Measuring Policy Content on the U.S. Supreme

Political scientists have developed increasingly sophisticated understandings of the influences on Supreme Court decision making. Yet, much less attention has been paid to empirical measures of the Court’s ideological output. We develop a theory of the interactions between rational litigants, lower court judges, and Supreme Court justices. We argue that the most common measure of the Supreme Court’s ideological output—whether the Court’s decision is liberal or conservative—suffers from systematic bias. We trace this bias empirically and explain the undesirable consequences it has for empirical analyses of judicial behavior. Specifically, we show that, although the Court’s preferences are positively correlated with the ideological direction of the justices ’ decision to reverse a lower court, the attitudes of the justices are negatively related—and significantly so—to the ideological direction of outcomes that affirm lower court decisions. We also offer a solution that allows scholars to work around this ‘‘affirmance bias.’’ O ver the last 50 years, political scientists and legal scholars have developed increasingly sophisticated models of judicial decision mak-ing, particularly for the U.S. Supreme Court. Scholars have demonstrated that a host of forces—including legal considerations (e.g., Richards and Kritzer 2002), the political preferences of justices (e.g., Rohde and Spaeth 1976; Schubert 1965; Segal and Spaeth 2002), strategic considerations arising from the relation-ships between the justices and between the Court and other policy makers (e.g., Epstein and Knight 1998