisprudential regime and across regimes, and these may play an important role in judicial decision-making independent of the revealed ideology of the judges (Clark & Lauderdale, 2010; Gillman, 1999; Gillman & Clayton, 1999a, 1999b; Ho & Quinn, 2010; Klarman, 2004; Kritzer & Richards, 2003; Richards & Kritzer, 2002). Indeed, norms of adherence to precedent may mitigate the effects of polarization within the strategic environment. Furthermore, focus on merit votes misses a great deal of information communicated by the Court in its decisions (Gillman, 1999; Ho & Quinn, 2010). This analysis does not address case salience or the impact of precedent in its measurements, except to the extent revealed ideology on the Court reflects these factors indirectly. Nor does this study go beyond the votes on the merits, given the exclusive use of measures of ideology on the Court. These are important innovations and improvements to be incorporated in future research.

Another theoretical drawback is the absence of a density component in the traditional polarization measures. As Esteban and Ray (1994) have argued, polarization is not merely a function of distance (inter-group alienation) but also a function of intra-group identification. It matters whether one is on an island, alone and distant from others, versus being in a “band of brothers” who share the same views or other relevant characteristics. On the Court, this could come into play in decisions to write solo opinions that represent ideological novelties compared with the views of the other justices, both in and out of the majority. Clark (2009a) developed a measure of judicial polarization that incorporates identification and alienation into the conceptual definition and uses it to assess polarization on the Court. However, this statistic is expressed at the natural court level in the Segal–Cover scores and the institutional level per term in the JCS, frustrating efforts to assess individual justice contributions to polarization on the Court.⁴ Although it cannot be used to assess change at the justice level of analysis, it is an excellent measure of judicial polarization at the court level of analysis, and hence I use it in the court-level analyses. I also use kurtosis and standard deviation as measures of polarization on the Court, with the appropriate caveats. Clark does find these measures correlate with his polarization statistic, particularly the standard deviation. I thus take a shotgun approach in this analysis: Where at all possible within the boundaries of methodological and data limitations, I use the traditional measures of polarization and the Clark polarization statistic to provide the most complete picture of court and justice polarization within the Court and across judicial terms. By deploying an assortment of distinct yet imperfect measures of polarization, the aggregate findings should reflect a whole truth greater than the sum of its parts.

Judicial Polarization Data and Research Questions

I use two distinct measures of justice ideology, one compiled by Segal and Cover (1989) and one from Martin and Quinn (2002). The Senate and presidential ideology data consist of the Poole-Rosenthal D-W NOMINATE scores from 1937 to 2008 (Carroll, Lewis, Lo, Poole, & Rosenthal, 2008). Data on the partisan composition of the Senate by congressional term were obtained from The Historical Atlas of Political Parties in the United States Congress (Martis, 2012). Data on the partisanship of the President were obtained through presidential biographies.⁵ The justice data set is a data file of ideological estimates and partisan environment variables for Supreme Court justices from 1937 to 2008. The court data consist of ideological estimates and partisan environment variables for the Court by term from 1950 to 2008. Polarization on the Court over time is examined using all available data (1937-2008). However, polarization in the SOP models is assessed from 1950 rather than 1937, given that most theories of political polarization in Congress use the 1950s as the earliest starting point for political polarization (e.g., Clark, 2009a; Poole & Rosenthal, 1984).

I test four basic research questions on U.S. Supreme Court ideological polarization.

Research Question 1: Has the Court become more polarized over time?

I examine polarization on the Court using two kinds of periodization—chief justice regimes and the Roe v. Wade jurisprudential regime.

Research Question 2: Has it done so in response to polarization in its institutional environment, specifically the Senate and the President?

Research Question 3: Do justices change ideologically the longer their careers are on the Court?

Research Question 4: Do justices change ideologically in response to polarization in the Court’s institutional environment, again specifically the Senate and the President?

I assess the first research question at the zero-order level of analysis. I assess the second, third, and fourth questions in the context of the institutional environment the Court exists in. I use two different types of measures of polarization in the Court’s strategic environment: partisan and ideological. For the partisan models, I use a measure of partisan polarization for the Senate and the partisanship of the President, and for the ideological models I use a measure of Senate ideological polarization and a measure of the distance between the median Senator and the President’s D-W NOMINATE score. Both types are used to assess the effect of that environment on the linear trend in polarization on the Court. To test the third and fourth research questions, I look at three different ways that justices can shift on the ideological spectrum over the course of their careers: ID, total ideological movement, and polarization (movement away from neutral point in the ideological space).⁶

Court and Justice Ideological Polarization Analyses—Data Structure

I conduct two different sets of analyses on judicial polarization, both of which are expressed at the level of the judicial term. The court polarization unit of analysis is an aggregation of justice ideological estimates for each judicial term included, whereas the justice polarization analysis data are structured with observations constituting the calculated difference between each justice’s last term and first term on all of the included variables. For the institutional environment analyses, it is particularly important to describe the data structure for both sets of analyses to show how the justice ideology is expressed over time in both the time-series and cross-sectional data sets. Variables included in the analyses are calculated over varying time scales. JCS scores are calculated based on case votes in each judicial term, thus these ideological estimates vary by judicial term. However, the Segal–Cover scores are a priori estimates of justice ideology, consequently, they are static for a justice’s career. Hence, an aggregate of justice ideology for a judicial term based on Segal–Cover scores value is constant across judicial terms within natural courts, only changing from natural court to natural court. Furthermore, congressional and presidential D-W NOMINATE–based variables are calculated at the level of the congressional term. Congressional terms span 2 years, thus there are two judicial terms for every congressional term. Across the 59 years of data included in the Court and justice polarization analyses, there are some 30 natural courts and 30 congressional terms. To maintain consistency across models and facilitate comparisons between the analyses, all of the Court judicial polarization models are expressed at the level of the judicial term. For the Segal–Cover models, the ideological estimates are expressed at the judicial term even though those values are constant from natural court to natural court. I do the same with respect to variables calculated at the level of congressional term—the values are expressed individually at the level of the judicial term but are constant across the two judicial terms inclusive of the congressional term (n = 59).⁷

Judicial Polarization Analysis 1—Court Polarization Over Time and Within Its Strategic Environment

Dependent Variables

Ideological Mean is the average ideological position for the Court, $\bar{I}$ , where $I$ is ideological position as measured using the Segal–Cover or JCS methods, $i$ is the judicial term, $j$ is the individual justice on the Court, $n_{i}$ is the number of justices for a given judicial term (typically 9), $i$ , and $i = 1 to 59$ , that is, each judicial term from 1950 to 2008.

{\bar{I}}_{i} = \frac{\sum_{j = 1}^{n_{i}} I_{i j}}{n_{i}} .

Ideological Standard Deviation is the standard deviation of the ideological distribution for the Court in a given judicial term.

I C S D_{i} = \sqrt{\frac{\sum_{j = 1}^{n_{i}} {(I_{i j} - {\bar{I}}_{i})}^{2}}{n_{i} - 1}} .

Ideological Kurtosis for a Supreme Court term, $k_{i}$ is a measure of bimodality for the distribution of Justices’ ideology scores in the given term. When kurtosis = 0, this distribution is centered on the normal distribution (mesokurtic). Negative kurtosis scores indicate thin tails, center, or both, or else multi-modality, and positive scores signify a distribution more peaked than the normal, with fat tails.

k_{i} = n_{i} {\frac{\sum_{j = 1}^{n_{i}} {(I_{i j} - {\bar{I}}_{i})}^{4}}{{(\sum_{j = 1}^{n_{i}} {(I_{i j} - {\bar{I}}_{i})}^{2})}^{2}}} - 3 .

Clark Polarization is a measure of polarization on the Court based on Esteban and Ray (1994), and is defined as a weighted index of the size of and distance among discrete groups, where a group is defined as one or more justices with the same ideal point. The total Court polarization for a judicial term is a linear function of the distance between all individual justices weighted by the mass of individual justices at each given ideal point. The measure weights the dispersion of the ideological estimates of the justices by group size, and thus accounts for heterogeneity between groups and homogeneity within groups of justices located at the same ideological point.⁸

Independent Variables

Court Term is the year that corresponds with each term of the U.S. Supreme Court in the data set. This variable is used to assess trends in polarization. A positive value for Court Term in the standard deviation model indicates increasing polarization, while a negative value would indicate depolarization. A positive value for Court Term in the kurtosis model indicates a more platykurtic distribution of ideology, which would thus indicate an increase in polarization, while a negative value would indicate a more leptokurtic distribution and suggest depolarization.

Democratic President (DP) is coded 1 if the President is a Democrat and a 0 if he is a Republican.

Presidential Ideological Polarization is the absolute distance between the D-W NOMINATE scores of the median U.S. Senator, $\overset{⌣}{S}$ , and the President, P, in the congressional term corresponding to judicial term $i$ .

P I P_{i} = | {\overset{⌣}{S}}_{i} - P_{i} | .

Percent Senate Democrat (PSD) is the percentage of Democrats in the U.S. Senate during judicial term $i$

Senate Partisan Polarization is the difference between the average D-W NOMINATE scores for Republicans in the Senate, ${\bar{S}}_{R}$ and the Democrats in the Senate ${\bar{S}}_{D}$ in the congressional term corresponding with judicial term $i$

S P P_{i} = {\bar{S}}_{R i} - {\bar{S}}_{D i} .

Senate Ideological Polarization is measured as the standard deviation of all D-W NOMINATE scores for senators in a given congressional term.

S I P_{i} = \sqrt{\frac{1}{n_{m i}} \sum_{m = 1}^{n_{m i}} {(S_{i m} - {\bar{S}}_{i})}^{2}} .

Method of Analysis

In the first cut at the data, I examine the trend in polarization using two distinct periodizations: chief justice regimes and the jurisprudential regime that demarcates the Court’s abortion rights cases, centered on the seminal case of Roe v. Wade. The motivation for examining polarization across chief justice regimes is fairly straightforward, as this is commonly used in the judicial behavior literature (Caldeira & Zorn, 1998; Kobylka, 1989; Maltzman & Wahlbeck, 1996; Slotnick, 1978; Walker, Epstein, & Dixon, 1988). Chief justice regimes serve as indicators of significant shifts in the Court’s membership and jurisprudential focus. The chief justice has a number of institutional prerogatives which allow him to influence the output of the Court. Two list just a few: The chief is always the most senior justice on the Court, irrespective of time served, the chief controls the institutional mechanisms of the Court (opinion assignment, case loads, etc.), the chief justice sets the agenda for the Court and has first privileges in conference, and the chief serves as an opinion leader among his fellow justices.

A second rationale for using chief justice regimes is the fact these regimes are concomitant with the political changes in the other branches of government—Indeed the shift in regimes is often correlated with, if not a consequence of, political and partisan change in the elective branches, particularly the President. As Skowronek (1993) argues, reconstructive presidents—those presidents who seek to alter the dominant political order rather than operate within it—attempt to recast the dominant constitutional regime. Chief justice regimes are a periodization of the history of the Court that captures these evolutionary and often revolutionary historical forces. The Hughes regime saw the last vestiges of the Lochner-era Republican justices, that is, the Four Horsemen, retire from the Court, and the Stone and Vinson Courts realized Franklin D. Roosevelt’s New Deal vision of constitutionalism, as he replaced or won over the recalcitrant justices who had frustrated his agenda (Skowronek, 1993; Whittington, 2009). Although Chief Justice Warren was a Republican nominee, he was integral to the civil liberties and social justice focus of the Court’s ascendant liberal regime that coincided with and was reinforced by the presidencies of Kennedy and Johnson. And in many respects the Burger regime, and its four Nixon-nominated, Republican appointees, is a function and embodiment of the reactionary response to the Warren and Johnson era liberalism. It was part and parcel of Nixon and Ford administrations efforts to institute a new constitutional regime (Skowronek, 1993; Whittington, 2009). And last, the Rehnquist and Roberts Courts reflect the constitutional innovations of the New Right, which emerged concomitant with Reagan and was solidified by the two Bush presidencies. In each case, the new political regimes—in particular the reconstructional regimes of Roosevelt, Nixon, and Reagan—changed the Court’s composition and interpretation of constitutional authority in ways captured by the chief justice regimes (Skowronek, 1993; Whittington, 2009).

The rationale for examining chief justice regimes dovetails with that of using jurisprudential regimes. Jurisprudential regimes are defined by the structuring of case factors relevant to decision-making, such as where the Court sets a level of scrutiny or a balancing test to be used in future cases (Richards & Kritzer, 2002). Richards and Kritzer (2002) posited that case factors would matter to justices differently after a new regime was established. This framework permits the assessment of judicial polarization as it relates to the Court’s involvement in the “culture war” which has influenced polarization in the elective branches, specifically abortion. The New Right’s emergence was both a function of and defined by the Court’s watershed decision in Roe v. Wade. As Whittington argues, it spurred Reagan and his attorney general, Edwin Meese, to adopt an “alternative constitutional discourse” that endorsed the oppositional views of the Court articulated by earlier reconstructive presidents. Whittington points to Roe as a polarizing event, particularly for the Democratic Party (Whittington, 2009). Hence, I define three distinct periods of jurisprudential regimes on abortion in the data set: pre–Roe v. Wade (1950-1973), before the Court defined the right to privacy as inclusive of the abortion decision by women and applied strict scrutiny within the trimester framework; Roe v. Wade to Planned Parenthood v. Casey (1973-1992), the period after Roe and before the Court shifted legal regimes to the “undue burden” hinging on viability rather than doctor’s rights in Casey; and Planned Parenthood v. Casey to Gonzales v. Carhart, where the Court rejected abortion rights arguments in upholding a ban on partial birth abortion. I examine changes in average polarization between these regimes of the Court.⁹

For the chief justice regimes, I report trends in polarization on the Court, in the Senate, and between the Senate and the President using Z-normalized averages for Clark polarization from Hughes to Roberts. The Z-normalization is necessary to depict the changes in polarization from regime to regime with each institution’s polarization depicted on the same scale. Though there are differences between the two measures, their commonality suggests that the measures are tapping into a real characteristic of the Court (Figure A-3).¹⁰

I use a difference of means test (independent-samples t test) to assess the difference between all polarization measures for the JCS and Segal–Cover scores for the Court between the jurisprudential regimes. For example, to compare the Pre-Roe regime from 1950 to 1973 (23 judicial terms) and the Roe to Casey regime covering the judicial terms from 1974 to 1992 (18 judicial terms), for a polarization measure (e.g., kurtosis), I calculate the mean polarization for the first regime, $r_{1}$ , the pre-Roe regime, and the mean polarization for the second regime, $r_{2}$ , the Roe to Casey regime, to conduct the difference of means t test between the two regimes.

t_{P (r 1, r 2)} = \frac{(P_{r 1} - P_{r 2}) - (μ_{r 1} - μ_{r 2})}{S_{P_{r 1} - P_{r 2}}},

where, $P_{r 1} - P_{r 2}$ is the polarization difference for the two sequential regimes, r₁ and r₂; $μ_{r 1} - μ_{r 2}$ is the zero under the null hypotheses; and, $S_{P_{r 1} - P_{r 2}}$ is the standard error of the difference between P_r₁ and P_r₂.¹¹

The second analysis of judicial polarization examines the Court’s trend in polarization in a strategic institutional context by estimating a multivariate autoregressive linear time trend model of the three polarization measures as well as the trend in mean ideology on the Court. I estimate full models for the mean, standard deviation, and Clark polarization measures, but estimate only the basic model for the kurtosis measure.¹² The models use two-, three-, or four-interval lagged dependent variables.¹³ I estimate Segal–Cover and JCS models using two different measures of Senate polarization: partisan polarization, which includes as predictors the difference between the Republican and Democratic D-W NOMINATE means in the Senate and the partisanship of the President, and ideological polarization, which uses as predictors the standard deviation of D-W NOMINATE scores in the Senate and the absolute difference between the median Senator on the floor and the President’s D-W NOMINATE score. As an example of the autoregressive models estimated hereafter, the full JCS model (controls + interactions) using the partisan polarization predictors is

J C S σ_{i} = α + β_{0} J C S σ_{i - 3} + β_{1} T_{i} + β_{2} P D_{i} + β_{3} P S D_{i} + β_{4} S P P_{i} + β_{5} (T_{i} \times S P P_{i}) + e_{i},

where $α$ = constant, $J C S σ_{i - 3}$ = lagged regressand, $J C S σ_{i}$ = standard deviation of the Court JCS scores for the $i th$ judicial term, $T_{i}$ = year of the judicial term, $P D_{i}$ = partisanship of the President in a congressional term for the $i th$ judicial term, $P S D_{i}$ = percent Senate Democrats in a congressional term for the $i th$ judicial term, $S P P_{i}$ = Senate partisan polarization in a congressional term for the $i th$ judicial term, $T_{i} \times S P P_{i}$ = interaction term between Court term and Senate partisan polarization, and $e_{i}$ = error term.

Judicial Polarization Analysis 2—Justice Ideological Evolution and Polarization

The second analysis shifts the unit of analysis from the judicial term of the Supreme Court to the justice, specifically the difference between the individual justice’s first term and last term on the Court. Thus, the data consist of a cross-section of the total justices who served on the Supreme Court from 1937 to 2008; where the observations consist of the difference between the Justice, Senate, and President characteristics for the first and last term of the $j th$ justice. This analysis assesses three difference changes in justice behavior: ideological directional change, ideological movement, and ideological polarization conditioned by changes in the strategic environment over the $i th$ justice’s career. As Segal–Cover scores are static, JCS scores are used exclusively.

Dependent Variables

The following measures test the justice ideological movement and polarization hypotheses. This analysis is conducted for Supreme Court justices on the Court from 1937 to 2008. The sample size, n = 44, is equivalent to the number of justices who served for more than 1 term during this period. The variables are indexed by j for each individual justice, and by f and l, respectively, for the first and last terms of the justice on the U.S. Supreme Court. Similarly, presidential partisanship for each President’s party is measured for the first and last terms of the justice. The U.S. Senate variables (percent Democrat and D-W NOMINATE–based measures) are likewise measured for the first and last terms of the justice.

Ideological Direction Change (IDC)—Last term-first term ideological difference for the $j th$ justice between their JCS ideology score in their last term, l, and their first term, f. A positive IDC reflects a conservative shift, while a negative IDC reflects a shift in the liberal direction from the first to last term.

Ideological Movement (IM)—Last term-first term absolute ideological difference for the $j th$ justice between their JCS ideology score in their last term and their first term. Larger values of IM correspond with greater ideological movement, irrespective of whether that movement is in a conservative or liberal direction from the first to the last term.

Ideological Polarization (IP)—Last term-first term ideological difference for the $j th$ justice between the absolute value of their JCS ideology score in their last term, and the absolute value of the JCS ideology score in their first term. Larger values of IP correspond with a movement toward the extremes of the ideological space and away from zero, the neutral point in the JCS ideological space. Justices can shift substantially in the ideological space without further polarizing from the first to the last term. For example, if a justice moves from a +2 JCS score to a −2 JCS score over the course of their careers, they have shifted four points in the liberal direction and had an ideological movement of four points. However, they would have a zero for ideological polarization. This polarization measure thus only assesses movement away from the neutral point (zero) in the ideological space.

Independent Variables

Supreme Court Justice Career—The length of the $j th$ justice’s Supreme Court career in number of years.

Change in Presidential Partisanship—The change in the partisanship of the President for the $j th$ justice from the term the justice was appointed to the Supreme Court bench, f, to the term the justice retired, l. This is a trichotomous variable with a +1 when the change is from a Republican President to a Democratic President from the first to the last term, a −1 when the change is from a Democratic President to a Republican President from the first to the last term, and a 0 when there is no change in the partisanship of the President from the first to the last term.

Change in Percent Senate Democrat—The change in the percentage of the Senate composed of Democrats for the $j th$ justice from the congressional term of the justice’s appointment, f, to the congressional term in which the justice retired, l. A positive value for this variable means the Senate became more Democratic, while a negative value means that it became more Republican from the first to the last term the justice sat on the Court.

Change in Senate Ideological Median—The change in the ideological median (D-W NOMINATE) of the Senate for the $j th$ justice from the congressional term of the justice’s appointment, f, to the congressional term in which the justice retired, l. A positive value indicates the Senate shifted to the Right over the course of the justice’s career, while a negative value means the Senate shifted to the Left.

Change in Senate Polarization ( ( $σ$ )—The change in the Senate’s polarization score for the $j th$ justice from the congressional term of the justice’s appointment, f, to the congressional term in which the justice retired, l, as measured by the standard deviation for all Senate D-W NOMINATE scores in the given congressional term. A positive value for the change in Senate polarization indicates that polarization increased from the first to the last term, while a negative value indicates that polarization decreased.

Method of Analysis

I estimate ordinary least squares (OLS) regression models of three different expressions of the change in ideological positioning of the justice from their term of appointment to their last term in office. This analysis will permit an assessment of the evolution of the justice’s views while on the Court and how institutional constraints affect justice ideology.

Judicial Polarization in Jurisprudential Regimes and Strategic Polarization Across Chief Justice Regimes

I begin with an examination of the trends in polarization in the jurisprudential and chief justice regimes. Figure A-1, depicts the distributions of the mean Court ideology score before and after Roe v. Wade. For both measures of ideology, there is a statistically significant shift between the pre-Roe and post-Roe periods. In the JCS and Segal–Cover, we see that the Court has shifted to the right of the ideological divide. Furthermore, we see a tighter distribution of ideological means for the Court’s terms after Roe than we do before it. Table 1 shows the mean difference t test between the pre-Roe and post-Roe periods for the JCS, the mean difference t test for Segal–Cover, and the three-regime variation for the JCS: Pre-Roe, Roe to Casey, and Casey to Carhart.¹⁴ Looking first to the two-regime comparisons, the shift to the ideological right by the post-Roe Court is substantively large and statistically significant (+0.74 and +0.35, respectively). The same is true for kurtosis. Recall that as kurtosis values move from positive to negative (platykurtic), a distribution is becoming approximately more bimodal. It is apparent that the post-Roe Court has moved to the right and the distribution of ideology on the court has become more platykurtic than it was prior to Roe v. Wade. The evidence on spread is mixed. Although there is an apparent increase in the standard deviation for both the JCS and Segal–Cover scores, it is only statistically significant for Segal–Cover (Table 1). The same increase is apparent in the Clark polarization measure, but it is only statistically significant for Segal–Cover. Figure A-2 graphically illustrates the distributions of the Clark polarization statistic and the average kurtosis for the JCS for the Court terms before and after Roe. Note the rightward shift in Clark polarization, indicating an increase in polarization from the pre-Roe to the post-Roe. The change in average kurtosis in the JCS is particularly striking. There is a strong shift in average per-term kurtosis post-Roe to a more platykurtic distribution. Figure A-3 shows the change in the distribution of the standard deviation in Segal–Cover scores between the two periods. The increasing trend in large standard deviations for Court terms in the post-Roe period vis-à-vis the pre-Roe period is apparent. Both Figures A-2 and A-3 show increasing polarization on the Court in the wake of the Roe v. Wad.

Table 1.

Paired t Test Comparisons of Jurisprudential Regime Mean Differences for All Court Polarization Measures.

	Segal–Cover pre–post t tests (two regimes)		JCS pre–post t test (two regimes)		JCS pre–post t test (three regimes)
	Pre-Roe \| Post-Roe		Pre-Roe \| Post-Roe		Pre-Roe \| Roe
Polarization characteristics	Estimate	p value	Estimate	p value	Estimate	p value
M	+0.35	<.001	+0.35	<.001	+0.45	<.001
SD	+0.07	.002	+0.07	.002	+0.21	.097
Kurtosis	−0.46	.077	−0.46	.077	−2.30	<.001
Clark PS	+0.12	.001	+0.12	.001	+0.32	.117
	Pre-Roe		Pre-Roe		Roe \| Casey
	Period M		Period M		Estimate	p value
M	0.70	—	−0.34	—	+0.35	<.001
SD	0.22	—	2.12	—	−0.18	.181
Kurtosis	−0.04	—	2.71	—	−1.01	.004
Clark PS	0.32		3.38		−0.34	.040
	Post-Roe		Post-Roe		Pre-Roe \| Casey
	Period M		Period M		Estimate	p value
M	0.35	—	0.29	—	+0.80	<.001
SD	0.29	—	2.24	—	0.04	.830
Kurtosis	−0.50	—	−0.12	—	−3.31	<.001
Clark PS	0.44		3.52		−0.02	.929

Note. JCS = judicial common space; PS = Polarization Statistic.

Turning to the three-regime comparison, Figure A-4 depicts mean difference comparisons between three separate periods, with the Casey-to-Carhart period a separate regime. I use a mean difference t test to assess the average differences between each of the bi-regime comparisons (Table 1). This analysis examines the paired regime differences. The shift to the Right from the pre-Roe period is apparent across all paired regime comparisons. The Court has become more conservative after Roe through Casey and from Casey to Carhart. The evidence on increased polarization over the three periods is mixed. The only consistent indicator of increased polarization is the average kurtosis of the ideological distributions, which became more platykurtic across the regimes. This is a statistically significant increase in bimodality from the pre-Roe period to Casey and from Casey to Carhart (−2.30 and −1.01, respectively). There is statistically significant evidence of an increase in the average standard deviation of ideology on the Court from Pre-Roe to Casey (+0.21). However, this does not hold when comparing the Roe-to-Casey period with the Casey-to-Carhart period. In fact, that comparison shows a decline, though statistically insignificant, in the average standard deviation for the Court (−0.18). Likewise, there is an increase in Clark polarization from the Pre-Roe to the Roe jurisprudential regime and an observed decrease in Clark polarization from Roe to Casey, though both fail to achieve statistical significance (p = .117 and p = .181, respectively). Thus, increasing polarization on the Court is apparent in both the bi-regime and tri-regime comparisons.

Figure A-5 depicts the polarization measures and average ideology for the 2008 term on the Court, the most recent term included in this study. As illustrated, Segal–Cover scores correlate with JCS scores, though not perfectly.¹⁵ These differences become more pronounced when examined over multiple terms. One reason for this is that Segal–Cover scores are static across judicial terms. Figure A-6 tracks the three Court polarization measures across chief justice regimes, from Hughes to Roberts. The trends show that polarization was on the decline from the Hughes through the Vinson regimes. The average standard deviation and Clark polarization declined, though the drop was more dramatic in the Clark polarization statistic. Increased polarization in these measures followed, from Vinson through the Burger regime. This has since declined somewhat in the Rehnquist and Roberts regimes, but overall rates of polarization are higher in the later regimes. The measure for bimodality does not track tightly with the other two trends in polarization. There is a sharp move from bimodality to unimodality from the Vinson to the Warren Courts followed by a steady increase in bimodality from Warren to Roberts (Figure A-6). This is not necessarily inconsistent as these measures are tapping into two distinct aspects of polarization: spread and shape. Although there may have been an increase in the spread of ideology on the Warren Court, a steady stream of Democratic appointments to the Court tilted that Court strongly leftward. The trends depicted in Figure A-6 demonstrate that the ideological right on the Court had dwindled in the post-Roosevelt era, and thus the distribution of ideology on the Court during the Warren regime is relatively unimodal. The reemergence of the Right as the Warren Court transitions to the Roberts Court is captured in the apparent spike in bimodality in the Court’s ideological distribution. Although the spread of Court ideology has somewhat declined since the Burger Court, the shape of the distribution of ideology best approximates bimodality in the Roberts regime of any included regime.

Figures 1 and 2 depict the Z-normalized polarization trends in the Court’s institutional strategic environment from Hughes to Roberts.¹⁶ It depicts polarization trends for the Supreme Court (Clark Polarization), the Senate (standard deviation of Senate D-W NOMINATE), and the President vis-à-vis the Senate chamber median (absolute difference between President D-W NOMINATE and median Senator’s D-W NOMINATE). There is a strong apparent linear trend across the chief justice regimes in the polarization of all three institutions, interrupted only by the drop in polarization observed between the Senate chamber median and the President during the Vinson regime (Figures 1 and 2). Although all three trends track fairly well together, the consistent upward trend of Senate polarization deviates from Court polarization, which declines slightly from the Burger regime to the Roberts Court. What is striking is the strength of the correlation between the President-Senate chamber median difference and that of Court polarization over the regimes. There is a remarkable consistency in the peaks and valleys of Court polarization and the ideological distance between the President and the Senate chamber median across chief justice regimes from Hughes to Roberts.

Figure 1.

Z-normalized trends in Senate, President-Senate, and court polarization (JCS) across chief justice regimes.

Figure 2.

Z-normalized trends in Senate, President-Senate, and court polarization (Segal–Cover) across chief justice regimes.

The strong linear trend across chief justice regimes apparent in the JCS is not nearly as evident using the Segal–Cover measure of ideology (Figure 2).¹⁷ Overall, there is a slight trend upward over the full series, though the Roberts and Rehnquist regimes mark a rather significant drop in polarization vis-à-vis the Burger regime. The Burger regime is far and away the most polarized regime in the series according to the Segal–Cover scores. The most striking difference between the Z-score for JCS Clark polarization versus the Segal–Cover scores are the Vinson and Warren regimes. The Vinson regime is the nadir of Court polarization in the JCS, while the Warren Court is second only to the Burger Court as most polarized regime. However, in the Segal–Cover scores, the Vinson Court has a higher level of polarization than the Warren Court does, and it is the Warren Court that has the lowest Clark polarization score. This incongruity is likely a function of the difference between the two ideological measurements. Many of the Republican nominees from the 1970s and 1980s were not the conservatives, or even centrists, that many Court observers expected them to be. This includes the newspaper editorial boards that provide the base data for the Segal–Cover scores. This reflects the significant difference between the expected ideology of the justice as captured by Segal–Cover and the actual behavior of the justice on the Court, as captured in the JCS.

Multivariate Analysis of Trends in Court Polarization Within the Strategic Institutional Environment

In the second part of the Court analysis, I estimate autoregressive regression models of the trends in polarization, as measured by the standard deviation, kurtosis, and Clark polarization of the JCS and Segal–Cover ideology scores from 1950 to 2008 (Tables 1 to 4).¹⁸ This broadens the time horizon of the analysis and permits the inclusion of additional factors in a multivariate model of the institutional environment. In the first set of models, I examine Court polarization in the autoregressive, multivariate context with partisan polarization predictors in the JCS (Table 2). There is clear and consistent evidence of judicial polarization on the Court from the 1950s to the present. The dispersion of JCS scores increases over time, as evidenced by the statistically significant (p < .001) term predictor in the standard deviation and Clark polarization models. Furthermore, the term predictor is negative and statistically significant in the kurtosis model, suggesting the Court has become more platykurtic (bimodal) over the past 60 years. All four models have relatively good model fits, with between a 20% and 40% reduction in error achieved in the estimation. The best performing model is the Clark polarization model. Nearly 40% of the variation in Clark polarization in the JCS is explained by variation in polarization in the Court’s strategic environment.

Table 2.

Determinants of JCS and Clark Court Polarization (Partisan Polarization Models), 1950-2008.

Variables	JCS M model	JCS SD model	JCS kurtosis model (1960)	Clark polarization model
Term	−0.041**	0.074***	−0.089***	0.104***
Term	(0.019)	(0.019)	(0.024)	(0.024)
Democratic President	−0.034	−0.066		−0.200
Democratic President	(0.074)	(0.073)		(0.127)
% Senate Democrats	0.232	−1.196*		−2.063*
% Senate Democrats	(0.701)	(0.691)		(1.160)
Senate Partisan Polarization	−22.382***	24.649***		39.715***
Senate Partisan Polarization	(5.7684)	(5.779)		(7.778)
SPP × Term	0.112***	−0.124***		−0.199***
SPP × Term	(0.029)	(0.029)		(0.039)
Intercept	81.003**	−142.932***		−20.219***
Intercept	(37.280)	(37.438)		(4.667)
Regress R²	.387	.316	.235	.395
N-Lag	4	3	3	3
Durbin–Watson	1.906	1.991	1.784	1.793
DFE	53	53	47	52

Note. JCS = judicial common space; SPP = . Senate Partisan Polarization; DFE = Degrees of Freedom Error.

p < .10. **p < .05. ***p < .01.

Table 3.

Determinants of JCS and Clark Court Polarization (Dispersion Models), 1950-2008.

Variables	M model	SD model	Kurtosis model (1960)	Clark polarization model
Term	−0.078***	0.107***	−0.089***	0.106***
Term	(0.026)	(0.027)	(0.024)	(0.039)
President—Average Senate Chamber Ideology Difference	−0.032	−0.072		2.017***
President—Average Senate Chamber Ideology Difference	(0.072)	(0.073)		(0.658)
% Senate Democrats	0.378	−1.248*		−1.793
% Senate Democrats	(0.689)	(0.691)		(1.152)
Senate Polarization	−55.619***	57.773***		64.834***
Senate Polarization	(13.949)	(14.460)		(21.475)
SPP × Term	0.279***	−0.291***		−0.327***
SPP × Term	(0.071)	(0.073)		(0.108)
Intercept	154.828***	−209.259***		−206.355**
Intercept	(51.782)	(53.78)		(77.347)
Regress R²	.409	.298	.235	.494
N-Lag	4	3	3	3
Durbin–Watson	2.078	2.003	1.784	1.768
DFE	53	53	47	52

Note. JCS = judicial common space.

p < .10. **p < .05. ***p < .01.

Table 4.

Determinants of Segal–Cover Court Polarization (Partisan Polarization Models), 1950-2008.

Variables	M model	SD model	Kurtosis model (1960)	Clark polarization model
Term	−0.016***	0.013***	−0.134***	0.018***
Term	(0.004)	(0.004)	(0.046)	(0.006)
Democratic President	0.018	−0.005	0.180	−0.008
Democratic President	(0.014)	(0.015)	(0.268)	(0.022)
% Senate Democrats	0.444***	−0.132	−4.289**	−0.040
% Senate Democrats	(0.138)	(0.140)	(2.433)	(0.207)
Senate Partisan Polarization	−11.488	27.357**	−38.207***	37.446**
Senate Partisan Polarization	(11.130)	(11.212)	(15.179)	(17.888)
SPP × Term	0.006	−0.014***	0.193***	−0.019**
SPP × Term	(0.006)	(0.006)	(0.076)	(0.009)
Intercept	30.738***	−25.070***	266.412***	−35.442***
Intercept	(6.878)	(7.215)	(90.708)	(11.643)
Regress R²	.805	.243	.257	.189
N-Lag	4	3	2	3
Durbin–Watson	1.938	1.682	2.054	1.823
DFE	52	53	54	53

p < .10. **p < .05. ***p < .01.

Table 3 introduces the alternative specifications for institutional polarization in the Senate and the President’s distance from the median Senator. The trend in polarization in these models is consistent with that reported in Table 2. The departure of FDR-appointed justices and the uniformly liberal Court of the Warren era are proximate determinants of the polarization trends modeled in Tables 1 and 2. Institutional factors also play a role, as evidenced by the term for the polarization of the U.S. Senate. It is highly significant, substantively impactful, and robust across all models. Generally, the ideological polarization models have slightly better fits to the data than do the partisan polarization models. However, there is little difference in the magnitude or direction of the coefficients. The exception is the Clark polarization model. The ideological polarization model explains 10% more of the variance in Clark polarization in the JCS vis-à-vis other models. Furthermore, whereas the presidential partisanship predictor was not significant in the partisan models, the President-median Senator absolute distance predictor is highly significant. This is important because that variable best approximates polarization in the confirmation process for Supreme Court justices. Thus, polarization on the Court is concomitant with polarization in the U.S. Senate.

The most parsimonious explanation for this result is that an increasingly polarized Senate, which has great influence over the composition of the Court through advise-and-consent, has produced a mirror image of itself on the Court. Both may be a function of polarization in the American electorate, as the effects of polarization are likely to be reciprocal (Hetherington, 2009). Although the Court is somewhat insulated from democratic forces, the Court is subject to macro-polarization trends both directly and indirectly through the institutional context. The Court is responsive to polarization entrepreneurial interest groups, such as the Federalist Society (conservative) and the Coalition for a Fair and Independent Judiciary (liberal), as well as polarization between the branches of government. This leads to a polarized confirmation process for the Court selected by and responsive to the polarized elective institutions which seek to place increasingly ideologically “pure” justices on the Court and whose policy demands in the institutional environment increasingly reflect a polarized policy space.

In the second set of models using the Segal–Cover scores, I obtain similar results for polarization: statistically significant increases in the dispersion and Clark polarization of Segal–Cover scores and in the bimodality of the Court in the later terms (Tables 3 and 4). Senate polarization covaries with Court polarization in the standard deviation, Clark polarization and kurtosis models and in the expected directions. However, there are significant differences in the fit for the Segal–Cover models vis-à-vis the JCS models. The term for the proportion of Senate Democrats drops out in the kurtosis model and barely achieves statistical significance in the standard deviation model. Generally, the Segal–Cover models are poorer fits than were the JCS models, particularly so for Clark polarization. The partisan polarization model for Clark polarization performs similarly to that of the JCS model in terms of the magnitude and direction of the coefficients and which predictors achieve statistical significance (Table 4). But this is not the case with the ideological polarization model. The Clark polarization model using the alternative ideological polarization predictors is the only model where Senate polarization fails to achieve statistical significance (Table 5). In that model, it is only the absolute distance between the President and the median senator that achieves statistical significance. Yet again, these differences may be a function of the differing measures of ideology.

Table 5.

Determinants of Segal–Cover Court Polarization (Dispersion Models), 1950-2008.

Variables	M model	SD model	Kurtosis model (1960)	Clark polarization model
Term	−0.016***	0.137**	−0.185***	0.011
Term	(0.004)	(0.005)	(0.070)	(0.009)
President—Average Senate Chamber Ideology Difference	0.020	−0.005	0.192	0.236**
President—Average Senate Chamber Ideology Difference	(0.014)	(0.014)	(0.271)	(0.108)
% Senate Democrats	0.429	−0.143	−4.294*	0.095
% Senate Democrats	(0.134)	(0.137)	(2.550)	(0.201)
Senate Polarization	−24.195	59.942**	−90.533**	40.561
Senate Polarization	(26.242)	(30.003)	(38.877)	(47.662)
SPP × Term	0.013	−0.031**	0.458**	−0.021
SPP × Term	(0.013)	(0.015)	(0.196)	(0.024)
Intercept	31.316***	−26.501**	367.191***	−22.323
Intercept	(9.539)	(11.225)	(137.993)	(17.901)
Regress R²	.826	.172	.234	.230
N-Lag	4	3	2	3
Durbin–Watson	2.050	1.738	2.046	1.909
DFE	52	53	53	53

p < .10. **p < .05. ***p < .01.

Justice Polarization and Determinants of Ideological Evolution on the Court

The results for the justice polarization models are reported in Table 6. Three different specifications of ideological change are used to assess whether justices evolve over their careers on the Court and to what extent this is determined by factors in the Court’s institutional environment: ideological movement, ID, and polarization. Conventionally, we would not expect justices to move much over time. Supreme Court justices come to the Court with well-developed political philosophies, ideological beliefs, partisan commitments, and legal and ethical principles. In many cases, they have already spent decades deciding legal issues and writing legal opinions. Thus, it is somewhat surprising that career length is positively correlated with justice ideological evolution. The basic and full IM models evidence excellent fit, with nearly half of the variance in ideological movement on the Court explained by the model $(R^{2} \approx . 45)$ . Furthermore, the ID models demonstrate that this movement tends to be in one direction—justices tend to become more liberal the longer they are on the Court (Table 6). This is empirical evidence of a pronounced leftward ideological shift the longer a justice is on the bench, dubbed in popular accounts as the “Greenhouse Effect” (Shafer, 2007). The polarization models uniformly fail in their tests of the two hypotheses. No variables achieve statistically significance, the model fits are very poor, as evidenced in the miniscule R² reported and the failed F tests. I find no evidence that the length of a justice’s career, the polarization trend in the Senate, the change in the partisan composition of the Senate, or the chance in the President’s partisanship has any influence over justice polarization in the ideological space.

Table 6.

Determinants of Ideological (JCS) Justice Polarization, 1937-2008.

Variables (Δ = change from last to first year on court)	Ideological direction basic model $(L - F)$	Ideological direction full model $(L - F)$	Ideological movement basic Model $\| L - F \|$	Ideological movement full model $\| L - F \|$	Polarization basic model $\| L \| - \| F \|$	Polarization full model $\| L \| - \| F \|$
Supreme Court Career (years)	−0.089***	−0.078***	0.048***	0.052***	0.024	0.024
Supreme Court Career (years)	(0.025)	(0.026)	(0.016)	(0.016)	(0.023)	(0.024)
Δ Senate Polarization ( $σ$ )	15.622***	14.361**	6.2662*	4.466	−3.343	−5.464
Δ Senate Polarization ( $σ$ )	(5.624)	(6.147)	(3.534)	(3.620)	(5.049)	(5.573)
Δ Senate % Democrat		−3.132		−2.468**		−2.865
Δ Senate % Democrat		(3.568)		(1.231)		(3.289)
Δ Presidential Partisanship		0.352		0.100		−0.190
Δ Presidential Partisanship		(6.041)		(0.214)		(0.323)
Δ Senate Median Ideology		−2.353		—		−4.193
Δ Senate Median Ideology		(6.130)				(5.558)
Intercept	0.3440	0.225	0.093	0.047	0.120	0.165
Intercept	(0.354)	(0.374)	(0.222)	(0.226)	(0.318)	(0.339)
R ²	.237	.2734	.432	.485	.027	.055
N	44	44	44	44	44	44
Pr > F	0.004	0.028	<.0001	<.0001	0.574	0.816

p < .10. **p < .05. ***p < .01.

Justices evolve ideologically over time. Two examples will suffice to illustrate this phenomenon, using the JCS. In 1970, Harry Blackmun’s first term on the Court, only Chief Justice Burger was more conservative than Blackmun. In his 10th term on the Court, Justice Blackmun crossed the ideological Rubicon, joining the Court’s left-leaning justices (Figure 3). By his final term in 1993, Justice Blackmun was as far to the Court’s ideological left as he had been to the ideological right when he began his career. His ideological score in the JCS was second only to that of Justice Stevens in its leftward location. Justice Blackmun’s ideological evolution is well-known and often cited as an example of presidential appointment regret (Biskupic, 1999; Ruger, 2005). Although Blackmun may be an unusual case, ideological movement on the Court is not. Justice Thomas is widely considered to be a stalwart of conservatism on the Court, and this is confirmed by his extreme rightward location in the JCS. However, Justice Thomas has evolved ideologically over his career. Unlike Blackmun, Thomas has moved further to the right. Justice Thomas’s absolute ideological evolution is only half that of Blackmun’s, but Thomas has become nearly twice as conservative as he was when he first joined the Court. As Epstein et al. noted in their study of ideological drift on the Court: Ideological evolution on the Court is the rule and not the exception (Epstein et al., 2007).

Figure 3.

Career trends in judicial common space scores for Justice Thomas and Justice Blackmun.

Why are some justices akin to leaves adrift on an ideological pond whereas others of their brethren take sail in chosen direction, and still others are anchors, holding firm no matter where the ideological winds blow? The answer is that these ideological sojourns are not made in a vacuum. Justices are part of the Brethren, a small clique of specialized judicial actors, and thus may be influenced by both the strategic environment and inter-personal influences within that institution. Without, the Court exists in a unique and competitive institutional context which constrains and influences their behavior. Within, justices may serve as ideological lodestones, drawing fellow justices to their own ideological viewpoint. Not only are justices aware of and may be responsive to the views of their fellow justices, but so too they may respond to mass and elite public opinion. Hence, it follows that justices may be more or less responsive to these internal and external pressures. Evidence of ideological movement in response to the institutional environment of the Court is presented in Table 6. Senate polarization is statistically significant with a positive coefficient in three of the four specifications for the IM and ID models. Although it drops out in the IM full model, it is still in the expected direction.¹⁹ Justices evolve ideologically on the Court in response to shifts in the ideological polarization in the Senate. The IM and ID models provide support for this susceptibility to external influence, especially the coordinate branches of the federal government. The tendency of justices to move to the Left is a function, in part, of the Senate role in the composition of the Court. That the Senate has been majority Democrat for the majority of the included justice’s confirmation hearings and tenure on the Court has undoubtedly influenced the appointment process and the sitting justices themselves. Other external factors create leftward headwinds. The Court is responsive to legal elites and legal institutions which shape the debate over judicial policy. Institutions such as the American Bar Association, which reflects the liberal tilt among legal professionals, help establish a range of judicial policies acceptable to the legal elite that is leftward shifted. Furthermore, favorable press coverage of justices becoming more liberal, the so-called “Greenhouse Effect,” may induce more justices to evolve in that ideological direction (Shafer, 2007; Wertheimer, 2008).

Judicial Polarization on the U.S. Supreme Court

I set out to assess the nature and extent to which the U.S. Supreme Court has polarized over time, to determine if the polarization trend in the Court was influenced by contextual factors within and without the Court, and to examine the ideological disposition of justices over their tenure on the Court to see whether and to what extent they change and what influences that change. The results of this analysis are summarized in Table 7. Depicted are the specific Court and justice hypotheses, the average approximate direction of the coefficients for each variable across the models, the average approximate statistical significant of the partial effects across the models, and the summary findings of the analysis. I find strong evidence of increasing Court polarization over time and that both congressional and presidential polarization has influenced this trend. This suggests Court polarization has not only become more pronounced over the last 50 or more years, but also that Court polarization appears to be responsive to coordinate institutions. I find strong polarization across chief justice regimes and that it is concurrent with trends in Senate and presidential polarization. I find mixed evidence that the Court has polarized across abortion rights jurisprudential regimes, and I do not find that individual justices become more polarized over time. However, justices do shift ideologically the longer they are on the Court, and this shift on the whole is to the left. The analysis suggests that this movement is not merely a function of drift but rather the consequence of the political environment within which the Court makes policy. Elite polarization entrepreneurs, public opinion shaped by media coverage, and institutional constraints may all play a part in causing justices to move Leftwards over their careers. These findings are robust across multiple specifications of the models, multiple alternative measurements, and controlling for other factors which might influence polarization.

Table 7.

Summary of Findings on Supreme Court Ideological Polarization, 1937-2008.

General hypotheses	Specific hypotheses	Average coefficient direction	Average statistical significance	Findings
Court polarization over time	Chief justice regimes	$↑$	***	There is a strong linear trend of increasing ideological polarization on the Court over chief justice regimes and the pre-Roe to post-Roe jurisprudential regimes.
Court polarization over time	Jurisprudential regimes	$↑$	***
Court polarization over time as a function of the institutional environment	President partisanship	$↓$		Taken on the whole, there is a substantively and statistically significant relationship between Court polarization and the strategic institutional environment within which it operates. Court polarization correlates with Senate ideological and partisan polarization as well as Senate-President polarization in the Clark polarization model.
	Senate partisan polarization	$↑$	***
	Senate ideological polarization	$↑$	***
	President-Senate polarization	$↑$	*
Justice career Ideological evolution	Ideological direction	$↓$	***	Justices evolve ideologically over the course of their careers, and the longer their careers the more they shift ideologically. Justices tend to become more liberal the longer they are on the Court. While justices polarized over the course of their careers, the trend is not statistically significant.
	Ideological movement	$↑$	***
	Ideological polarization	$↑$
Justice career Ideological evolution as a function of the institutional environment	President partisanship	$↑$		There is mixed evidence of justices shifting ideologically as a function of the strategic institutional environment of the Court. Senate polarization is strongly associated with ideological direction change and somewhat with ideological movement. And Senate partisanship influenced ideological movement. No apparent relationship was found between justice polarization and the institutional environment.
	Senate ideological polarization	$↑$	**
	Senate median Ideology	$↓$
	Senate partisanship	$↓$	*

p < .10. **p < .05. ***p < .01.

The implications of this analysis are various. Court polarization and polarization in the Court’s strategic environment influences justice ideological disposition and evolution. It may have wide-ranging effects on the judicial nomination process, the Senate judicial confirmation process, and thus the policy output of the Court. Ideological polarization influences competition between the branches over policy. Justices becoming more liberal over time may influence policy debates over the life tenure of justices. It could change how presidents, particularly Republican presidents, choose nominees for the Court. The emergent ideological polarization on the Court has profound implications for how the Court will figure in the national political discourse and how its interactions with polarization forces will affect the constitutional order. Whittington posits that the judiciary’s interpretation of the Constitution is an implicit challenge to presidential authority and juxtaposes it with presidential efforts to define the constitutional order (Whittington, 2009). A polarized Court is more vulnerable to the attacks reconstructive presidents tend to direct at the Court. Reconstructive presidents may take advantage of Court polarization to politicize Court decisions in a bid to detract from the Court’s constitutional authority and to challenge judicial supremacy. Polarization bodes ill for the prospects of forging harmony between the branches in a consensus on constitutional meaning necessary to avoid reconstructional moments (Whittington, 2009). President Obama’s State of the Union challenge to the Supreme Court over Citizens United and Justice Alito’s “not true” rejoinder is illustrative (Barnes, 2010). Polarization may lead to further and more frequent attacks on judicial authority and judicial supremacy in the constitutional order.

Judicial polarization is an important phenomenon of the U.S. Supreme Court and a topic ripe for further study. Factors ripe for examination include the institutional and regime exogenous shocks which can spur or mitigate ideological polarized behavior on the Supreme Court. Future research should investigate the issue and regime-specific polarization on the Court. Further research into the nature and causes of ideological polarization on the Court and at other levels of the judiciary is also warranted.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Supplementary Material

Online appendices are available on the American Politics Research website at .

Notes

Author Biography

Donald Michael Gooch is an Assistant Professor of Political Science in the Department of Government at Stephen F. Austin State University. He received his MA in political science from the University of Arkansas, Fayetteville in 2002, and his PhD in political science from the University of Missouri-Columbia in 2009. His research agenda includes political polarization, the Supreme Court from both behavioral and institutional perspectives, campaign finance regulation, civic education, formal theory and the spatial theory of voting.

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