The
Restatement at Section 102(2) reads "Customary international law results from a general and consistent practice of states followed by them from a sense of legal obligation". The notes to the section go on to explain "Subsection (2), includes diplomatic acts and instructions as well as public measures and other governmental acts and official statements of policy, whether they are unilateral or undertaken in cooperation with other states...." and it continues "For a practice of states to become a rule of customary international law it must appear that the states follow the practice from a sense of legal obligation (
opinio juris sive necessitatis ); a practice that is generally followed but which states feel legally free to disregard does not contribute to customary law." This idea is reinforced in Section 103(2)(d), discussing evidence of a rule, accepting as evidence "pronouncements by states that undertake to state a rule of international law, when such pronouncements are not seriously challenged by other states."
The overall concept is referred to as the "practice" of international law by such-and-such jurisdiction. It may be useful to consult the reporter’s notes to the
Restatement for an extended discussion of the issues, including an acknowledgment of the possible circularity of definition and the process underlying the definition.
It follows that the sources used to substantiate Custom can be quite broad. The clearest sources are those documents from the appropriate institutions which assert that they are statements of customary international law. The institution should be the one within a given jurisdiction which has the power to make such statements, such as the Department of State for the United States. Thus diplomatic papers involving dispute resolution are prime source materials for international law.
In recognition of this, the Department of State periodically produces information about its work which includes letters and other documents relevant to international law. The best products were conscious digests of the work of the department, each covering a set period and each edited by an international law specialist. Early compilations tend to be known by the editor’s name, Wharton (1887), Moore (1906), and Hackworth (1944). The version known as
"Digest of International Law" is edited by Marjorie Whiteman and is generally known as "Whiteman’s Digest". This is in 15 volumes, covering 1963-73, (at JX237 W588). Later years are covered by the
Digest of United States Practice in International Law, 1974-2003 (with gaps), available in paper with documents available on the Department of State website
http://www.state.gov/s/l/c8183.htm . Another resource is the publication of a kind of chronicle in each issue of the
American Journal of International Law called "Contemporary Practice of the United States relating to International Law" written in consultation with the Department of State.
The website for the
Department of State has a lot of raw materials for current activities, some of which is useful for international law purposes. The University of Chicago has an archive of DOS web materials at
http://dosfan.lib.uic.edu/ERC/law.html . The unit within the Department of State which handles international law issues is the Office of the Legal Adviser. Their website is at
http://www.state.gov/s/l/ .
Because of the fuzziness of sources in this area, the
Restatement itself, especially given its explanatory text and reporter’s notes, is a strong statement as to U.S. practice. It should be noted, however, it was not written with the
imprimatur of the State Department.
The best parallel tools for other jurisdictions tend to be the non-official yearbooks of international law for particular countries. These tend to combine academic articles on international law with a solid review of current practice and selected reprinted documents.
Leading Examples Include:
Multi-jurisdictional examples include the
African yearbook of international law and the
Asian yearbook of international law . Yearbooks are also important in documenting the role of international organizations within international law. (See below) The best yearbooks lead you to more solid materials by citing the actual texts involved with the perceived practice.
Some of this kind of privately edited material is starting to appear on the web. The text of the
German Yearbook 's section on practice is available at
http://www.mpil.de/ww/en/pub/research/details/publications/institute/prax.cfm .
Many countries now have websites for their equivalent of the Department of State. The contents vary enormously. Examples include:
The sources presented above are related to practice in its narrowest sense. In fact, the totality of a nation’s "presentation of self" is its practice. This includes treaty participation and execution, actions of its administrative agencies, decisions of its courts, etc. All of this can be used as evidence of customary international law. Arguably, any institution which has the power in a given situation to decide that an assertion or action has the imprimatur of customary international law is in fact making it so. The limitation on this power is a jurisprudential one similar to the limits on the lawmaking power of a judge in Common Law.
