International Custom

International Custom

Unlike domestic legal systems, where legislation plays a central role in regulating legal relationships, international law assigns a special status to custom. This is largely because states themselves participate in its formation and therefore do not view it as contrary to national independence. In many fields, the lack of treaties is compensated for through customary rules. In numerous instances, treaties are also drafted as codifications of existing customary rules.

Under Article 38 of the Statute of the International Court of Justice, international custom is defined as general practice accepted as law. More precisely, international custom may be described as the repeated conduct of subjects of international law, particularly states, which over time becomes binding in their mutual relations and acquires legal force. The Permanent Court of International Justice also recognized the legal validity of custom after treaties in the Lotus case.

Customary international law is generally understood as consisting of two elements:

• The material element.
• The mental element.

Important Examples of International Custom in Relations Between States

The Material Element of Custom

The material element refers to the repetition of a legal act in a consistent and stable manner over a relatively long period, without objection by subjects of international law. According to the International Court of Justice, a short period of repetition does not prevent the formation of custom if the practice is uniform and continuously carried out by the states concerned. Furthermore, abstention from an act may also contribute to the formation of a customary rule if it is shown that the abstention occurred with awareness and intent.

The Mental Element of Custom

A practice regularly and continuously carried out by states becomes a customary rule only if there is a belief in its obligatory character and necessity, i.e., it is accepted as a legal rule. Otherwise, the conduct may amount to nothing more than an international habit. One example is the practice of vessels raising flags when encountering one another. If the conduct is not uniform and varies across situations, it is described as international comity. An example is the practice of states refraining from claiming reimbursement of costs incurred in the extradition of criminals.

The requirement of a belief in obligation was emphasized in the Court’s judgment in the Asylum case involving Colombia and Peru.

Objection to Custom

If a state consistently objects to a customary rule from the beginning of its formation, that customary rule will not be binding upon that state, because the belief in its obligatory nature is not present in respect of that state. In modern custom, unlike classical custom, where the length of repetition was considered the main factor, belief in obligation has become more significant. The mental element is given priority over the material element. This approach is commonly traced to the Court’s judgment in the North Sea Continental Shelf cases.

Foundations of International Custom

The Objective School

Some scholars of international law, including Savigny, consider custom to be an objective rule that stands above the will of states and arises from the necessities of social life. Under this view, both the material and mental elements are required for the formation of a customary rule.

The Will-Based School

Followers of this school consider the basis of custom, like other legal rules, to be the will of states. The roots of this approach may be traced to Grotius and later developed by Anzilotti and Triepel. Under this view, the binding character of customary rules derives from the implicit concordant will of states. Therefore, such rules cannot be imposed upon third states without their consent. This theory emphasizes the mental element and tends to disregard the material element.

In general, international judicial practice has adopted the objective school, and only in the Lotus case was the will-based approach accepted.

Relationship Between Custom and Treaty

Although the Statute of the International Court of Justice lists custom after treaties, and Court decisions, including the Lotus case, indicate that where no treaty exists, one may refer to custom, custom and treaty have equal legal validity, and neither can be treated as inherently superior. In contemporary practice, there is a continuing effort to transform customary rules into treaty form.

Several situations may be considered regarding the relationship between custom and treaty:

• A treaty may repeal a customary rule, for example, the Vienna Convention in relation to prohibiting the opening of diplomatic bags, or the 1890 Brussels Act, which repealed the customary practice relating to the purchase and sale of Black persons.
• A treaty may be prior to custom, for example, the agreement relating to the Moon adopted by the General Assembly.
• A treaty may codify customary rules, such as the 1958 Law of the Sea Conventions.

From the equality of custom and treaty, it follows that the later treaty rule or later customary rule is valid. Peremptory norms, whether customary or treaty-based, prevail over other rules in any event.

The Status of Custom in Today’s World

Given the rapid pace of developments in international relations, custom is no longer sufficient to respond to new needs. States have therefore increasingly turned toward treaty-making. Newly established states, which played no role in the formation of earlier customs, tend not to readily accept customary rules. The International Court of Justice has also indicated that recognition of customary rules should be approached cautiously. For these reasons, there has been a growing trend to codify customary rules in international treaties. The absence of custom in relation to newly emerging issues has also intensified attention to the development of international law.

Frequently Asked Questions About International Customs