We review the substantial literature on estimating judicial ideology, from the US Supreme Court to the lowest state court. As a way to showcase the strengths and drawbacks of various measures, we further analyze trends in judicial polarization within the US federal courts. Our analysis shows substantial gaps in the ideology of judges appointed by Republican Presidents versus those appointed by Democrats. Similar to trends in Congressional polarization, the increasing gap is mostly driven by a rightward movement by judges appointed by Republicans. We conclude by noting important avenues for future research in the study of the ideology of judges.
Bonica, Adam, and Maya Sen. "Estimating Judicial Ideology." Journal of Economic Perspectives 35.1 (Winter 2021): 97-118.