Realist and (liberal) institutionalist thinkers in international relations have long used rational choice assumptions to explain states’ behavior, but they only more recently entered into legal intricacies or specific legal questions, especially in the United States. By now, this has become a joint enterprise by international lawyers, economists, and rational-choice political scientists. Ever more empirical research is generated, informing also about the empirical validity hypotheses held by rationalist scholars. The research mostly stands on two pillars: the rational choice assumption and, following the traditional international law assumptions in the aftermath of the Westphalian peace, the treatment of the nation-state as a unitary actor, or what has been described as a “black box” state. Rational choice analysis has been used to conceptualize or reframe international law generally, including its sources.