Assessing Preference Change on the US Supreme Court

Abstract

The foundation upon which accounts of policy-motivated behavior of Supreme Court justices are built consists of assumptions about the policy preferences of the justices. To date, most scholars have assumed that the policy positions of Supreme Court justices remain consistent throughout the course of their careers and most measures of judicial ideology—such as Segal and Cover scores—are time invariant. On its face, this assumption is reasonable; Supreme Court justices serve with life tenure and are typically appointed after serving in other political or judicial roles. However, it is also possible that the worldviews, and thus the policy positions, of justices evolve through the course of their careers. In this article we use a Bayesian dynamic ideal point model to investigate preference change on the US Supreme Court. The model allows for justices' ideal points to change over time in a smooth fashion. We focus our attention on the 16 justices who served for 10 or more terms and completed their service between the 1937 and 2003 terms. The results are striking—14 of these 16 justices exhibit significant preference change. This has profound implications for the use of time-invariant preference measures in applied work.