Supreme Court Justices Move Left Over Time (More than They Move Right)

Mark Fisher, Nicholas Georgakopoulos

We modify a 2002 IRT model applied to Supreme Court justices’ votes, adopting a nonconjugate prior for case parameters and allowing justices' locations on a simple left/right spectrum to vary with each change in the court's composition. We examine the joint distribution of justices' locations, showing the tendency to move left over the course of their tenures.