Benjamin E. Lauderdale and Tom S. Clark, “The Supreme Court’s Many Median Justices”, American Political Science Review, 106(4):847-866.
Theoretical and empirical research on the US Supreme Court has become increasingly channeled through a unidimensional spatial model of judicial decision-making. We argue that limitations in quantitative measurement of justices’ preferences have obscured the ways in which justices’ preferences vary across areas of the law and constrained the set of questions the research can pursue. We introduce a new approach, using multiple indices of substantive similarity among cases and a kernel-weighted optimal classification estimator to recover estimates of judicial preferences that are localized to particular legal issues as well as periods of time. Allowing preference variation across legal areas significantly improves the predictive power of estimated preference orderings versus a model that only allows for variation in preferences over time. We find judicial preferences are not reducible to simple left-right ideology and, as a consequence, there is substantial variation in the identity of the median justice across areas of the law during all periods of the modern court. These results suggest a need to reconsider empirical and theoretical research that hinges on the existence of a unitary and well-identified median justice.