What factors allow some international courts (ICs) to rule against the express preferences of powerful member states, whereas others routinely defer to governments? While judicial independence is not the only factor explaining the strength of a given international institution, it is a necessary condition. The paper first develops three sets of competing explanatory variables that potentially can explain variations in the judicial independence of ICs. The causal effects of these explanatory variables upon variance in judicial independence are investigated in a comparative analysis of the ACJ, ECJ, ECtHR and IACHR. It is found that the threat of governmental noncompliance and the strength of the constituency possessed by an IC have the most explanatory power, although there is still a significant residual that can only be explained by looking at factors relating to judicial choices and agency.