Unfolding Judicial Ideology: A Data-Generating Priors Approach with an Application to the Brazilian Supreme Court

Spatial models, in the form of latent response theories (ideal point estimation), have been widely used to study the voting behavior of judges in collegial courts. In some specific institutional contexts, building coherent testable hypotheses with conventional methodology is challenging. We set up a non-parametric method to identify the number and nature of the latent ideological traits allegedly orienting judicial voting behavior in the absence of prior information regarding the nature of their preferences. We draw information from explorative cluster analysis conducted on votes cast by judges in the decisions of the court to construct priors in the context of Item Response Theory. We concentrate on the Brazilian Supreme Court in the period 2009–2018. We primarily find that votes express a split which groups judges into two distinct clusters. On one side, we find judges appointed further back in time and with longer tenure on the bench; on the other side, we observe judges more recently appointed and with shorter experience. Judges are likely to respond to the presidential appointer and to elements related to their origin, university education, and career background (aside from the guidance of their own experience). Our study provides an original empirical approach that is not limited to the Brazilian Supreme Court, but is suitable to investigate judicial voting behavior when the nature of potential ideological drivers is debatable, controversial, or unknown.