Evidence of International Law

(1) Whether a rule has become international law is determined by evidence appropriate to the particular source from which that rule is alleged to derive (§ 102).

(2) In determining whether a rule has become international law, substantial weight is accorded to
(a) judgments and opinions of international judicial and arbitral tribunals;
(b) judgments and opinions of national judicial tribunals;
(c) the writings of scholars;
(d) pronouncements by states that undertake to state a rule of international law, when such pronouncements are not seriously challenged by other states.
Since the adoption of the ICJ statute in 1946, the post World War II growth of a wide variety of Inter-Governmental Organizations (IGOs) has injected the work product of these IGOs into the mix as well.