International Judicial Monitor
Published by the American Society of International Law and the International Judicial Academy
December 2006, Volume 1, Issue 5

General Principles of International Law

Customary International Law

A key concept in international law is that of customary international law (CIL).  This precept is based upon the notion that states are bound to a given norm of international law through custom and action indicating their obligation to be bound, opinio juris.  The International Law Association has defined a “rule of customary international law” as one that is “created and sustained by the constant and uniform practice of states in circumstances that give rise to the legitimate expectation of similar conduct in the future.”  It is a practice that states follow, not in blind pursuit of political approbation, but rather, because they believe that they have a legal obligation to do so.

          The American Law Institute’s 1987 Restatement of Foreign Relations Law §102, defines “customary international law” as resulting “[f]rom a general and consistent practice of states followed by them from a sense of legal obligation.”  In Article 38 of the Statute of the International Court of Justice, the Court is obliged to examine “i[n]ternational custom, as evidence of a general practice accepted as law,” in addition to treaties, general principles of law in the major legal systems, judicial decisions, and the writings of scholars, in its determination of disputes in international law.

          The practice of States.  What might we examine to determine whether states are engaging in “uniform practice”?  We may, for example, consider how states act in their diplomatic relations, and what sorts of official statements of policy are made.  It is worth emphasizing here, that for a practice to be considered customary international law, it does not have to be universally followed, but must nevertheless have obtained widespread acceptance.

          Opinio Juris.  How do we determine whether states act from a sense of legal obligation or rather from a sense of courtesy or mere habit?  As the comment to the Restatement of Foreign Relations Law § 102 indicates, it may be difficult to ascertain when this precise shift takes place, when a state acts not merely from custom but from a sense of legal obligation.  Such a sense of responsibility may therefore be implied from a state’s acts or omissions.

          A concept related to that of customary international law, is that of being a persistent objector.  If, while a norm of international law is in its embryonic stages, a state consistently and openly objects to it, after time, while a norm may apply to other states that have consistently and regularly followed it with a belief that they are legally obligated to do so, the norm will nevertheless not apply to the state that objected to it in its formative stages.

          Another key idea in this discussion is that of a jus cogens norm.  These are preemptory norms from which derogation is not allowed.  Professor Sean Murphy describes jus cogens norms as being “super” customary international law—international law so fundamental to the inter-relationship of states that a state cannot, through its treaty practice or otherwise, deviate from the law.” [1]   Because of the overarching importance of jus cogens norms, it is generally accepted among courts and scholars that there are only a limited number of them including genocide, slavery, and torture.  The concept of jus cogens is relevant to our discussion of customary international law and persistent objectors, because states cannot (and it would be difficult to imagine them doing so) object to being bound by norms against which they cannot derogate.

          A number of courts in landmark decisions have discussed the concept of customary international law.  For example, in Filartiga v. Pena-Irala, 630 F.2d 876  (2d. Cir. 1980), the first case in which a court applied the Alien Torts Claims Act (the Act) since 1789, the Second Circuit stated that the Act provides a jurisdictional basis for the suit, and that it recognized the existence of a private right of action for aliens seeking to remedy violation of customary international law or of a treaty of the U.S.  Filartiga was also the first decision in the post-Erie period to hold that customary international law has the status of Federal common law. In Military and Paramilitary Activities in and against Nicaragua, (Nicar. v. U.S.), 1986 I.C.J. 14.., the International Court of Justice held that a number of the provisions in the U.N. Charter are customary international law norms.

          During the contemporary era, a number of commentators have criticized the concept of customary international law as being imprecise, and the sense of opinio juris, difficult to determineCustomary international law nevertheless remains a critical underpinning of international law jurisprudence.

For Further Reading:

Barry E. Carter, Philip R. Trimble, Curtis A. Bradley, International Law (Aspen Pub., 2003)

Sean D. Murphy, Principles of International Law, (Thomson West, 2006).

[1] Sean D. Murphy, Principles of International Law 78-83 (Thomson West, 2006)

Susan Notar, Esq, International Legal Materials (ILM) Managing Editor, ASIL 

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