| Abstract: | The foundation upon which accounts of policy-motivated behaviorof Supreme Court justices are built consists of assumptionsabout the policy preferences of the justices. To date, mostscholars have assumed that the policy positions of Supreme Courtjustices remain consistent throughout the course of their careersand most measures of judicial ideology—such as Segal andCover scores—are time invariant. On its face, this assumptionis reasonable; Supreme Court justices serve with life tenureand are typically appointed after serving in other politicalor judicial roles. However, it is also possible that the worldviews,and thus the policy positions, of justices evolve through thecourse of their careers. In this article we use a Bayesian dynamicideal point model to investigate preference change on the USSupreme Court. The model allows for justices' ideal points tochange over time in a smooth fashion. We focus our attentionon the 16 justices who served for 10 or more terms and completedtheir service between the 1937 and 2003 terms. The results arestriking—14 of these 16 justices exhibit significant preferencechange. This has profound implications for the use of time-invariantpreference measures in applied work. |
