Criticisms of the ICJ’s Approach to Examining Opinio Juris: Case Studies of All ICJ Case to Date

To identify the existence and content of a rule of customary international law, it is necessary to ascertain whether there is a general practice that is accepted as law (opinio juris). Opinio juris is a qualitative, subjective and psychological element based on a notion of legal right or obligation. As such, the question of opinio juris is highly controversial in determining its existence. This was the same for the ICJ’s latest judgment delivered in the Nicaragua v. Colombia case, where the ICJ found opinio juris from the relevant state practice, even if such practice may have been motivated in part by considerations other than a sense of legal obligation. Such an attitude was contrary to the fundamental significance of the subjective element in identifying customary international law, which has been consistently emphasised by the ICJ itself. This article aims to highlight the general tendency of the ICJ in examining opinio juris through case studies of all ICJ cases to date, including the Nicaragua v. Colombia judgment. For this purpose, first, this article assesses the latest judgment of the ICJ. Subsequently, this article expands the scope of analysis to the entire case-laws of the ICJ up to date, including both contentious and advisory opinion cases. This analysis reveals that, among the categorised approaches, the ICJ has examined opinio juris separately in the least number of cases. Given the importance of identifying customary international law, one of the major sources of international law, many concerns and criticisms can be raised over such a general tendency of the ICJ. In this respect, this article contends that the ICJ should further elaborate its reasoning and criteria to examine opinio juris in determining customary international law in actual proceedings.