Supreme Court Ideology Project

Alexander Tahk

Department of Political Science

University of Wisconsin–Madison

Stephen Jessee

Department of Government

University of Texas at Austin

Estimates updated on April 26th, 2015

This website uses Bayesian ideal point estimation to update our beliefs about the location of Supreme Court justices, following the work of Martin and Quinn and others. We begin with a prior belief about the location of each justice and update these beliefs after each opinion delivered by the court, learning from the justices’ votes what their underlying ideologies are likely to be. Our prior beliefs for Justices Rehnquist, O’Connor, Breyer, Ginsburg, Scalia, Thomas, Kennedy, Stevens and Souter are based on their voting history over the past two years. By contrast, our prior beliefs about the ideologies of Roberts, Alito, Sotomayor, and Kagan are somewhat more subjective—based on our knowledge about the appointing president’s beliefs and past information about the justice’s history and confirmation hearings.

The main questions of interest focus on these new justices and what we can infer about their ideological position relative to the other sitting justices and to the justices they replaced. We are able to obtain estimates of these relative locations as well as estimates of each justice’s probability of being the most conservative justice or being, for example, more conservative than the justice he replaced.

After updating our estimates through the present, we find that both Roberts and Alito seem to have judicial ideologies that are on the conservative side of the Court, but appear to be less conservative than both Scalia and Thomas. Both of these newly appointed justices seem to be more conservative than both Rehnquist and O’Connor (see the trend lines page for more information), but it is highly unlikely that either is the most conservative justice on the Court today. Justice Alito seem to be slightly more conservative than Chief Justice Roberts, but this finding is fairly uncertain and more cases must be observed before we can answer this question with some certainty.

The animation on the left side of the page shows the updating process at work. If you don’t see a moving picture to the left, you can download Flash here. It plots the posterior density for each justice’s ideal point—what our belief should be about each justice’s location—updating after each vote to the present time.