Forecasting Opinion Assignment on the U.S. Supreme Court Paul J. Wahlbeck George Washington University Alyx Mark George Washington University Ryan Krog George Washington University Phillip J. Wininger George Washington University The chief justice does not possess extraordinary powers by virtue of that leadership post. Instead, the chief “presides over a conference not of eight subordinates, whom he may direct or instruct, but of eight associates who, like him, have tenure during good behavior, and who are as independent as hogs on ice.”1 One prerogative that the chief enjoys is the authority to assign the majority opinion. This responsibility gives the chief justice the ability to shape the Court’s agenda. By assigning the opinion, the chief justice is able to determine which justice will make the first policy proposal in the opinion-writing process with a unique ability to shape the law. Since the assigned author circulates a draft opinion before dissenting or concurring opinions, that author can secure commitments from colleagues.2 Because of the importance of opinion authorship, legal commentators follow each decision announcement with interest. Opinion assignment patterns give legal commentators clues that allow them to predict the outcomes of the remaining cases on the Court’s docket that term. Numerous legal and media outlets, such as the prominent SCOTUSblog, regularly highlight these conjectures in the days prior to notable Supreme Court decisions. For instance, approach the finale of the 2011 term, a whirlwind of speculation and guessing led up to the Court’s highly-anticipated decision regarding the constitutionality of the Affordable Care Act of 2010. A week prior to the Court’s announcement various legal commentators began predicting Chief Justice Roberts as the likely author of the case’s opinion, since the other predicted candidate for opinion author, Justice Kennedy, delivered the majority opinion in the highly salient Arizona immigration case immediately prior to the health care ruling.3 1 Of course, interest in opinion assignment extends beyond journalists and commentators. Recognizing opinion assignment as central to agenda-setting and case outcomes, political scientists, too, have long shown interest in the Court’s opinion assignment process. Following Walter Murphy’s 1964 classic The Elements of Judicial Strategy, an immense body of research has developed on the subject.4 Employing data derived from assignment sheets found in the justices’ papers, this research assesses the assignments of some previous chief justices—Vinson, Warren, Burger, and Rehnquist— and uncovers various causal factors that may underlie the process.5 However, despite this wealth of knowledge, political scientists have not applied their theoretically grounded explanations to forecast current opinion assignments, even as commentators and journalists continue to toss around their predictions. This comes as no surprise; political scientists have long eschewed forecasting methods. With the exception of a study by Martin and colleagues6 forecasting applications to the Supreme Court have been quite limited. In this chapter, we undertake a unique endeavor: we construct a theoretically- grounded model of Supreme Court opinion assignment and use it to predict assignments in out-of-sample data. Our research utilizes data from 2004 to 2009 Supreme Court terms to build an explanatory opinion assignment model. To cross-validate the model, we then make assignment predictions with respect to cases decided during an out-of-sample 2010 term. This attempt at making a real-world application is both methodologically and theoretically important. Forecasting and out-of-sample predictions provide significant analytical leverage, as testing predictions against future events is one of the most stringent checks of statistical models. To this end, Beck, King, and Zeng write, “even if 2 we have no interest in prediction per se, the poor forecasting performance of standard models indicates that we can improve on them to provide better knowledge of real-world causal relationships.”7 Consequently, we argue that it is important to revisit (and perhaps question) our understanding of the opinion assignment process. Testing our models of opinion assignment against future observations helps accomplish this goal. It is this challenge that we undertake as our principal responsibility in this project. More so than a model that seeks to predict the outcomes of cases alone, a model of opinion assignment allows us to think not only about the policy implications of a particular outcome, but how the predicted assigned author will incorporate their own unique insights into the drafting of the decision. This might have implications for court watchers, speculating on the impact of cases, and outside parties who have a stake, financial or otherwise, in the case. Moreover, our study also allows us to investigate opinion assignment practices during the current Roberts Court. It is important to understand Chief Justice Roberts’ decision-making process, as he will have many opportunities to develop and change legal policy during his presumably lengthy tenure on the Court. Thus, our model can potentially provide a first glimpse into how he will lead the Court and on how law will develop under his leadership in the coming years. In the sections that follow, we review extant research on the opinion assignment process and use this literature to derive hypotheses about the forecasting of opinion assignment. Next, we describe our data and methods and specify the series of statistical models designed to test both in-sample and out-of-sample forecasts. Along the way, we pay special attention to the challenges we face in testing our hypotheses given a lack of available data associated with several of our measures. 3 Opinion Assignment Decision Making Given the strategic environment within which the justices operate, political scientists have long attempted to uncover underlying motivations behind the chief justices' opinion assignment decisions.8 An array of causal factors has been explored. But the literature has sorted these factors into two primary goals in opinion assignment: (1) the strategic pursuit of policy preferences and (2) the desire to satisfy the Court’s organizational needs.9 Highlighting the first goal, political scientists David Rohde and Harold Spaeth aptly state, “the rational strategy for the assignor is to assign the opinion to the justice whose views are most like his own on the issue being decided.”10 This statement, of course, portrays the chief as a policy-minded justice who seeks to choose a writer who will construct a written opinion—and, thus, the content of law—that most closely reflects the chief’s preferred policy. This policy-centered view of opinion assignment transcends from a large body of research demonstrating that policy motivations guide an array of judicial behavior by Supreme Court justices.11 There may be more to the story, however. The second goal—the pursuit of organizational needs—suggests that the chief looks beyond ideological preferences in many cases. More specifically, research indicates that formal and informal norms confer upon the chief a special job obligation: the role of an administrator and representative of the Court.12 Internalizing this role, the chief may use opinion assignment to preserve judicial legitimacy, enhance harmony amongst justices, and promote smooth operation of the Court.13 These factors uniquely constrain the chief justice. Associate justice assignors do not weigh organizational factors.14 4 When these policy and organizational components of opinion assignment are viewed collectively, a nuanced process of opinion assignment unfolds in which the chief justice balances multiple goals.15 To help illustrate, this review highlights a body of scholarship exploring opinion assignment as a function of both goals. And, building upon this scholarship, it iterates several empirical hypotheses relevant to a forecasting model of opinion assignment. The Pursuit of Policy Goals Scholars readily acknowledge that the pursuit of policy preferences underlies a variety of Supreme Court behavior, including certiorari decisions, bargaining and accommodation over opinion content, decisions to write concurring or dissenting opinions, and general voting patterns.16 Thus, it is reasonable to surmise that opinion assignment is not exempt from policy-motivated behavior. Moreover, given that opinion assignment constitutes one of the chief’s primary responsibilities when voting with the majority, the chief may be able to exercise special policymaking power over the Court.17 In order to demonstrate how this power may operate, opinion assignment must be placed within the context of the dynamic give-and-take of opinion writing. Notably, a majority opinion generally requires five supporting votes; the opinion writer cannot institute his or her preferred policy by fiat. The opinion writer’s need to collect and maintain a majority leads to a process of bargaining and accommodation whereby justices haggle over how an opinion should textually unfold.18 An opinion writer may have to draft or alter the opinion to accommodate the views of other justices, especially when the majority appears to be small or fragile. Maltzman, Spriggs, and Wahlbeck find, for example, that writers circulate more opinion drafts—and, thus, engage in more 5 bargaining—when the majority coalition is small, majority coalition members are ideologically distant from the author,