Peer Effects on the United States Supreme Court

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. ∗Holden: UNSW Business School, email: [email protected]. Keane: Oxford University and UNSW Australia Business School, email: [email protected]. Lilley: Harvard University Department of Economics, email: [email protected]. We are grateful to Rosalind Dixon, John Friedman, Christopher Malloy, Emily Oster, Jesse Shapiro and Andrei Shleifer for helpful discussions, and to seminar participants at Harvard and MIT.