Ideal Point Estimation and Selection Bias on the United States Supreme Court

Abstract

This paper addresses a long-standing limitation of analyses of Supreme Court ideology, which is the fact that the Court's docket is discretionary and thus renders the pool of votes a highly non-random sample. While many scholars rely on ideology measures that rely on information drawn from justices' votes, I argue that the political conditions in which the Court operates has the capacity to fundamentally reshape the Court's incentives to take on certain types of cases (e.g., those of higher or lower salience), the votes on which we observe. In addition to demonstrating the dynamic nature of the substantive composition of the Court's docket, the paper goes on to show that justices' propensity to vote in liberally or conservatively is quite dependent upon the policy domain in which they are judging. Thus, if we base our analyses on justices' votes, we will have systematic bias in our inferences about Supreme Court justices' ideology and their relation to the other branches in the American separation-of-powers system. The paper additionally sheds new light on the asymmetric power of congressional and public opinion constraint on the Supreme Court, and the complications that it can present in using vote-based measures of judicial preferences.