What Difference Does a New Justice Make?

Abstract: 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. We find direct effects from new justices: the majority opinions of the new justice differ from those of the old justice even absent a move in the median voter. In addition, the new justice's appearance on the Court induces strong but varying peer effects among the other justices. 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.