GENERAL PRINCIPLES

According to the definition of opinio juris I have suggested, both multilateral and bilateral treaties can serve as weighty evidence of the existence of opinio juris. As noted in Chapter 3, the International Court of Justice (ICJ) has recognized that treaties, although not creating customary international law per se, can constitute such evidence. To recall, in the North Sea Continental Shelf Cases, the Court affirmed that there is “no doubt” that a rule that was originally embodied in a treaty can pass into the corpus of customary international law. And in the 1985 Continental Shelf Case (Libya v. Malta), it declared, “[M]ultilateral conventions may have an important role to play in recording and defining rules deriving from custom, or indeed in developing them.” We have seen that legal scholars have adopted a similar view. Despite these assertions, however, the precise reasons for giving treaties weight in determining opinio juris remain obscure. The analysis so far indicates a number of reasons why treaties are deserving of this weight.

First, treaties are acts, and therefore express the views, of heads of state or government and parliaments in one of their areas of competence, foreign affairs. Second, these officials or bodies are often elected, in which case we should give their views greater weight. Furthermore, the ratification of treaties is usually the result of some degree of open-minded consultation, both among members of parliaments that must approve a treaty and between parliaments and heads of state or government.