The Limits of Custom in Constitutional and International Law

50 San Diego L. Rev. 867 (2013)

San Diego Legal Studies Paper No. 113-116

28 Pages Posted: 12 Apr 2013 Last revised: 9 Mar 2021

See all articles by Michael D. Ramsey

Michael D. Ramsey

University of San Diego School of Law

Date Written: April 11, 2013


This essay explores the difficulty of using customary practices to establish legal rules in constitutional and international law. Many arguments styled as appeals to custom do not rest on custom alone, and as a result require justification on the basis of authority other than (or in addition to) custom. In particular, there are two basic types of arguments from custom. One is that certain practices have been accepted in the past and should be understood to establish law with respect to those particular practices in the future. The second is that certain practices have been accepted in the past and should be understood as the basis for law regarding other practices in the future, either because the other practices are (somewhat) analogous or because the prior practices support a general principle which then can be applied to new circumstances.

Only the first of these forms of argument is purely an argument from custom because – unlike the second form – it does not require contested value judgments to administer it (other than the initial judgment that custom itself should have legal force). In contrast, the second form of argument necessarily involves value judgments not just with respect to the force of custom, but also on the question whether a particular customary practice should be extended to a new and arguably distinct circumstance. Put another way, the first form of argument rests, as a normative matter, only on the proposition that custom can or must be binding; the second involves further normative determinations regarding the particular new circumstances at issue, external to the idea of custom in the abstract.

This distinction may have particular force for custom-based arguments in constitutional and international law. Unlike common law adjudication, neither field readily accepts that adjudicative decisionmakers have authority to make law on the basis of normative considerations. To take the more obvious example of customary international law, it is said that customary practices establish law between nations because by their practices sovereigns consent to longstanding ways of acting. To the extent sovereign consent provides the authority for unwritten international law, arguments depending on extensions of custom lack this authority and must draw authority from some other less congenial source. Similarly, in constitutional law, arguments from custom may draw authority from the nation’s (or a branch’s) consent to (or acquiescence in) a practice manifested by longstanding acceptance of it, even if the practice is not consistent with the Constitution’s text. Again, to the extent an argument is based not upon pure custom but upon an extension of custom, the argument has changed the basis of its authority because it no longer arises solely from widespread historical consent.

This essay illustrates these concerns by reference to two modern debates in which arguments from custom are commonly invoked: in constitutional law, the claim that the President has independent power to engage in low-level uses of force, as in the 2011 intervention in Libya; and in international law, the claim that private businesses may be sued for complicity in human rights violations of the governments of nations in which they do business. It concludes that both claims are examples of arguments from extensions of custom, and thus cannot draw authority from ideas related to consent or acquiescence.

Keywords: Custom, customary international law, constitutional law, war powers, international human rights

Suggested Citation

Ramsey, Michael D., The Limits of Custom in Constitutional and International Law (April 11, 2013). 50 San Diego L. Rev. 867 (2013), San Diego Legal Studies Paper No. 113-116, Available at SSRN:

Michael D. Ramsey (Contact Author)

University of San Diego School of Law ( email )

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