Assessing Preference Change on the U.S. Supreme Court

To understand policy-motivated behavior of Supreme Court justices it is necessary to measure their policy preferences. To date, most scholars have assumed the policy preferences of Supreme Court justices remain consistent throughout the course of their careers, and most measures of judicial ideology – such as Segal and Cover (1989) scores – are time invariant. This assumption is facially valid; Supreme Court justices serve with life tenure, and are typically appointed after serving in other political or judicial roles. However, it also possible that the world views, and thus policy preferences, of justices evolve through the course of their careers. In this paper we use a Bayesian dynamic ideal point model to investigate preference change on the Supreme Court that allows for justices’ ideal points to change over time in any smooth fashion. We focus our attention on the sixteen justices who served for ten or more terms and completed their service between 1937 and the present. The results are striking – fourteen of these sixteen justices exhibit significant preference change.