Customary International Law Interpretation: The Role of Domestic Courts

Abstract

This chapter analyses a large dataset of domestic court decisions relevant to customary international law (CIL). It finds that domestic courts at times engage in CIL interpretation rather than mere ascertainment, even if they largely refrain from using that term. Domestic courts may interpret CIL autonomously, may defer to and validate international courts’ CIL interpretations, or interpret written documents, such as treaties, codifying CIL norms. Such practices bear out that domestic courts may consider some core CIL norms to be relatively stable and amenable to further refinement through interpretation. When interpreting CIL, domestic courts appear to resort mainly to systemic interpretation and interpretation on the basis of subsequent State practice. This reflects earlier findings by Panos Merkouris with respect to CIL interpretation by international courts. Domestic courts deciding cases on the basis of CIL may want to be more explicit regarding whether they engage in de novo CIL norm-identification or rather in the interpretation of pre-existing and stabilised customary norms. In case of interpretation, they may want to improve the methodological quality of their reasoning by pinpointing the canon of construction which they apply, and by relying on sufficient international (including foreign) legal practice.