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Customary International Law and Statutory Interpretation: An Empirical Analysis of Federal Court Decisions

76 Pages Posted: 20 Jul 2013 Last revised: 14 Oct 2014

Bart M.J. Szewczyk

German Marshall Fund

Date Written: August 2014


This Article provides the first comprehensive empirical analysis of the use of customary international law by federal courts in statutory interpretation — a particularly important issue given the growth of subject-matter areas covered by international custom and the increased likelihood of its potential overlap with statutes. The analysis shows that courts utilize international custom across a diverse range of fields, not only to construe ambiguous statutes but also to review clear legislation. But some judges and commentators have recently challenged the practical determinacy and democratic legitimacy of this interpretive modality, and have sought to abrogate it altogether. Acknowledging the partial validity of these concerns, the Article argues that courts should interpret statutes consistently with custom that is clear and accepted (“established custom”) based on what could be called the Sosa-Charming Betsy doctrine. Conversely, courts should construe statutes independently of vague or disputed custom (“emerging custom”) and articulate statutory interpretation as persuasive evidence of the formation of a new customary norm. Recognizing the constraint of established custom in statutory interpretation, in turn, increases U.S. influence over emerging custom. By engaging in this interpretive exercise within the international community and taking established custom seriously, U.S. judicial opinions regarding emerging custom also will be taken more seriously. These dual interactions provide a previously unexplored power-maximizing justification for this canon of construction. To be sure, important questions remain regarding the sources of evidence and the uniformity of state practice and extent of opinio juris necessary to identify established custom. But the scholarly and judicial debate should shift to these issues, which might be resolvable only in context and on a case-by-case basis assessment of particular norms, rather than seek overall nullification of this interpretive modality.

Keywords: international law, statutory interpretation, federal courts, custom, empirical legal studies, foreign relations, transnational law, Charming Betsy, Sosa

Suggested Citation

Szewczyk, Bart M.J., Customary International Law and Statutory Interpretation: An Empirical Analysis of Federal Court Decisions (August 2014). George Washington Law Review, Vol. 82, 2014; Columbia Public Law Research Paper No. 13-360. Available at SSRN:

Bart Szewczyk (Contact Author)

German Marshall Fund ( email )

1744 R Street NW
Washington, DC 20009
United States

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