The interaction between treaty and custom
The possible interactions between treaty and custom are generally considered by reference to the well-established framework set out by the International Court of Justice in the North Sea Continental Shelf Cases of 1969; namely, the declaratory, crystallising and generating effects of a treaty (as for a resolution) on a customary rule. The impact of codification on the time it takes for a customary rule to develop, as well as the nature of the substantive rules that treaty codification might generate (obligations of means in addition to those of result; as a consequence of the acceleration in the rule formation process) are also well canvassed. But other issues relative to the treaty-custom interplay warrant consideration today.
With the International Law Commission focussed on the proper identification of customary international law, and separately on the role of subsequent practice in treaty interpretation, one question that arises is how do we differentiate between what might be practice for the purposes of custom as opposed practice for the purposes of treaty interpretation? Each type of practice is different, with its own conditions, thresholds and effects.
Second, some codification issues stand out as being of particular contemporary relevance, even if they are not new. For instance, one can still ask in relation to the ever-flourishing field of international investment law when and whether an investment treaty operates to derogate from or generate customary rules (‘Baxter’s paradox’). Other issues of codification pose practical dilemmas, such as the out-sourcing of codification projects today, especially where the private codifier’s product (such as the NATO Cooperative Cyber Defence Centre for Excellence’s Tallinn Manual 2.0) appears to carry more weight than that of a United Nations organ (specifically, the UN Group of Governmental Experts). Another enduring issue, as ever highlighted by current State behaviour, is the question of custom’s potential scope beyond the treaties when it comes to the laws of armed conflict.
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