Customary International Law

• Author: John H. Currie
• Pages: 185-217

185

CHAPTER 5

CUSTOMARY

INTERNATIONAL LAW

A. NATURE OF CUSTOMARY

INTERNATIONAL L AW

Recalling the largely anarchic state of t he international legal system

and the resulting conundrum of how to “make” (or, for natural law-

yers, how to “discover”) international law, one potential solution is to

borrow a page from domestic legal systems which sometimes treat cer-

tain habitual or customa ry societal practices as ev idence of a form of

law. For example, the modern common law system could be considered

such a form of customary l aw in th at it represents the cumulative a nd

generally consistent practice of a certain set of actors (namely, courts)

over time.1 Moreover, such consistent practice is self-conscious in that

the relevant actors subjectively believe (for whatever reason) that t heir

consistent practice is required as a matter of law (usually expres sed as

the rule of stare decisis). That is, common law judges would not explain

their patter n of deciding like cases alike as a random or merely prac-

tical phenomenon, but would r ather consider t heir consistent practice

to be legally required.2

1 On the simi larity between custom ary legal systems and t he common law trad-

ition, see J.L. Brierl y, The Law of Nations: An Int roduction to the Internatio nal Law

of Peace, 6th ed. by H. Waldock (Oxford: Cl arendon Press, 1963) at 70.

2 Caution is in order so a s not to take this illust rative analogy too far. The refer-

ence to common law as a t ype of customary la w is not meant to suggest that

PUBLIC INTERNATIONA L LAW186

The apparent circularit y of such a customary system of law (con-

duct is legal ly required because it is reg ularly engaged in and believed

to be legally required) is in fact well suited to a n international society

composed of sovereign entities. The theory requires no legislator but

rather derives the law from the consistent actions and subjective con-

victions or perceptions of its own subjects.

Superf‌icially such a source of law would also appear to be conson-

ant with t he positiv ist theory of consent, particularly if consistent prac-

tice and subjective belief are considered law-creating rather than simply

law-evidencing. That is, states that consistently engage in certain con-

duct which they subjectively bel ieve to be legally required can be con-

sidered to consent, in some sense, to the resultant rules of customary

international law. In fact, as we shall see, t he consistency of customary

international law with the theory of consent depends crucially on what

is meant by “consent” and on cer tain other factors, such a s t he me an-

ing given to “consistently,” or the number of subjects which must be

found to “believe.” However, the idea of customary international law can

also be argued to be consistent with natural law theories, particularly if

practice and subjective belief are considered law-evidencing rather than

law-creating. More fundamentally, natural law theories might furnish

an explan ation for why states would believe certain practices are legally

obligatory, whereas positivism might not. Thus, customary international

law mirrors the moder n eclecticism of internat ional legal theory.3

In any case, the idea t hat binding r ules of international law can be

derived from the behaviour and beliefs of states has long been accepted

by international legal theorists4 as well as by states themselves. Some

twentieth and twenty-f‌irst century theorists have questioned the con-

tinued importance of custom as a source of internat ional law given the

proliferat ion of multilateral codif‌ication or law-making treatie s, which

occupy much of the “territory” formerly covered only by custom. Some

have wondered if t his tendency to codify is a natural result (and indica-

tor) of international law’s evolution from its primitive customary orig ins

to a more deliberate, thought-out system. Notwith standing these mus -

ings, customary intern ational law continues today to be widely accepted,

along with treaties, as one of the two principal sources of international

customar y international law i s a form of international common l aw. As we have

seen (Chapter 3, Section C(5)), there is no concept of binding judici al precedent

in internat ional law. In customary inter national law, as we shall see, it i s not the

consistent pract ice of judges that is legally sig nif‌icant, but rather the con sistent

practice of state s.

3 See Chapter 3, Sect ion B(3).

4 See Chapter 3, Sect ions B(1) & (2).

Customary Int ernational Law 187

law. Article 38(1)(b) of the Statu te of the Inter natio nal Cou rt of Ju stice g ives

modern expression to this acceptance by identifying “international cus-

tom, as evidence of a general practice accepted as law” as a source of inter-

national law upon which the Intern ational Court of Justice is to rely.

It would be diff‌icult to overstate the profoundly important role

played by the concept of customary international law in the inter-

national legal system. In contrast to treaty law, customary international

law is, with rare exceptions to be noted below,5 universally binding on

all states. It thus constitutes the substratum of common legal rights

and obligations of the entire community of states, upon which their

more particularized legal relationships (usually in t he form of treaties)

are built. What is more, customar y international law is an enormously

powerful normative concept in that it can have the effect, under certain

circumstances, of binding states which have not actually consented

to its content at all. It thus challenges positivist interpretations of the

theory of consent. At t he same time, it receives universal endorsement

as a valid and binding source of inter national law.

The other striking implication of such a source of l aw is that it is

forever evolving under the inf‌luence of the actions of its own subjects. In

other words, the legal signif‌icance of a state’s behaviour is not limited to

its consis tency o r incon sisten cy wit h the l aw. Its sig nif‌ic ance ca n exte nd,

in t he right circumstances, to inf‌luencing t he content of the law itself.

This can, of course, pose problems for the application of the law in par-

ticular i nstance s. If the content of customary i nternation al law is by def-

inition dynam ic, how can its content be determined or applied with any

precision at any given point in time? Should a state’s behaviour which is

not in conformit y with past state practice be condemned as contrary to

customary international law, or should it r ather be con sidered evidence

of a new customary rule? The requirements for the formation and deter-

mination of customary international law rev iewed below provide partial

answers to the se and simil ar questions, but the fundamental paradox

persists — a system of law which governs the behaviour of its subjects

while itself being subject to modif‌ication by such behaviour.

B. ELEMENTS OF CUSTOMARY

INTERNATIONAL L AW

Even viewing the expres sion “international custom” in par agraph

38(1)(b) as a convenient short form for the more accurate expression

5 See Sections C & D, below i n this chapter.