2.2 Custom

Definition: unwritten law formed by states' conduct — the two-element doctrine: Objective element — state practice: general, consistent, reasonably continuous conduct (diplomatic acts, legislation, treaties, votes, military and naval practice, pleadings); duration matters less than density (the "instant custom" debate for continental shelf and space law); Subjective element — opinio juris sive necessitatis: the conviction that the practice is followed as legal obligation, not habit, comity or convenience — the line between la coutume and la courtoisie (the Paquete Habana's line; the North Sea Continental Shelf cases' requirement that opinio juris be shown, not presumed). The refinements that earn higher marks: Persistent objector rule: a state consistently objecting from a custom's formation is not bound — but never against jus cogens; Regional and special custom: custom among a region or group (the Asylum case's failed regional-custom proof — the burden of proving consistency); Jus cogens: peremptory norms — prohibition of aggression, genocide, slavery, torture; self- determination — accepting no derogation, voiding conflicting treaties, overriding consent (VCLT Arts. 53/64); Codification's triple effect on custom: a codifying convention declares existing custom (binding non-parties as custom); a crystallizing convention fixes developing practice; a generating convention creates new law for parties only (North Sea Continental Shelf).