Has the National Treatment Treaty Standard Evolved into a Customary Rule of International Law?

The present paper examines whether the consistent inclusion of the National Treatment standard in various BITs has led to the emergence of a customary international law rule of similar content. To analyse this issue, the paper tailors the test established by the ICJ in the North Sea Continental Shelf cases, which examines the process of development of a treaty rule into a customary law rule. The article concludes that the mere presence of the National Treatment standard in numerous BITs does not inherently imply that it has evolved into a customary international law rule. The article argues that this is primarily because the assessment of customary international law necessitates state practice to exist separately and autonomously from treaty practice. Hence, the replication of the rule across BITs alone does not provide sufficient grounds for presuming the existence of a customary law rule. By assessing and tailoring the criteria set forth by the ICJ and considering the scholarly perspectives on this matter, this paper offers an understanding of the conditions required for the transformation of a treaty-based rule into a customary law principle. The aim is to contribute to the ongoing scholarly discourse on the development and identification of customary international law, specifically in the context of investment treaties and the National Treatment standard.