LibGuides: International Law Research Guide: Customary International Law

Customary International Law

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Definitions of Customary International Law

Custom is evidence of a general practice accepted as law according to article 38, paragraph 1, of the Statute of the International Court of Justice . Custom is an expression of a claim, demand, or expectation. The two generally accepted constituent elements in customary international law are state practice and opinio juris sive necessitatis or opinio juris [legal opinion of a state]. It is also commonly agreed that custom refers to the practice and opinions of states, and not individuals and other collective entities... - Doebbler, Curtis F. J.

Understanding Customary International Law

Interactive Tutorial

Customary International Law - CALI Lesson
On completion of the lesson, the student will be able to:
1. Define customary international law.
2. Describe a research strategy for locating customary law.
3. List the tools used in customary international law research.

Scholarly Articles

Katie A. Johnston, The Nature and Context of Rules and the Identification of Customary International Law, 32 Eur. J. Int'l L. 1167 (2021)
The International Court of Justice (ICJ) regularly invokes a two-element test for the identification of customary international law: state practice accompanied by opinio juris. Yet the Court's application of this test has appeared inconsistent. In some cases, an absence of evidence of opinio juris leads to a finding that an alleged rule of customary international law does not exist; yet in other cases, there is no clear evidence of opinio juris on the part of states participating in the practice, but the Court nevertheless concludes that the alleged customary rule exists. In other cases, the Court concludes that a customary rule exists apparently based on evidence of opinio juris alone. This article argues that these judgments do not undermine the ICJ's claim to be applying a two-element analysis but reveal something fundamental about how customary international law is identified--namely, that how the existence of state practice and opinio juris is evaluated may vary depending on the nature of the customary rule under investigation (for example, whether it is a permissive or prohibitive rule) and the context of underlying international law rules in which that alleged new customary international law rule is located.
William Thomas Worster, The Inductive and Deductive Methods in Customary International Law Analysis: Traditional and Modern Approaches, 45 Geo. J. Int'l L. 445 (2014)
Contemporary customary international law analysis is currently understood to be a struggle between traditional and modern approaches, implicating significant normative outcomes. These opposing approaches are supposed to reflect the use of the inductive and deductive methods respectively, with the former excusing the freedom of action of the state and the latter limiting the freedom of the state. This image of two schools, possibly in struggle, however, does not fully capture the ways in which the inductive and deductive methods are actually intertwined in customary international law analysis. The methods are not two opposing monolithic techniques. Instead, in practice, the methods are intermixed, combining a variety of choices. This Article will suggest the complex ways inductive and deductive analyses are layered in the assessment of customary international law and refute the notion that the inductive and deductive approaches can be so easily separated.
More articles on Customary International Law

Books: Customary International Law Theory

Custom As a Source of Law by David J. Bederman
Call Number: Online or 4th Stacks ; K282 .B43 2010
ISBN: 9780511900853
Publication Date: 2010-08-16
Customary International Law: a new theory with practical applications by Brian D. Lepard
Call Number: 5th Stacks ; KZ1277 .L47 2010
ISBN: 9780521138727
Publication Date: 2010-01-11
Custom's Future: International Law in a Changing World by Curtis A. Bradley (Editor); Ingrid B. Wuerth (Editor)
Call Number: 5th Stacks ; KZ1277 .C87 2016
ISBN: 9781107443105
Publication Date: 2016-02-15
Reexamining Customary International Law by Brian D. Lepard (Editor)
Call Number: 5th Stacks ; KZ1277 .R44 2017
ISBN: 9781107146914
Publication Date: 2017-02-16
The Rule of Unwritten International Law: Customary Law, General Principles, and World Order by Peter G. Staubach
Call Number: 5th Stacks ; KZ1277 .S73 2018
ISBN: 9781351207300
Publication Date: 2018-04-17

Customary International Law Research

Article 38 of the International Court of Justice Statute. 108 says that one primary source of international law is custom. Customary international law is made up of the rules of law that come from the consistent actions of states, driven by their belief that the law compels them to act, or to refrain from acting in a certain manner. This sense of obligation is called opinio juris, while state practice refers to the official actions of a government, as seen in a range of possible activities such as official statements during international conferences, diplomatic communications, formal directives to diplomatic agents, decisions made by national courts, legislative actions, or any other measures adopted by governments to address international issues.

This means that you’re looking for two things, first state practice, that is, an action that many states are all doing (or not doing) and second, some kind of legal obligation, opinio juris, that is the reason why they are all doing or not doing the action.

Finding evidence of customary international law can be tricky because the sources are not easy to identify or to find. Moreover, there isn't just one place where all these types of law sources are found. They're spread across different sources like treaties, laws of countries, official government papers, documents produced by intergovernmental organizations and so on. This page gives some basic direction on where to look for these sources of customary international law. It is best to start with secondary sources on your topic and let these publications lead you to the most relevant documents.