Measuring Ideological Polarization in State Supreme Courts 1970–2019

ABSTRACT Objectives This article provides an estimation of ideological polarization for each state's supreme court between 1970 and 2019. Methodology We measure ideological polarization using the Esteban and Ray polarization model with Party‐Adjusted Judge Ideology (PAJID) scores. Our measure covers all 50 state supreme courts from 1970 to 2019. We validate the measure by showing it is associated with a higher probability of observing a dissenting opinion. We then investigate the extent to which institutional and environmental factors are associated with state court polarization. Findings Our measure reveals substantial variation in polarization across states and over time. Courts with higher levels of ideological polarization are more likely to issue dissenting opinions. This relationship supports the validity of the polarization measure. Further, we find that courts with partisan selection systems are less polarized than courts using other selection systems. However, we find that environmental factors, defined as the polarization observed in the state's federal Congressional delegation, have the biggest association with state court polarization. Conclusions This study introduces the first validated, longitudinal measure of ideological polarization in state supreme courts. It provides a foundation for future research on judicial behavior, institutional dynamics, separation of powers, and judicial legitimacy in the states.