Customary international law (CIL) is perhaps the main source of international law. It is defined in article 38 of the Statute of the International Court of Justice as “evidence of general practice accepted as law”. Two elements stem out of this definition: a widespread practice among states (State Practice); and a sense of legal obligation that prompts states to accept the practice as law (Opinio Juris). However, the exact characterization of each of these elements remain uncertain. Hence, the ongoing debate over what are the criteria for CIL.

Which factors indicate State Practice? Scholars recognize a non-exhaustive list of evidence, including treaties, governmental statements, decisions by domestic courts and the enactment of national legislation. But, how many states need to be engaged into the relevant practice and for how long? In respect to these enquiries, there are not absolute criteria, so their analysis has to be conducted on case-by-case basis. For instance, in the Right of Passage Over Indian Territory, the International Court of Justice (ICJ) admitted the existence of a local custom amid the parties in the dispute, despite India´s objection that no custom could be established between only two states. Conversely, in the Asylum Case, the ICJ was reluctant to recognize the existence of a regional custom, by imposing a high threshold for determining the existence of a practice, peculiar to Latin-American countries, about the right to qualify political offenses for the purpose of conceding an asylum.

In relation to Opinion Juris, the ICJ stated in the North Sea Continental Shelf cases, that “[t]he states concerned must therefore feel that they are conforming to what amounts to a legal obligation.” The problem, however, is how to demonstrate what a state believe when it decides to embrace a specific action. Moreover, the classic formulation of Opinio Juris entails a circular argument: CIL certainly requires Opinio Juris, understood as a sense of legal obligation. But if a state believes that some conduct is legally binding, it is only because such conduct already entailed a legal obligation. Then, why would we need the Opinio Juris in the first place?

The above-mentioned issues affect the analysis of CIL and its formation process. Professor Andrew Guzman came up with an alternative-rational approach, in order to explain CIL. According to him, if a state is aware that, by acting contrary to the expectations of other States with which it interacts, it will face reputational sanctions and reprisals, then that state will have incentives to behave the opposite way, i.e., the conduct expected by those other states. Consequently, the Opinio Juris is not about what a state considers is (customary) law; rather, the believe of other states is what counts for determining whether a specific conduct entails a violation of a rule of CIL. Hence, only to the extent that other states within the international community consider there is a legal obligation, a potential violator faces a rule of CIL. Furthermore, within this alternative approach, the importance of State Practice is limited at serving as indicator of the perception of states about the existence of a legal obligation (i.e., the Opinio Juris).

© Rafael Tamayo, 2014.