Note: This lesson should not be considered as a comprehensive overview on the subject. It was used as a supplement to classes conducted in 2010.
An Introduction to Sources of Law
Article 38 (1) of the Statute of the International Court of Justice (ICJ) is considered as an authoritative statement on sources of international law.
The ICJ is requested to form its opinion based on customary international law evidenced by “general practice accepted as law”. This sets out the two essential components necessary to form customary international law, which the Court interpreted as State practice and opinio juris.
In the North Sea Continental Shelf Cases, the ICJ held that in order to argue that a customary rule has emerged one needed to prove:
(1) The objective element (State practice).
In other words:
(a) widespread and representative participation, including States whose interests were specially affected (i.e. generality); and
(b) virtually uniform practice (i.e. consistent and uniform usage) undertaken in a manner that demonstrates;
(2) a general recognition of the rule of law or legal obligation (i.e. opinio juris).
In the North Sea Continental Shelf Cases the ICJ held that the passage of a considerable period of time was unnecessary (i.e. duration) to form a customary law.
In summary, two elements are necessary before one could argue that a customary rule has emerged:
1. The Objective Element: The existence of State practice. In other words, the actions or omissions by the State must support the custom; and
2. The Subjective Element: Acceptance as law. In other words, States when performing a custom must do so because they feel that they are legally bound to perform the custom. We call this opinio juris.
Figure 1: Elements of Customary International Law
State practice can be seen in the actual actions performed by the State (both by way of acts and omissions), statements made by authorized representatives in international fora, or through national laws and judicial decisions that deal with international relations. ✐ See further the ILA report on the formation of CIL paras. 7-11.
“State practice” that is necessary for the formation of a CIL must be (1) consistent and uniform; (2) generally accepted by States; and (3) of a certain duration. The latter, as we saw in the North Sea Continental Shelf Cases, does not need to be excessively long.
On State practice, the jurisprudence of the Nicaragua case contained several important clarifications in respect of inconsistent State practice (para 186). The ICJ held that:
(1) For a customary rule to come into force, it is not necessary to have complete consistency in State practice in respect of the rule.
(2) Inconsistent State practice does not affect the formation or existence of a customary principle so long as the inconsistency is justified by the State as a breach of the rule.
(3) This attempt at justifying a violation would only make the rule’s customary law nature stronger.
Click here for the blog post on opinio juris.
Customary international law can be general or particular. Particular customary law embodies local or regional customs. This type doesn’t bind all States; but, binds some States that share a common interest or that are in a similar geographical location. See the Asylum Case (Summary) where the ICJ had to decide on the existence of a regional custom.
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