Why the Supreme Court Issues Plurality Opinions

Many of the most important Supreme Court decisions of the past 20 years have failed to produce a majority opinion of the Court. Examples abound, including such pathbreaking decisions as Parents Involved in Community Schools v. Seattle School District No.1, Casey v. Planned Parenthood, and Rapanos v. United States. The reasons for the rise of plurality opinions are not well understood, either in the political science or legal literature. To fill this gap, this paper presents the first empirical study of why coalitions on the Supreme Court break down and, specifically, what factors contribute to the issuance of plurality opinions. The authors present the results of two empirical models. The first seeks to uncover the case-level determinants of plurality opinions using a universe of 5,711 cases from the United States Supreme Court Database. The authors measure three types of variables in the case-level model: (1) ideological factors such as the opinion author distance from the majority coalition and ideological heterogeneity of the coalition; (2) legal variables such as the classification of the case and whether it involves lower court conflict; and (3) contextual factors such as the number of amicus briefs filed and the number of issues involved in a case. Among other things, the authors find that cases involving constitutional interpretation are nearly 250% more likely to result in plurality opinions. Similarly, cases that are reargued or where the Court strikes a federal law as unconstitutional are also much more likely, on average, to result in plurality opinions. The second part of the model focuses on the influence of the independent variables on the actual choice that results in a plurality opinion - a Justice's decision to concur in the judgment rather than join the majority, join the majority and concur, or dissent. Using a multinomial logit model, the authors apply the independent variables in the case-level model (plus several more) to a universe of 44,647 votes by Justices occurring between October Term 1953 and October Term 2006. The single greatest determinant of whether a Justice will write an opinion concurring in the judgment is a Justice's distance from the author of the majority opinion. Significantly, many of the same variables studied in the author's case-level model are also statistically and substantively significant in the Justice-level model too.