The Statute of the International Court of Justice refers to custom as follows: “international custom, as evidence of a general practice accepted as law.” The wording of this definition has been the source of some confusion, as custom itself is not evidence of general practice but rather evidenced by general practice. Unlike treaties, custom is not written. Thus, one must find evidence of custom in other sources, establishing a "general practice accepted as law." See Karol Wolfke, Custom in Present International Law 6, 169 (1993). Available at the UIC Law Library 10th Floor - KZ3405.P6W6 C87 1993.
There are two components of custom: an objective part (consistent practice) and a subjective part ("opinio juris" or sense of obligation). See Guide to International Legal Research. 10th Floor - K85.G85 & via Lexis.
As customary law is not written, the researcher aims to find evidence of custom. Evidence of custom can be found in the records of a state’s foreign relations and diplomatic practice, compiled state papers or diplomatic archives, such as the Foreign Relations of the United States. It can also be found in a state’s internal legislation (i.e. session laws, gazettes, statutory compilations) concerning its international obligations. Evidence of custom may also be found in resolutions, declarations, and legislative acts of intergovernmental organizations as recorded in yearbooks such as the United Nations Juridical Yearbook (available via Hein).
International practices are often documented in international law digests, collections of state papers, and the official publications of national governments and international organizations. See John W. Williams, Research Tips in International Law, George Washington Journal of International Law & Economics (1986). Available via Hein & UIC Law Library 10th Floor - KZ 3.W55 1981.
Per the International Law Commission, the following six sources may contain evidence of custom:
Yearbooks provide surveys of state practice through treaties and international activities. They may also provide information on national case law involving questions of international law or national legislation concerning matters of international law. Many are available via HeinOnline's Foreign & International Law Resources Database. If you have difficulty finding an article from a yearbook, use the tools discussed under the Treatises & Law Review Articles tab of this LibGuide.
Marci Hoffman. International & Foreign Legal Research Researching Customary International Law and Generally Recognized Principles (Bibliography 2007)
Silke Sahl. Researching Customary International Law, State Practice and the Pronouncements of States regarding International Law (2007).
Ralph Gaebler & Maria Smolka-Day (eds.). Sources of State Practice in International Law. 10th Floor - KZ64.S67 2001 (with eBook option).
Restatement of the Law: The Foreign Relations Law of the United States. 10th Floor - KF4651.Am35 3d. Also available via Westlaw & Lexis. The Restatement is often cited in federal decisions having an extraterritorial impact and is highly regarded by judges and legal scholars. According to the Guide to International Legal Research, it is a "key resource concerning the U.S. incorporation of international law." According to the Restatement, customary international law is “the sense of legal obligation that States generally and consistently follow.”
The Foreign Relations of the United States (FRUS) and Digest of United States Practice in International Law are both State Department publications. It should be noted that the FRUS is a historical record of the foreign policy of the United States. According to the State Department website, it is the “official documentary historical record of major U.S. foreign policy decisions and significant diplomatic activity.” Also available via HeinOnline's FRUS Library.
The Digest of United States Practice in International Law dates back to 1886, but it ceased publication from 1989 to 1999 and then restarted in 2000. According to the State Department website, "the Office of the Legal Adviser publishes the annual Digest of United States Practice in International Law to provide the public with a historical record of the views and practice of the Government of the United States in public and private international law." It contains all types of documentary sources, including treaties, executive agreements, legislation, regulations, federal court decisions, congressional statements, and diplomatic internal memoranda. Materials are divided into chapters that cover topics from nationality to terrorism. Also available at UIC Law Library 10th Floor - KZ21.D54.
