Shaping Supreme Court Policy Through Appointments: The Impact of a New Justice

Different theories of decision making on the U.S. Supreme Court make radically different predictions about the impact of a new Justice on the Court. Using a new method for locating average majority opinion locations in a policy space, we test the predictions in a case study: the replacement of Justice Potter Stewart by Justice Sandra Day O’Connor. We find a direct effect from the new Justice: O’Connor’s majority opinions were more conservative than Stewart’s and neither Justice’s majority opinions were located at the Court’s median. In addition, O’Connor’s appearance on the Court induced strong but varying peer effects among the other Justices: conservatives and most moderates wrote more conservative majority opinions while some liberals wrote somewhat more liberal opinions. These findings appear quite harsh to the Median Voter Model of Supreme Court decision making and diverge in important ways from the predictions of the Median Majority and Monopoly Author models. They appear somewhat friendlier to recent “author influence” theories. We discuss the implications for the President’s ability to shape the Court’s policy through appointments. The evidence suggests those opportunities are substantial.