UPDATE: Researching Customary International Law, State Practice and the Pronouncements of States Regarding International Law

By Silke Sahl

Update by Catherine Deane

Catherine A. Deane is the Research Specialist for the Bay Area Offices of Shearman & Sterling LLP. She has a B.A. in Cultural Anthropology with a Certificate in Latin American Studies from Princeton University, an M.A. in Cultural Anthropology, a J.D. with a Certificate in International and Comparative Law from the University of Tulsa in Oklahoma, and an M.L.I.S. degree from San Jose State University, School of Library and Information Science.

Published November/December 2018

(Previously updated by Catherine Deane in July 2016)

See the Archive Version!

1. Introduction

This research guide is intended to be an introduction to the concept of international custom and its place as a source of international law. The primary focus is on researching state practice and the pronouncements of states regarding international law as evidence of custom. While treaties, state law and the actions of international organizations can also contribute to customary international law, this guide does not assist with researching these areas. References to some of the excellent research guides already written on these areas are included.

The guide introduces the researcher to titles that provide texts of the pronouncements of states regarding international law, both U.S. and international. There are also recommendations for secondary sources and finding aids helpful in describing state practice and in tracking down additional resources. Lastly, a list of additional research guides on customary international law is also provided. These alternate research guides were used extensively in preparation for writing this guide and are highly recommended as additional resources on the subject.

2. What is Customary International Law?

“Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation.”[1]

This definition was published in §102 (2) of the Restatement of the Law, Third, Foreign Relations Law of the United States, published by the American Law Institute in 1987. The Restatement’s reporters’ notes for this section state that “No definition of customary law has received universal agreement, but the essence of Subsection (2) has wide acceptance” and goes on to explain various difficulties in defining custom.”[2]

When is state practice considered to be customary international law? The Restatement calls for two-pronged approach to determining custom requiring both a general and consistent practice and a sense of legal obligation (opinion juris sive necessitates). J.L. Brierly describes it as follows: “Custom in its legal sense means something more than mere habit or usage; it is a usage felt by those who follow it to be an obligatory one. There must be present a feeling that, if the usage is departed from, some form of sanction probably, or at any rate ought to, fall on the transgressor.”[3]

Obviously, terms such as “a feeling that”, “will probably” and “ought” are difficult to prove. As Mark Janis puts it in his book, An Introduction to International Law, “The determination of customary international law is more an art than a scientific method.”[4] This is a complex and fascinating area of law that is addressed by the many excellent books and articles on customary international law. This guide focuses on finding the resources that provide evidence of international custom.

For additional information on the legal issues relating to customary international law, see the following sources:

3. Custom as a Source of Law

Customary law is listed as a source of international lawin the Restatement (Third) of Foreign Relations Law in section 102(1)(a), along with international agreements and general principles:

§ 102 Sources of International Law states that “(1) A rule of international law is one that has been accepted as such by the international community of states: (a) in the form of customary law; (b) by international agreement; or (c) by derivation from general principles common to the major legal systems of the world.”[5]

The Restatement’s description of the sources of international law is derived in part from the Statute of the International Court of Justice,[6] described in the Restatement as an “authoritative statement of the ‘sources’ of international law.”[7] Article 38 of the Statute describes what the court should consider in making decisions in accordance with international law:

Article 38 of the ICJ states that “(1) The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

(a) international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
(b) international custom, as evidence of a general practice accepted as law;
(c) the general principles of law recognized by civilized nations;
(d) subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.”[8]

The concept of customary international law is briefly explained by ICJ Judge Sir Christopher Greenwood in his video lecture Sources of International Law, available in the Audiovisual Library of International Law. This concept is more fully explained by attorney and law professor, Mr. Anthony D’Amato in his video lecture The Sources of International Law - Part 2: Customary International Law also available in the Audiovisual Library of International Law.

4. Evidence of Customary International Law

The Restatement describes the evidence of international law in §103. Of particular interest to the researcher of custom is §103(2)(d) which describes the evidence of pronouncements of states:

§ 103 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.[9]

The Restatement comments state, “Thus, for customary law the “best evidence” is the proof of state practice, ordinarily by reference to official documents and other indications of governmental action.”[10] In Principles of Public International Law, Ian Brownlie lists the following sources as evidence of custom: “The material sources of custom are very numerous and include the following: diplomatic correspondence, policy statements, press releases, the opinions of official legal advisers, official manuals on legal questions, e.g. manuals of military law, executive decisions and practices, orders to naval forces etc., comments by governments on drafts produced by the International Law Commission, state legislation, international and national judicial decisions, recitals in treaties and other international instruments, a pattern of treaties in the same form, the practice of international organs, and resolutions relating to legal questions in the United Nations General Assembly.”[11]

4.1. State Practice

The practice of states must be both general and consistent and combined with a state’s sense of a legal right or a legal obligation.The Encyclopaedic Dictionary of International Law (3 ed., 2009) Edited by John P. Grant and J. Craig Barker has a definition of state practice that includes references to authoritative International Law treatises.

For an extensive, but not exhaustive list of sources of state practice, see Treves T. Customary international law, Part C7 The Sources of Knowledge of Practice, Max Planck Encyclopedia (Oxford University Press, 2006).

4.2. Pronouncements of States

Official pronouncements by states that undertake to state a rule of international law are one of the best places to find evidence of international custom as they often describe both prongs of the definition of custom, the general and consistent practice of the state and an explicit description of the state’s sense of legal obligation. The majority of this guide is dedicated to finding these resources.

4.3. Treaties

According to the Restatement, “International agreements constitute practice of states and as such can contribute to the growth of customary law under Subsection (2).”[12] There are many excellent guides to treaty research available. See for example the GlobaLex guide UPDATE: An Introduction to Sources for Treaty Research,[13] the Arthur W. Diamond Law Library Guide to Treaty Research,[14] or the Alyne Queener Massey Law Library Treaty Research Guide.[15]

4.4. State Law

A country’s law (legislation, court decisions, regulations, etc.) are sources of state practice to the extent that they might seek to describe the country’s practice and obligations under international law. The many excellent research guides available in the foreign law section of GlobaLex and in the Foreign Law Guide, edited by Marci Hoffman. serve as resources for those researching domestic laws of all countries. Often you can find foreign law research guides among the research guide offerings at high ranking law schools.[16]

4.5. International Organizations and International Courts

As described in the Restatement, “The practice of states that builds customary law takes many forms and includes what states do in or through international organizations.”[17] In particular, the Restatement refers to resolutions and other documents of the United Nations. The decisions of international courts likewise are a good source of information. Research guides to the United Nations and other international organizations describe the documents of these organizations. See for example, the GlobaLex guide Update: Researching the United Nations: Finding the Organization’s Internal Resource Trails,[18]the Arthur W. Diamond Law Library Research Guide: The United Nations[19] and the Alyne Queener Massey Law Library, Find UN Documents.[20]

5. Pronouncements of States—Resources

When looking for pronouncements of states describing international law, one can research the publications of the state itself. Alternatively, using the many excellent digests and secondary sources available will provide the researcher with explanatory notes regarding the pronouncements as well as good citations to these documents and sometimes reprints of the texts themselves. The following sections describe both of these types of resources.

5.1. Pronouncements of States—The United States of America

In the United States, the State Department publications include pronouncements that undertake to state the rules of international law and as such are excellent resources for the evidence of custom. Examples of these documents include memorandums, letters from legal advisors, U.S. briefs, position papers and remarks. Today, the State Department and the Office of the Legal Advisor provide many of these resources on their websites. For more information on researching foreign relations of the United States, there are many good U.S. Foreign Policy Research Guides, such as:

While the resources in this section can be excellent resources, it is sometimes more practical to use resources that have already compiled and analyzed domestic documents that relate to international law, such as the digests and secondary sources described in the subsequent sections.

Following is a list of resources for U.S. diplomatic papers in more or less reverse chronological order:

5.2. Digest of U.S. Practice in International Law

Over the years, an official digest of U.S. practice in international law has been published in various forms, with different authors, sometimes annually. These digests include editorial descriptions of international law in various areas as well as selections of official U.S. documents that are the evidence of customary international law. Published by the Department of State, authorship has changed over the years, and recent issues are published with the International Law Institute under the auspices of the Department of State, Office of the Legal Advisor.

The Digest is one of the best sources for determining the official State Department’s view of international law. On its website, the State Department describes this publication as follows: “The Office of the Legal Adviser publishes the annual Digest of United States Practice in International Law to provide the public with a ready source of current information on the views and practice of the Government of the United States in the arena of public and private international law.” Many documents cited in editions of the Digest since 1989 are available on the website of the Department of State.

Following is a list of these digests:

5.3. Secondary Sources—United States

Secondary sources are sometimes the best way to find descriptions of a state’s practice in international law. Also, they usually provide excellent citations to the documents themselves and sometimes excerpts or texts of the documents.

Following is a list of secondary sources on the practice of the U.S. in international law:

5.4. Pronouncements of Non-U.S. States

When researching state practice from the perspective of countries other than the United States, the same principles apply as in U.S. research. The best resources will usually be publications of a government’s State Department, Ministry of Foreign Affairs or Ministry of External Affairs. Many foreign ministries are putting a significant amount of information on their websites. Alternatively, you can use the name of the ministry as the author in a catalog search or to limit an internet search.

A selective list of foreign ministries and their websites is included below, but a Google search for the country name and the word Foreign affairs will usually bring up the appropriate agency for any country:

5.5. International Yearbooks

An excellent way to find information about state practice in other jurisdictions is using yearbooks on international law. Many states publish such a yearbook. They are unofficial, often published by a national law society, and include articles about international law and state practice. They also include citations to official documents, and sometimes include indexes and full-text documents. They often have bibliographies of books and articles published in that country on international law. Many are published (or translated into), at least in part, in English and are therefore a great resource for those who cannot read the vernacular.

International law yearbooks can be found in a library catalog by searching for both “yearbook” and “year book” (as one word or two), and the term “international law”. Occasionally, publications including the classic content of an international law yearbook may be called by another name (annual, etc.). While subject headings in catalogs and indexes often include the term “yearbook” or “year book”, in case they do not it is also useful to search catalogs for the term in the vernacular: “annuaire”, “anuario”, “jahrbuch”, etc.

A selective list of yearbooks is included below.

5.6. State Yearbooks

5.7. Multi-Jurisdictional Yearbooks

5.8. Secondary Sources—Other Jurisdictions

There are many excellent secondary sources on customary international law from other jurisdictions. This section covers sources that include multiple jurisdictions. These sources can be used to find titles and citations for country-specific sources. Alternatively, searches for “international law” combined with terms like “practice”, “applied”, “digest” or “interpreted” and the name of the country in question can be used in catalogs and other resources for finding country-specific titles.[26]

6. Additional Research Guides and Scholarship on Customary International Law

6.1. Online Research Guides

6.2. Print Research Guides

6.3. Scholarship

[1] American Law Institute. Restatement of the Law, Third, the Foreign Relations Law of the United States. St. Paul, Minn.: American Law Institute Publishers, 1987. §102(2).

[2] Id. §102, Reporters’ Notes, 2.

[3] Brierly, J. L. The Law of Nations: An Introduction to the International Law of Peace, 6th ed. Oxford; New York: Oxford University Press, 1963. p. 59.

[4] Janis, Mark W. An Introduction to International Law, 4th ed. New York, Aspen Publishers, 2003. p. 44.

[5] Id. §102(1)(a).

[6] Statute of the International Court of Justice. 3 Bevans 1179; 59 Stat. 1031; T.S. 993; 39 AJIL Supp. 215 (1945).

[7] Restatement, §102, Reporters’ Notes 1.

[8] Statute of the International Court of Justice, Article 38, § 1.

[9] Restatement, §103.

[10] Restatement, §103, comment a.

[11] Brownlie, Ian. Principles of Public International Law, 6th ed. Oxford; New York: Oxford University Press, 2003. p. 6.

[12] Restatement, §102, comment i.

[13] Engsberg, Mark & Chappell, Mary Beth. UPDATE: An Introduction to Sources for Treaty Research March 2016.

[14] Arthur W. Diamond Law Library. Guide to Treaty Research. Written by Simon Canick. Maintained by Silke Sahl. Last Updated April 2018.

[15] Alyne Queener Massey Law Library. Treaty Research Guide. Written by Catherine Deane.

[16] Bodleian Libraries Legal Resources for Jurisdictions. Written by Ruth Bird, Elizabeth Wells, et al.

[17] Restatement, §102, Reporters’ Note 2.

[18] Tashbook, Linda. Update: Researching the United Nations: Finding the Organization’s Internal Resource Trails. Last Updated: November/December 2016.

[19] Arthur W. Diamond Law Library. Research Guide: The United Nations. Written by Silke Sahl.

[20] Alyne Queener Massey Law Library. Research Guide: Find UN Documents. Written by Catherine Deane. Last Updated by Catherine Deane: May 21, 2017.

[21] Maintained by David Azzolina, Collection Development and Liaison Services Librarian, University of Pennsylvania.

[22] Maintained by Jenny Kusmik, Reference Librarian, Johns Hopkins University.

[23] The Federal Depository Library at the Richard J. Daley Library, University of Illinois at Chicago maintains a database that includes the Secretary of State speeches as far back as 1993.

[24] See 53 Am. J. Int’l. L. (1959) at 896.

[25] Restatement, Forward, p. IX.

[26] For examples, see Kindred, Hugh M. and Phillip M. Saunders, eds. International Law, Chiefly as interpreted and Applied in Canada. 7th ed. Toronto: Emond Montgomery Publications, 2006; and “Völkerrechtliche Praxis der Bundesrepublik Deutschland” in Zeitschrift für Ausländisches Öffentliches Recht und Völkerrecht. Stuttgart: W. Kohlhammer, 1929-. Also available online at the website of the Max Planck Institute for Comparative Public Law from Volume 1, 1929 forward.