Over the last ten years, judges, scholars, and policy makers have argued— quite vehemently at times—about whether U.S. courts should use transnational sources of law to interpret domestic legal doctrine. All eyes in this debate focus on the U.S. Supreme Court and its alleged use and misuse of transnational law. And almost all the debates are normative. Some scholars and judges argue the Court is correct to use transnational law. Others believe to do so is constitutional apostasy.

Still, the controversy seems to have generated more heat than light. Among the clamor can be found little empirical work on the conditions under which Supreme Court Justices actually use transnational law. Is it in fact the case that only liberal Justices employ transnational law—or do conservatives as well? In addition, there is little work on which countries Justices cite when they do use transnational law. Do they cherry pick whichever country works best in a given case, or is there a constraint via legitimacy on which countries to examine and cite?

The authors provide one of the most systematic empirical explorations of the Court’s use of transnational law to date. Their results challenge conventional wisdom and upend the existing debates over transnational law. The data show that Justices are more likely to reference transnational law when they exercise judicial review and when they overturn precedent, which likely explains much of the controversy around the practice. Importantly, the data show, further, that all Justices cite transnational law. Liberals cite transnational law when they render liberal decisions, and conservatives cite transnational law when they render  conservative decisions. Liberals and conservatives alike employ such law be- cause both are ideologically conscious and strategic judicial actors who seek to support their decisions with as much persuasive material as possible.

Finally, the results suggest that Justices cite countries with regard to their political and legal characteristics. They cite what the public would consider to be among the most legitimate countries across the globe. In other words, on the whole, Justices seem to borrow from countries most like the United States. Whether these results are good or bad is unclear; what is clear, however, is that the normative debate over using transnational law must take a turn and address the authors’ findings. Scholars can then pay more attention to best practices and the policy implications of cases that cite transnational law.