Panel 2.2 / The Theory & Philosophy of Customary International Law and its Interpretation / 24 - 25 May 2019 / Faultlines and Weak Spots in the Edifice of CIL Customary international law comprises general rules binding States generally, and particular rules binding only a limited number of States. My paper focuses on what particular customary law is, and on its relationship with general customary law. This is not a marginal topic. I will suggest that the distinction is profoundly important to customary international law. I will also suggest that the distinction is challenging to draw, in part because of difficulties in interpreting State practice.
What is particular custom? A common view is that it is geographically confined – eg arising among neighbouring States, or with regard to a matter falling within a certain geographical area. Hence writers and jurists have spoken of “regional” or “local” custom. Another view, recently endorsed by the ILC in its 2018 draft conclusions on custom, is that particular custom need not be geographically confined: “there is no reason in principle why a rule of particular customary international law could not also develop among States linked by a common cause, interest or activity other than their geographical position”.
This approach faces problems where State practice and opinio juris diverge. Suppose that 80 States recognise the immunity of incumbent Heads of State from foreign criminal jurisdiction, and 60 States deny such immunity. The practice is too inconsistent to infer a general rule. But might we infer a particular customary rule, at least among the 60 States? (And perhaps another particular rule among the other group of 80 States?)
The ILC’s response is that particular custom depends on a practice among “the States concerned that is accepted by them as law (opinio juris) among themselves”. Its commentary explains that “the practice must be general in the sense that it is a consistent practice ‘among the States concerned’, that is, all the States among which the rule in question applies”. Perhaps, then, no particular customary rules may be inferred in the above example if each group of States (80 and 60) think that the rule applies to all States, not just among members of their own group.
But how do we tell? How should we interpret State practice given that it is typically vague or silent on this matter? Should interpretative presumptions be used and, if so, how and why?
The ILC’s proposal raises further concerns. Suppose that a particular customary rule (as the ILC understands it) emerges among 20 States, who do recognise this rule among themselves. Later, 5 additional States begin engaging in practice that seems to align with this rule; must each of the 5 be recognised by the group of 20 so as to establish a rule “among themselves” (the 25)? And again, how is this to be inferred from the State practice given that it is normally silent or vague?
The alternative is to dismiss the ILC’s proposal, in which case we return to the problem that, where practice is divergent or inconsistent, various particular rules could be inferred.
This is a major faultline for custom. Although questions about particular and general custom are not new, their relationship requires further analysis. The paper will set out the problem and attempt to suggest a way forward.
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