At a time when the world needs more, and more complex, international legal rules and institutions to address major cooperation problems, customary international law (“CIL”) has several important limitations: (i) it cannot be made in a coordinated manner in advance of events, (ii) it cannot be made with sufficient detail, (iii) it cannot be made with sufficiently heterogeneous reciprocity between states, (iv) it cannot be made with specifically-designed organizational support, (v) it is generally not subject to national parliamentary control, (vi) it purports to bind states that did not consent but failed to object to its formation, and (vii) it provides excessive space for auto-interpretation by states, or for sometimes insufficiently disciplined interpretation by judges.
Treaty, and legislation produced by international organization voting, can perform better on all of these dimensions. Today, the vast majority of rules of CIL are codified: of 300 identified rules of CIL, only 13 have not been codified either in treaties or in International Law Commission instruments. To a remarkable extent, due to the growth of treaty law incorporating the same norms, if CIL were abolished today, most of the international legal system would remain intact. Although I cannot prove that the rise of treaty rules has resulted from the factors discussed above, those factors are a good set of reasons why states might have determined to produce treaties that include, but are not limited to, the rules that exist in CIL.
- customary international law
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