Taxes and Custom: Tax Treaties as Evidence for Customary International Law

Scholars of public international law have not paid attention to international tax law in the past. This article seeks to fill this vacuum and to foster cross-field research by studying customary international law in international tax law. It assesses the value of international tax law’s most prominent feature for the identification of custom: the dense network of almost identical, bilateral double tax treaties. The primacy of source-based taxation for business profits serves as a test case for this purpose. The International Law Commission’s conclusions on the identification of customary international law constitute the theoretical reference point that informs the empirical analysis. Thus, this article simultaneously serves as a treadmill test to appraise whether the International Law Commission’s conclusions actually offer practical guidance. The analysis culminates in the conclusion that tax treaties have only little value for the identification of customary international law. First, tax treaties alone do not entail representative state practice. Second, tax treaties give rise to the pitfalls of the Baxter paradox. Third, the tax treaty network yields no evidence that any state practice originates from opinio juris. Judging by the evidence brought into play so far, states likely display uniform treaty practice in international taxation because they believe it is in their best interest, not due to any legal conviction.