The constitutional rules that govern how states engage with international law have profound implications for foreign affairs, yet we lack comprehensive data on the choices countries make and their motivations. We draw on an original dataset that covers 108 countries over a nearly two-hundred-year period to map countries’ foreign relations law choices and trace their evolution. We find that legal origins and colonial legacies continue to account for most foreign relations law choices. A small number of models emerged in the nineteenth and early twentieth century in Western Europe, subsequently spread through colonial channels, and usually survived decolonization. Departures from received models are rare and usually associated with major political shifts. Prominent political science accounts that emphasize how states design their foreign relations law strategically to enhance their international credibility or entrench democracy or human rights appear to have limited explanatory power over the bulk of foreign relations law today.

At first blush, “comparative international law” might sound like an oxymoron. In principle, international law—at least when it arises from multilateral treaties or general custom—applies equally to all parties or states. As a result, international lawyers often resist emphasizing local, national, or regional approaches due to the field’s aspirations to universality and uniformity. Comparativists, meanwhile, frequently overlook the potential to apply comparative law insights to international law on the basis that “rules which are avowedly universal in character do not lend themselves to comparison.”

International legal scholars have long recognized the importance of the rules and processes by which states adhere to international legal obligations and “translate” them into their domestic legal systems. Research by political scientists on specific issue areas likewise increasingly recognizes that domestic implementation is crucial to international law compliance and effectiveness. Yet the lack of systematic data makes it difficult to assemble an overall picture of the relationship between international law and domestic law around the world, let alone to document its evolution over time. Recent qualitative surveys of state practice have begun to fill that gap, but provide only a snapshot in time and are limited to relatively few countries. Some quantitative projects cover more countries, but address only a limited number of questions based solely on the text of national constitutions.

Customary international law (CIL) is widely recognized as a fundamental source of international law. While its continued significance in the age of treaties was once contested, it is now generally accepted that CIL remains a vital element of the international legal order. Yet CIL is also plagued with conceptual and practical difficulties, which have led critics to challenge its coherence and legitimacy. In particular, critics of CIL have argued that it does not meaningfully affect state behavior. Traditional CIL scholarship is ill equipped to answer such criticism because its objectives are doctrinal or normative—namely, to identify, interpret, and apply CIL rules, or to argue for desirable changes in CIL. For the most part, that scholarship does not propose an explanatory theory in the social scientific sense, which would articulate how CIL works, why states comply, and why and how rules change.