Locating Supreme Court Opinions in 'Doctrine Space'

Abstract

A significant limitation in the empirical analysis of judicial politics has been the difficulty of measuring judicial policy. Theories of bargaining and opinion writing make predictions about where an opinion will fall in policy space, but empirical tests of those theories have not benefited from direct measures of opinion location. This paper develops a scaling model to estimate opinion locations and justice ideal points along a common, continuous dimension using the citations between opinions as data. We assume that each opinion has a fixed location in this unidimensional doctrine space and that the probability of a citation that affirms rather than disputes the doctrine of the precedent decreases as the doctrinal distance between them increases. This proximity citation model is applied to original datasets of citations in search and seizure and freedom of religion opinions written by the Warren, Burger and Rehnquist Courts. We use the resulting estimates of opinion content to evaluate median and non-median voter theories of Supreme Court bargaining and opinion writing. We find striking empirical support for theoretical models that predict the majority opinion will fall at the ideal point of the median member of the majority coalition as opposed to the court's median or the opinion author's ideal point. Given the centrality of theories of judicial policymaking to various substantive problems in political science, the method of scaling opinions developed in this paper can facilitate a variety of future research.