SCOTUS Appointments Bring More Than Their Own Vote, They Have Peer Effects

As a new appointment to the U.S. Supreme Court looms, it is worth noting that there is well documented evidence that the way judges who sit in multiple judge panels vote in cases is influenced by who else is on the panel of judges deciding the case (continuing the theme that judges are people too).

Using data on essentially every US Supreme Court decision since 1946, we estimate a model of peer effects on the Court. We consider both the impact of justice ideology and justice votes on the votes of their peers. 

To identify these peer effects we use two instruments. The first is based on the composition of the Court, determined by which justices sit on which cases due to recusals or health reasons for not sitting. The second utilizes the fact that many justices previously sat on Federal Circuit Courts and are empirically much more likely to affirm decisions from their “home” court. 

We find large peer effects. Replacing a single justice with one who votes in a conservative direction 10 percentage points more frequently increases the probability that each other justice votes conservative by 1.63 percentage points. In terms of votes, a 10 percentage point increase in the probability that a single justice votes conservative leads to a 1.1 percentage increase in the probability that each other justice votes conservative. Finally, a single justice becoming 10% more likely to vote conservative increases the share of cases with a conservative outcome by 3.6 percentage points – excluding the direct effect of that justice – and reduces the share with a liberal outcome by 3.2 percentage points. In general, the indirect effect of a justice’s vote on the outcome through the votes of their peers is typically several times larger than the direct mechanical effect of the justice’s own vote.

Richard Holden, Michael Keane, & Matthew Lilley, Peer Effects on the United States Supreme Court (February 14, 2017).

President Trump's current nominee for the U.S. Supreme Court is roughly in the same point in the ideological spectrum as Justice Scalia whose death created the vacancy, so the pre-vacancy and post-appointment court can be expected to be pretty similar ideologically.

But, if the new appointee is more of a "team player" than Scalia was, the peer effects might be greater. And, this also suggests that the current eight member U.S. Supreme Court is more liberal than one might naively assume simply from the loss of Scalia's vote, because Scalia's peer influences on the other eight justices are also now absent.