The European Journal of International Law Vol. 28 no. 2
EJIL (2017), Vol. 28 No. 2, 357–385	 doi:10.1093/ejil/chx024
© The Author, 2017. Published by Oxford University Press on behalf of EJIL Ltd.
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The International Court of
Justice and the Judicial Politics
of Identifying Customary
International Law
Niels Petersen* 
Abstract
It is often observed in the literature on customary international law that the identification practice
of the International Court of Justice (ICJ) for customary norms deviates from the traditional
definition of customary law in Article 38 (1) lit. b of the ICJ Statute. However, while there are
many normative and descriptive accounts on customary law and the Court’s practice, few studies
try to explain the jurisprudence of the ICJ. This study aims at closing this gap. I argue that
the ICJ’s argumentation pattern is due to the institutional constraints that the Court faces. In
order for its decisions to be accepted, it has to signal impartiality through its reasoning. However,
the analysis of state practice necessarily entails the selection of particular instances of practice,
which could tarnish the image of an impartial court. In contrast, if the Court resorts to the consent
of the parties or widely accepted international documents, it signals impartiality.
1 Introduction
Customary law is international law’s most controversial source. Coinciding with the
International Law Commission’s (ILC) study on customary international law,1
there
have recently been several studies trying to shed new light on the normative underpinnings
of customary law.2
There have been further studies analysing the jurisprudence
*	 Professor of Public Law, International Law and European Union Law, University of Münster, Münster,
Germany. Email: niels.petersen@uni-muenster.de.
1
	 See M.  Wood, First Report on Formation and Evidence of Customary International Law, UN Doc. A/
CN.4/663 (2013); Second Report on Identification of Customary International Law, UN Doc. A/CN.4/672
(2014); Third Report on Identification of Customary International Law, UN Doc. A/CN.4/682 (2015).
2
	 See,e.g.,B.D.Lepard,CustomaryInternationalLaw:A NewTheorywithPracticalApplications(2010);Bradley
and Gulati, ‘Withdrawing from International Custom’, 120 Yale Law Journal (2010) 202; Blutman,
‘Conceptual Confusions and Methodological Deficiencies: Some Ways That Theories on Customary
International Law Fail’, 25 European Journal of International Law (EJIL) (2014) 529; Verdier and
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358 EJIL 28 (2017), 357–385
of the International Court of Justice (ICJ) regarding customary law. These often find
a divergence between the definition of customary law in Article 38(1) lit. b of the
ICJ Statute3
and the actual practice of the ICJ.4
Two papers have received particular
attention. On the one hand, Stephen Choi and Mitu Gulati provocatively argue that
the Court completely ignores the traditional definition when identifying customary
norms.5
On the other hand, Stefan Talmon reveals that the ICJ, in the majority of
cases, ‘has simply asserted the rules that it applies’.6
However, if legal doctrine does not determine the ICJ’s identification of customary
international law, which are the factors that shape the Court’s decision making? The
existing studies do not explore this question in detail but only offer some speculation.7
Choi and Gulati suggest that judges might be driven by efficiency concerns or a homestate
bias.8
Talmon considers judicial assertion as a ‘gateway for judicial legislation’
and warns that the Court should not ‘overstep the [methodological] limits’.9
This article
aims to close this gap in the existing research. It analyses which factors drive the
Court’s decision making. I argue that fears, according to which courts use the identification
of customary international law as a means for judicial legislation according
to their political preferences, are largely unfounded. Even though the judges have only
weak legal constraints, they face significant institutional constraints. For this reason,
their identification strategies aim at gaining legitimacy in order to preserve their judicial
authority.
The article consists of three main parts. First, it sets out the conceptual framework
of the article and explains the research design. Second, it analyses the two main
Voeten, ‘Precedent, Custom and Change in Customary International Law: An Explanatory Theory’, 108
American Journal of International Law (AJIL) (2014) 389; C.A. Bradley (ed.), Custom’s Future: International
Law in a Changing World (2016).
3
	 Statute of the International Court of Justice 1945, 1 UNTS 993.
4
	 See Baker, ‘Customary International Law in the 21st Century: Old Challenges and New Debates’, 21
EJIL (2010) 173; Geiger, ‘Customary International Law in the Jurisprudence of the International Court
of Justice: A Critical Appraisal’, in U. Fastenrath et al. (eds), From Bilateralism to Community Interest:
Essays in Honour of Bruno Simma (2011) 673, at 692; G.I. Hernández, The International Court of Justice
and the Judicial Function (2014), at 91; Talmon, ‘Determining Customary International Law: The ICJ’s
Methodology between Induction, Deduction and Assertion’, 26 EJIL (2015) 417; Choi and Gulati,
‘Customary International Law: How Do Courts Do It?’, in C.A. Bradley (ed.), Custom’s Future: International
Law in a Changing World (2016) 117. See also Ginsburg, ‘Bounded Discretion in International Judicial
Lawmaking’, 45 Virginia Journal of International Law (VJIL) (2005) 631, at 640 (arguing that it is ‘fair to
characterize much customary international law as actually being declared by judicial bodies rather than
arising from the explicit agreement of states’). Contra, Alvarez-Jiménez, ‘Methods for the Identification
of Customary International Law in the International Court of Justice’s Jurisprudence: 2000–2009’, 60
International and Comparative Law Quarterly (ICLQ) (2011) 681, at 711 (arguing that the ‘flexible, deductive
approach’ has lost in importance in the recent jurisprudence of the International Court of Justice
[ICJ]).
5
	 Choi and Gulati, supra note 4, at 147.
6
	Talmon, supra note 4, at 441.
7
	 See also Shaffer and Ginsburg, ‘The Empirical Turn in International Legal Scholarship’, 106 AJIL (2012)
1, at 12 (remarking a lack of empirical scholarship on customary international law).
8
	 Choi and Gulati, supra note 4, at 147.
9
	 Ibid.
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The ICJ and the Identification of Customary International Law 359
constraints that judges face in their decision-making and develops a hypothesis on
how these influence the ICJ’s jurisprudence on customary international law.The third
part, finally, consists of an empirical analysis of the strategies of the ICJ in identifying
customary international law. For this purpose, I have analysed all of the instances in
which the Court has identified a norm of customary international law and classified
the arguments upon which the Court has based its decisions. The result shows that
institutional constraints play a significant role in the judges’ decision-making.
2  Concept and Measurement
The main aim of this article is the analysis of factors that influence judicial decisionmaking.
If we assume that legal norms do not completely determine judicial decision-
making,10
the question of judicial motivation becomes imminent. There are, broadly,
three explanations on offer. The approach that is arguably the most provocative for traditional
legal scholars argues that judges mainly follow their political preferences. The
most examined court in this respect is the Supreme Court of the United States, for which
some studies suggest a significant correlation between the political preferences of the
judges and their judicial decision making.11
In international law scholarship, there is
a corresponding discussion on whether judges have a home, state or a regional bias.12
A second approach argues that, even though judges enjoy certain discretion, their
decisions are largely determined by legal norms and legal doctrine. The argument is
supported by empirical studies on the US Supreme Court, finding that legal regimes or
legal tools have a significant influence on the decision-making of the Court.13
Finally,
10
	 See von Bogdandy and Venzke, ‘Beyond Dispute: International Judicial Institutions as Lawmakers’, 12
German Law Journal (2011) 979, at 984.
11
	 See Segal and Cover, ‘Ideological Values and the Votes of U.S. Supreme Court Justices’, 83 American
Political Science Review (APSR) (1989) 557; Segal et al., ‘Ideological Values and the Votes of U.S. Supreme
Court Justices Revisited’, 57 Journal of Politics (1995) 812; J.A. Segal and H.J. Spaeth, The Supreme Court
and the Attitudinal Model Revisited (2002). See also Brennan, Epstein and Staudt, ‘The Political Economy
of Judging’, 93 Minnesota Law Review (2009) 1503 (according to whom decisions are also influenced by
the economic environment); George and Epstein, ‘On the Nature of Supreme Court Decision Making’, 86
APSR (1992) 323; (proposing an integrated model according to which judicial decisions are influenced
both by legal and extra-legal factors).
12
	 See, on the one hand, Posner and de Figueiredo, ‘Is the International Court of Justice Biased?’, 34 Journal
of Legal Studies (2005) 599 (finding such a bias for the judges of the ICJ) and, on the other hand, Voeten,
‘The Impartiality of International Judges: Evidence from the European Court of Human Rights’, 102
APSR (2008) 417 (arguing that such a bias is largely absent in the European Court of Human Rights).
13
	 See Richards and Kritzer, ‘Jurisprudential Regimes in Supreme Court Decision Making’, 96 APSR (2002)
305 (arguing that US Supreme Court decision-making is structured by jurisprudential regimes but recognizing
that these regimes are themselves human constructs); Brenner and Stier, ‘Retesting Segal and Spaeth’s
Stare Decisis Model’, 40 APSR (1996) 1036 (finding that the doctrine of precedent has a certain influence
on the decision making of Supreme Court justices); Lindquist and Klein, ‘The Influence of Jurisprudential
Considerations on Supreme Court Decision Making: A Study of Conflict Cases’, 40 Law and Society Review
(2006) 135 (arguing that ‘the desire to find legally sound, persuasive solutions to legal questions plays a
significantrole’inthedecision-makingof theUSSupremeCourt);BaileyandMaltzman,‘DoesLegalDoctrine
Matter? Unpacking Law and Policy Preferences on the U.S. Supreme Court’, 102 APSR (2008) 369 (arguing
that legal factors play a role in the Supreme Court’s decision-making but that this effect varies across justices);
M.A. Bailey and F. Maltzman, The Constrained Court: Law, Politics, and the Decisions Justices Make (2011).
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360 EJIL 28 (2017), 357–385
there is an institutionalist approach that assumes that judicial decisions are shaped
by the institutional setting.14
According to this explanation, judicial power depends
on the legitimacy of the judicial institutions so that judges strive to enhance their
legitimacy through their decision-making.15
These three approaches do not exclude
each other. To the contrary, it is rather likely that all three factors influence judicial
decision-making to some extent. However, the extent may vary depending on the normative
or institutional context. This study wants to examine which of these factors is
the dominant one in the field of customary international law.
If we want to analyse judicial motivation, we face a problem of measurement – we
cannot observe it directly. Therefore, we have to find indirect ways of measuring moti-
vation.16
What we can observe are the outcome of the case and the legal reasoning.
The mere outcome tells us very little about judicial motivation. If we want to determine
whether an outcome has been motivated by adherence to legal doctrine or political
preferences, we would need a baseline regarding the expected outcome in order to
compare the actual with the expected result.
However, it is extremely difficult to establish such a baseline for either an expected
‘legal’ or ‘political’ result. In order to determine the expected ‘legal’ result, we would
need to compare the actual outcome with the ‘right’ legal outcome, which is something
that is impossible to determine in a world where we have reasonable disagreement about
legal interpretation. Studies on the US Supreme Court often take the expected political
preferences of judges as a baseline and compare their actual voting behaviour with the
voting behaviour that should be expected if they were driven by political preferences.17
However, such a solution is not viable in the field of international relations, where political
preferences cannot easily be represented in a one-dimensional left/right policy space.
14
	 On the concept of institutionalism, see, in general, March and Olsen, ‘The New Institutionalism:
Organizational Factors in Political Life’, 78 APSR (1984) 734; Steinmo, ‘Historical Institutionalism’,
in D. della Porta and M. Keating (eds), Approaches and Methodologies in the Social Sciences: A Pluralist
Perspective (2008) 118.
15
	 See M. Shapiro, Courts: A Comparative and Political Analysis (1981); Stone Sweet, ‘Constitutional Politics:
The Reciprocal Impact of Lawmaking and Constitutional Adjudication’, in P. Craig and C. Harlow (eds),
Lawmaking in the European Union (1998) 111; A. Stone Sweet, Governing with Judges: Constitutional Politics
in Europe (2000); K.J. Alter, Establishing the Supremacy of European Law: The Making of an International Rule
of Law in Europe (2001); G. Vanberg, The Politics of Constitutional Review in Germany (2005). Specifically
for international courts, see also Ginsburg, ‘Political Constraints on International Courts’, in C. Romano,
K. Alter and Y. Shany (eds), The Oxford Handbook of International Adjudication (2014) 483, at 487–494;
Helfer and Alter, ‘Legitimacy and Lawmaking: A  Tale of Three International Courts’, 14 Theoretical
Inquiries in Law (2013) 478; K.J. Alter, The New Terrain of International Law: Courts, Politics, Rights (2014);
Voeten, ‘Public Opinion and the Legitimacy of International Courts’, 14 Theoretical Inquiries in Law
(2013) 411; Alter, Helfer and Madsen, ‘How Context Shapes the Authority of International Courts’, 79
Law and Contemporary Problems (2016) 1.
16
	 On such problems of measurement when concepts cannot be observed directly, see, e.g., K.A. Bollen,
Structural Equations with Latent Variables (1989); Brady, ‘Doing Good and Doing Better: How Far Does the
Quantitative Template Get Us?’, in H.E. Brady and D. Collier (eds), Rethinking Social Inquiry: Diverse Tools,
Shared Standards (2004) 53; Collier, Brady and Seawright, ‘Critiques, Responses, andTrade-Offs: Drawing
Together the Debate’, in H.E. Brady and D. Collier (eds), Rethinking Social Inquiry: Diverse Tools, Shared
Standards (2004), 195, at 202–209.
17
	 See, e.g., Segal and Cover, supra note 11, at 559–561.
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The ICJ and the Identification of Customary International Law 361
Instead,thisstudywillfocusontheCourt’slegalreasoning.Certainly,legalreasoning
is no perfect proxy of judicial motivation. Judges can justify their opinions with different
reasons than the ones that motivated them. Even if judges were motivated by political
considerations, they would rarely openly say so in a judicial opinion. Nevertheless,
the reasons given for an opinion may contain certain information about judicial motivation.
If judges do not give any reasons for a specific conclusion, it is unlikely that they
derived the result through a logical deduction from abstract legal norms. Otherwise,
they would have disclosed the reasons that guided their decision-making. Similarly, if
judges give certain signals through their reasoning that would not be necessary from
a purely doctrinal point of view, then we can interpret these signals and draw conclusions
on the factors that were driving their judicial decision making.
This study will look at the identification of customary international law by the ICJ.
I will establish a classification of different arguments that the Court uses to identify
customary norms. This classification contains information in two dimensions. On the
one hand, I will look at the different arguments in detail and use the context in order
to justify why the use of certain arguments indicates a specific motivation.18
On the
other hand, I will quantify the number of times that the Court uses a specific argument
in order to observe the relative importance of a specific argument in the Court’s
jurisprudence.
3  Constraints of Judicial Decision Making
In the following section, I will elaborate theoretical predictions to what extent the three
factors identified in the previous section influence the identification of customary
international law. In the first part, I will argue that legal constraints are rather weak.
For this reason, one might expect that the judges’ political preferences on what constitutes
a ‘good’ international order play a significant role in the jurisprudence. This
is supported, in particular, by the debate on ‘modern’ customary law. Nevertheless,
in the second part, I will argue that institutional constraints will prevent judges from
merely following their political preferences.
A  Doctrinal Constraints and the Discussion on ‘Modern’ Customary
Law
There is a lot of conceptual confusion and uncertainty about custom.19
Scholars disagree
on the constitutive elements of customary law and on the methods of their iden-
tification.20
In particular, there has been a widespread discussion on the emergence
18
	 On the importance of context in qualitative studies, see Adcock and Collier, ‘Measurement Validity:
A Shared Standard for Qualitative and Quantitative Research’ 95 APSR (2001) 529, 534.
19
	 Kadens andYoung, ‘How Customary Is Customary International Law?’, 54 William and Mary Law Review
(2013) 885, at 906. See also Koskenniemi, ‘The Pull of the Mainstream’, 88 Michigan Law Review (1990)
1946, at 1947 (referring to customary law as a ‘theoretical minefield’).
20
	 M. Byers, Custom, Power and the Power of Rules (1999), at 129–146; Kelly, ‘The Twilight of Customary
InternationalLaw’,40VJIL(2000)449,at498–517;Kolb,‘SelectedProblemsintheTheoryof Customary
International Law’, 50 Netherlands International Law Review (2003) 119; Hestermeyer, ‘Access to
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362 EJIL 28 (2017), 357–385
of ‘modern’ approaches to customary international law.21
In contrast to ‘traditional’
approaches that are primarily concerned with the identification of patterns of state
practice,22
modern customary law is rather based on interpretative techniques and
opinio juris.23
Modern approaches to customary law do not only differ from traditional
ones regarding the method of interpretation but also on the role of judges in identifying
customary rules. Many proponents of modern approaches advocate a more
active role of courts, asking them not merely to find, but actively to shape and develop,
customary law.
In a recent paper, Curtis Bradley likened the development of customary international
law to be similar to the development of common law rules.24
For this reason,
he proposed a common law account in which judges actively develop custom, taking
into account state preferences and consequentialist considerations.25
Similarly, Eyal
Benvenisti advocates that judges have a legislative function when they identify norms
of customary law.26
His account has both a normative and a descriptive dimension. He
argues that courts, when exercising this legislative function, should be, and actually
are indeed, guided by efficiency considerations.27
Some authors use the weakening of the practice requirement to infuse customary
international law with ethical values. For example, Brian Lepard argues that
norms that objectively promote fundamental ethical principles should be presumed
to have legal authority.28
Furthermore, John Tasioulas offers ‘a moral judgment-based
Medication as a Human Right’, 8 Max Planck Yearbook of United Nations Law (2004) 101, at 158;
D. Terris, C. Romano and L. Swigart, The International Judge: An Introduction to the Men and Women Who
Decide the World’s Cases (2007), at 113; Postema, ‘Custom in International Law: A Normative Practice
Account’, in A. Perreau-Saussine and J.B. Murphy (eds), The Nature of Customary Law: Legal, Historical
and Philosophical Perspectives (2007) 279, at 281–282; Petersen, ‘Customary Law without Custom?
Rules, Principles, and the Role of State Practice in International Norm Creation’, 23 American University
International Law Review (2008) 275, at 276–277; Bradley and Gulati, ‘Customary International Law and
Withdrawal Rights in an Age of Treaties’, 21 Duke Journal of Comparative and International Law (2010) 1,
at 3–5; Charlesworth, ‘Law-Making and Sources’, in J. Crawford and M. Koskenniemi (eds), The Cambridge
Companion to International Law (2012) 187, at 193–194; Blutman, supra note 2; Bradley, ‘Customary
International Law Adjudication as Common Law Adjudication’, in C.A. Bradley (ed.), Custom’s Future:
International Law in a Changing World (2016) 34, at 35–38.
21
	 The term is often ascribed to Roberts, ‘Traditional and Modern Approaches to Customary International
Law: A Reconciliation’, 95 AJIL (2001) 757. However, similar terms have been used before; see, e.g.,
Abi-Saab, ‘Cour général de droit international public’, 207 Recueil des Cours (1987) 9, at 176–178 (distinguishing
between ‘coutume traditionnelle’ and ‘nouvelle coutume’). But see also Tomka, ‘Custom and
the International Court of Justice’, 12 Law and Practice of International Courts and Tribunals (2013) 195,
who argues that the distinction ‘take[s] the point too far by insisting on theorizing this development’.
22
	 See, e.g., Mendelson, ‘The Formation of Customary International Law’, 272 Recueil des Cours (1998) 155.
23
	Roberts, supra note 21, at 758; Worster, ‘The Inductive and Deductive Methods in Customary
International Law Analysis: Traditional and Modern Approaches’, 45 Georgetown Journal of International
Law (2014) 445, at 470.
24
	Bradley, supra note 20.
25
	 Ibid., at 49–50.
26
	 Benvenisti, ‘Customary International Law as a Judicial Tool for Promoting Efficiency’, in E. Benvenisti
and M. Hirsch (eds), The Impact of International Law on International Cooperation (2004) 85, at 87.
27
	 Ibid., at 88–114.
28
	Lepard, supra note 2, at 110–111.
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The ICJ and the Identification of Customary International Law 363
account of customary international law’.29
Tasioulas argues that ‘customary norms
can come into being despite the absence of general state practice, or at the extreme,
even in the teeth of considerable countervailing practice’.30
He allows for a trading-off
of state practice against opinio juris if the norm in question is of high moral importance
for the legitimacy of international law.31
Even though the term ‘modern’ customary law is rather new, the tendency to
infuse customary international law with moral values is no recent development.
Louis Sohn wanted to consider certain United Nations General Assembly (UNGA)
resolutions, such as the Universal Declaration of Human Rights, as automatically
binding because they represented the consensus of the international community.32
Fernando Tesón argues that it was inappropriate to require state practice for the
establishment of customary rules enshrining fundamental moral principles.33
Most
often, however, the attempt to charge customary law with moral principles has
not been made explicit. Instead, morally desirable outcomes have been presented
as results of an objective application of legal methodology. This is particularly the
case for the field of international human rights law. This tendency is probably best
summarized by Martti Koskenniemi in a review article of Theodore Meron’s book on
human rights as customary law: ‘The feeling is, in other words, that Professor Meron
has quite strong opinions about which norms should be included among those that
are binding even beyond specific treaties, and that he uses whichever arguments are
available to support them.’34
Consequently, ‘modern’ customary international law is often primarily characterized
not by methodological rigour but, rather, by an attempt to reconcile legal
interpretation with considerations of efficiency or moral intuitions about human
rights and the international community that most international lawyers share.35
In the absence of methodological constraints, customary law thus seems to be an
entry gate for the ‘progressive’ development of international law and a tool for judicial
law-making.
29
	 Tasioulas, ‘Custom, Consent, and Human Rights’, in C.A. Bradley (ed.), Custom’s Future: International
Law in a Changing World (2016) 95, at 95. See also earlier accounts making a similar point: Tasioulas,
‘In Defence of Relative Normativity: Communitarian Values and the Nicaragua Case’, 16 Oxford Journal
of Legal Studies (1996) 85; Tasioulas, ‘Customary International Law and the Quest for Global Justice’, in
A. Perreau-Saussine and J.B. Murphy (eds), The Nature of Customary Law: Legal, Historical and Philosophical
Perspectives (2007) 307. See also Meron, ‘International Law in the Age of Human Rights: General Course
on Public International Law’, 301 Recueil des Cours de l’Academie de Droit International (2003) 9, at 388
(arguing that there is ‘a direct relationship between the importance attributed by international community
to particular norms and the readiness to lower the burden of proof required to establish custom’).
30
	 Tasioulas, ‘Custom, Consent’, supra note 29.
31
	 Ibid., at 101–102.
32
	 Sohn, ‘The Human Rights Law of the Charter’, 12 Texas International Law Journal (1977) 129, at 133.
Universal Declaration of Human Rights 1948, UN Doc. A/810 (1948).
33
	 Tesón, ‘Two Mistakes about Democracy’, 92 ASIL Proceedings (1998) 126, at 127.
34
	Koskenniemi, supra note 19, at 1952, referring to T. Meron, Human Rights and Humanitarian Norms as
Customary Law (1989).
35
	 See Kelly, supra note 20, at 497–498.
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364 EJIL 28 (2017), 357–385
B  Judicial Politics and the Search for Legitimacy
However, courts do not only face doctrinal, but also institutional, constraints. They
are usually unable to implement their own decisions.36
For this reason, courts cannot
act independently of the preferences of the political actors who are affected by
their decisions.37
States have several ways to impose sanctions on courts and thus to
constrain judicial decision-making indirectly.38
First, they can choose not to comply
with a judgment.39
In the history of the ICJ, there are several examples of such failures
to comply.40
Non-compliance may hurt the Court in two ways: on the one hand,
the decision it has rendered will be ineffective and, on the other hand, frequent noncompliance
may damage the reputation of the Court and thus weaken its institutional
position.
Second, states can sometimes exit the jurisdiction of an international court by withdrawing
their acceptance of the Court’s compulsory jurisdiction or refuse to accept
the jurisdiction in the first place.41
The fewer states that have accepted the compulsory
jurisdiction of the Court, the weaker the Court’s institutional position. In the context
of the ICJ, exit from compulsory jurisdiction is relatively easy.There are two prominent
examples in this respect.42
France withdrew from the compulsory jurisdiction of the
ICJ after the decision in the Nuclear Tests cases in the 1970s,43
and the USA withdrew
after the ICJ had found to have jurisdiction in the Nicaragua case in 1984.44
Despite this sanctioning potential, however, international courts also have a certain
level of independence from state governments.45
First, withdrawing from the jurisdiction
of the ICJ is not without costs. Once a state has withdrawn from the jurisdiction of
the ICJ, it cannot bring claims against other states. Therefore, if a state values the ICJ
as a means of dispute resolution, one unfavourable decision does not automatically
lead to a withdrawal. Second, governments may face external constraints concerning
the compliance with a specific judgment. Internationally, states may jeopardize their
36
	 This is not a specific feature of international courts but applies to domestic courts as well. See Alter, New
Terrain, supra note 15, at 32. See also Staton and Moore, ‘Judicial Power in Domestic and International
Politics’, 65 International Organization (2011) 553; Ginsburg, supra note 15, at 486 (arguing that institutional
distinctions between domestic and international courts are overstated).
37
	Ginsburg, supra note 15, at 487.
38
	 See Steinberg, ‘Judicial Lawmaking at the WTO: Discursive, Constitutional, and Political Constraints’, 98
AJIL (2004) 247, at 263–267.
39
	Ginsburg, supra note 15, at 491–492.
40
	 See C. Schulte, Compliance with Decisions of the International Court of Justice (2004), at 271–275.
41
	Ginsburg, supra note 4, at 657; Ginsburg, supra note 15, at 490.
42
	 Oda, ‘The Compulsory Jurisdiction of the International Court of Justice: A Myth? – A Statistical Analysis
of the Contentious Cases’, 49 ICLQ (2000) 251, at 264; Keohane, Moravcsik and Slaughter, ‘Legalized
Dispute Resolution: Interstate and Transnational’, 54 International Organization (2000) 457, at 480;
Ginsburg, supra note 15, at 490.
43
	 Nuclear Tests (Australia v. France), Judgment of 20 December 1974, ICJ Reports (1974) 253; Nuclear Tests
(New Zealand v. France), Judgment of 20 December 1974, ICJ Reports (1974) 457.
44
	 Military and Paramilitary Activities in and against Nicaragua (Nicaragua v.  U.S.), Jurisdiction and
Admissibility, 26 November 1984, ICJ Reports (1984) 392.
45
	 Voeten, ‘International Judicial Independence’, in J.L. Dunoff and M.A. Pollack (eds), Interdisciplinary
Perspectives on International Law and International Relations: The State of the Art (2013) 421; Alter, New
Terrain, supra note 15, at 52–58.
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The ICJ and the Identification of Customary International Law 365
reputation if they do not comply with a judgment of an international court or tribu-
nal.46
On the domestic level, they may be forced to comply with judgments of the ICJ
by national courts if the latter decide that the government is bound by international
law,47
or they may be influenced by transnational or domestic pressure groups that are
in favour of the ICJ ruling.48
What consequences do these considerations have on the decision-making of international
courts and tribunals? Martin Shapiro has pointed out that courts transform
dispute resolution from a dyadic into a triadic relationship.49
However, they have to be
careful that this triadic relationship is not perceived to break down into a dyad, with
the court seemingly becoming an ally of one of the parties.50
Consequently, in order to
preserve their acceptance, international tribunals have to appear as neutral arbiters
whose decisions are based on ‘legal’ principles and not on a political agenda.51
If a
court is not regarded as legitimate, non-compliance with its decisions will neither hurt
a state’s reputation nor entice domestic courts to force the government into compliance
with international law.52
Furthermore, states would be less willing to bring new
cases to the court or might even withdraw from the court’s jurisdiction entirely.
If we apply these considerations to the identification of customary international
law, then it is unlikely that the ICJ will show significant activism. Instead, the Court will
try to signal impartiality. It will base its decisions only on those customary norms that
are generally acceptable to states. However, such general acceptability can be based on
several grounds. A norm can be generally acceptable because its identification stands
46
	 On reputation as compliance-promoting mechanism, see R.O. Keohane, After Hegemony. Cooperation and
Discord in the World Political Economy (1984), at 105–108; Downs and Jones, ‘Reputation, Compliance,
and International Law’, 31 Journal of Legal Studies (2002) 95; A.T. Guzman, How International Law
Works: A Rational Choice Theory (2008), at 71–117; Brewster, ‘Reputation in International Relations
and International Law Theory’, in J.L. Dunoff and M.A. Pollack (eds), Interdisciplinary Perspectives on
International Law and International Relations: The State of the Art (2013) 524; Stein, ‘The Engines of
Compliance’, in Dunoff and Pollack, ibid., 477, at 481–483.
47
	 See Dothan, ‘How International Courts EnhanceTheir Legitimacy’, 14 Theoretical Inquiries in Law (2013)
455, at 463–467; Alter, New Terrain, supra note 15, at 53.
48
	 On the influence of transnational and domestic pressure groups, see M.E. Keck and K. Sikkink, Activists
beyond Borders: Advocacy Networks in International Politics (1998); Risse and Ropp, ‘International Human
Rights Norms and Domestic Change: Conclusions’, inT. Risse, S.C. Ropp and K. Sikkink (eds), The Power of
Human Rights: International Norms and Domestic Change (1999) 234; Risse, ‘‘Let’s Argue!’: Communicative
Action in World Politics’, 54 International Organization (2000) 1; Sikkink, ‘Restructuring World Politics:
The Limits and Asymmetries of Soft Power’, in S. Khagram, J.V. Riker and K. Sikkink, Restructuring World
Politics (2002) 301; Trachtman, ‘International Law and Domestic Political Coalitions: The Grand Theory
of Compliance with International Law’, 11 Chicago Journal of International Law (2010) 127.
49
	Shapiro, supra note 15, at 1. On the triadic structure of dispute resolution by international tribunals, see
also Sandholtz and Stone Sweet, ‘Law, Politics, and International Governance’, in C. Reus-Smit (ed.), The
Politics of International Law (2004) 238, 247–248.
50
	Shapiro, supra note 15, at 8.
51
	 Stone Sweet, Governing with Judges, supra note 15, at 199–200; Shapiro, ‘The Success of Judicial Review
and Democracy’, in M. Shapiro and A. Stone Sweet (eds), On Law, Politics, and Judicialization (2002) 149,
at 165; Posner and Yoo, ‘Judicial Independence in International Tribunals’, 93 California Law Review
(2005) 1, at 21; Dothan, supra note 47, at 459. Specifically regarding the ICJ, see alsoY. Shany, Assessing
the Effectiveness of International Courts (2014), at 171–174.
52
	 See Helfer and Alter, supra note 15, at 483.
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366 EJIL 28 (2017), 357–385
on firm methodological grounds; it can be acceptable because states signalled acceptance
of the norm in the past or during the proceedings before the ICJ or the content of
the norm is of such a universal moral appeal that states will at least not openly oppose
its application. It is thus not important for the Court to develop a coherent methodology
of identifying customary international law. Instead, identification strategies may
differ depending on the circumstances of each individual case and the preferences of
the affected parties.
4  Judicial Strategies to Identify Customary International
Law: An Empirical Analysis
The hypothesis that judges face institutional constraints in their decision-making and
that, for this reason, they try to signal impartiality through their identification strategy
of customary norms is an attempt to predict and explain judicial identification strategies
of customary international law. But is this theoretical hypothesis reflected in the
practice of the ICJ? In order to test the hypothesis empirically, I performed a qualitative
analysis of all judgments and advisory opinions of the ICJ, since its establishment in
1949, in which the Court positively identified a rule of customary international law.
A  Research Design
The analysis was not limited to instances where the ICJ explicitly referred to ‘customary
law’ or ‘custom’. Instead, it also looked at passages where the Court was dealing
with rules of ‘general international law’, the analysis of ‘state practice’, or with norms
of ‘jus cogens’ or ‘erga omnes’ character. The study focused only on cases where the
Court positively identified a customary norm. I assume that the burden of justification
for confirming a customary rule is higher than for rejecting a customary norm.
On the one hand, denying the existence of a customary norm will rarely be associated
with judicial law-making and thus pose legitimacy problems except if the Court
rejects a norm that is widely believed to exist. On the other hand, the analysis of state
practice that leads to the positive identification of a norm has to be more detailed,
while it would, in principle, suffice to cite a few examples that are inconsistent with
the normative proposition in order to reject the existence of custom. For this reason,
using the same classification scheme for both types of situations would not do justice
to the reasoning of the Court.
I limited the systematic analysis to cases in which the ICJ confirmed the positive
existence of a customary norm. In contrast, I did not construct a separate database
for situations in which the Court rejected the customary status of a norm because I do
not believe that this would have added valuable information to the analysis. However,
I will occasionally highlight differences in the qualitative analysis of the specific arguments
where this reveals interesting information. The arguments that were used for
positively identifying a customary norm were categorized according to a specific classification
scheme. The categories of this scheme are not exclusive. Instead, the identification
of one specific customary norm could be based on different arguments at the
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The ICJ and the Identification of Customary International Law 367
same time. In total, the analysis includes 48 decisions from the Corfu Channel case in
1949 to the Costa Rica v. Nicaragua case in 2015, and, in total, 95 instances in which
the Court positively identified the existence of a customary norm.53
I have to make two further specifications. First, the analysis counted every rule of
international law that was identified only once. For example, the ICJ held in several
judgments that Article 31 of the Vienna Convention on the Law of Treaties (VCLT)54
formed part of customary international law.55
However, only the first of these statements
was included in the database in order not to inflate the count of one argument
because the same situation arises over and over again. At the same time, however,
references to different rules of the VCLT were counted as individual instances as long
as they appeared for the first time. Second, the analysis only comprises generalizable
rules that can also be applied in other contexts. This excludes references to binary
customary law or the analysis of the concrete practice of the two parties regarding
the dispute in question.
Furthermore, there are several caveats to the analysis. First, the status of a rule of
international law in the argumentation of the Court is not always clear. For example,
the judges do not specify in every case whether they are dealing with a rule of customary
law or with a general principle in the sense of Article 38(1) lit. c of the ICJ Statute.
If in doubt, I qualified a rule as customary. Second, the category of state practice only
includes the analysis of individual state practice. Treaties or resolutions of the UNGA,
which are often considered to be multilateral state practice in international law doctrine,
have been measured in separate categories. Third, the classification is not free
53
	 Corfu Channel (U.K. v. Albania), Judgment of 9 April 1949, ICJ Reports (1949) 4; Certain Activities Carried
out by Nicaragua in the Border Area (Costa Rica v. Nicaragua); Construction of a Road in Costa Rica along the
San Juan River (Nicaragua v. Costa Rica), Judgment of 16 December 2015.
54
	 Vienna Convention on the Law of Treaties (VCLT) 1969, 1155 UNTS 331.
55
	 Arbitral Award of 31 July 1989 (Guinea-Bissau v. Senegal), Judgment of 12 November 1991, ICJ Reports
(1991) 53, para. 48; Land, Island and Maritime Frontier Dispute (El Salvador v. Honduras), Judgment of
11 September 1992, ICJ Reports (1992) 351, para. 380; Territorial Dispute (Libya v. Chad), Judgment
of 3 February 1994, ICJ Reports (1994) 6, para. 41; Oil Platforms: Preliminary Objections (Iran v. United
States), Judgment of 12 December 1996, ICJ Reports (1996) 803, para. 23; Kasikili/Sedudu Island
(Botswana/Namibia), Judgment of 13 December 1999, ICJ Reports (1999) 1045, para. 18; LaGrand
(Germany v. United States), Judgment of 27 June 2001, ICJ Reports (2001) 466, para. 99; Sovereignty
over Pulau Ligitan and Pulau Sipadan (Indonesia v. Malaysia), Judgment of 17 December 2002, ICJ Reports
(2002) 625, para. 37; Avena and Other Mexican Nationals (Mexico v. U.S.), Judgment of 31 March 2004,
ICJ Reports (2004) 12, para. 83; Legality of Use of Force (Serbia and Montenegro v. Belgium), Judgment
of 15 December 2004, ICJ Reports (2004) 279, para. 100; Legal Consequences of a Wall in the Occupied
Palestinian Territory, Advisory Opinion, 9 July 2004, ICJ Reports (2004) 136, para. 94; Application of
the Convention on the Prevention and Punishment of the Crime of Genocide (Bosnia v. Serbia), Judgment
of 26 February 2007, ICJ Reports (2007) 43, para. 160; Certain Questions of Mutual Assistance in
Criminal Matters (Djibouti v. France), Judgment of 4 June 2008, ICJ Reports (2008) 177, para. 112;
Navigational and Related Rights (Costa Rica v. Nicaragua), Judgment of 13 July 2009, ICJ Reports (2009)
213, para. 47; Pulp Mills on the River Uruguay (Argentina v. Uruguay), Judgment of 20 April 2010, ICJ
Reports (2010) 14, paras 64, 65; Maritime Dispute (Peru v. Chile), Judgment of 27 January 2014, ICJ
Reports (2014) 3, para. 57; Application of the Convention on the Prevention and Punishment of the Crime
of Genocide (Croatia v. Serbia), Judgment of 3 February 2015, para. 138, available at www.icj-cij.org
(last visited 15 March 2016).
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368 EJIL 28 (2017), 357–385
of subjective evaluation. Some classifications are rather simple; it is relatively easy to
see whether the Court refers to a treaty or a resolution of an international institution
when identifying a norm of customary law. However, in other cases, one may
debate whether an argument is a functional, a deductive, or an equitable consideration.
However, this is a general problem in the social sciences. For this reason, I have
listed all cases in which the Court refers to a specific argument in the corresponding
footnotes. This allows every reader to check my classification and to come to different
conclusions.
B  Summary of the Results
The results of the classification are shown in Table 1. They confirm the hypothesis
developed in the theoretical part. The main identification mechanism is the reference
to treaties and UNGA resolutions.56
This reference has two aims. First, it is used as a
legitimation device. The ICJ has been relying on treaties or UNGA resolutions in order
to show that the specific principle was accepted by the vast majority of the international
community. Second, the Court has been using written texts to add specificity to
the often vague, unwritten principles. Furthermore, the Court frequently relies on the
consent of the parties. It painstakingly analyses whether the parties to the particular
case have consented to the norm in question. If the Court finds consent, it is less rigorous
in establishing that the customary norm in question is indeed a norm of universal
scope. From a doctrinal perspective, this finding is surprising since the consent of the
parties cannot be a sufficient reason for the identification of a customary norm. After
all, the norm that is to be identified is a general norm with an effect erga omnes. The
mere consent of the parties of a particular case is not more than the indication of an
opinio juris of the involved states. If the Court thus heavily relies on consent, it shows
two things: first, it is a sign that the Court is rather more concerned with resolving
the specific dispute at issue than with developing and shaping international law as
56
	 This finding is confirmed by other authors; see, in particular, Choi and Gulati, supra note 4, at 131–136.
See also Condorelli, ‘Customary International Law: The Yesterday, Today, and Tomorrow of General
International Law’, in A. Cassese (ed.), Realizing Utopia: The Future of International Law (2012) 147 (arguing
that in certain areas of international law we assume that what is proclaimed in the corresponding
treaties is, at the same time, also part of customary international law).
Table 1:  Identification of Customary Law
Type of Argument Frequency (%)
Treaties 47.4
Consent of parties 30.5
Precedent 17.9
Resolutions of international institutions 15.8
General reference to state practice 12.6
Assertion 10.5
Functional arguments 6.3
Equitable considerations 3.2
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The ICJ and the Identification of Customary International Law 369
a whole and, second, highlighting the consent of the parties is a signal of impartiality
that is supposed to promote the acceptance of the decision and, thus, the Court’s
legitimacy.
These two strategies cover most of the identification activity of the ICJ. However,
in certain cases, the Court also relies on other arguments. It sometimes refers to individual
state practice. But it usually does so in order to show that a customary norm
does not exist. If it refers to state practice when confirming the existence of a customary
norm, this reference is usually of a very general character.There is not one case in
which the positive identification of a customary norm was based on a detailed analysis
of individual state practice. In other cases, it refers to functionalist arguments, to
equitable principles or relies on a simple assertion – without much further reasoning.
Finally, the Court sometimes bases customary norms on precedents – either from
its own jurisprudence or from the jurisprudence of other international courts or
tribunals.
C  Analysis of the Specific Arguments
The quantitative assessment of the different arguments has only demonstrated
the relative use of different types of arguments by the ICJ. In this part, I will add a
qualitative dimension analysing the different arguments in their context in order
to examine whether we can find further indications for the factors driving judicial
decision-making.
1  General Consent of the Parties
Even though it is second to treaties in quantitative terms, consent of the parties to the
dispute is arguably the most important consideration to identify a norm of customary
international law.57
In some judgments, the ICJ is painstakingly concerned with
pointing out that all parties to a particular dispute have consented to the norm in
57
	See Corfu Channel, supra note 53, at 22, 28; Reservations to the Convention on the Prevention and Punishment
of Genocide, Advisory Opinion, 28 May 1951, ICJ Reports (1951) 15, at 23; Fisheries Case (U.K. v. Norway),
Judgment of 18 December 1951, ICJ Reports (1951) 116, at 136–137; Interhandel (Switzerland v. U.S.),
Judgment of 21 March 1959, ICJ Reports (1959) 6, at 27; Barcelona Traction (Belgium v. Spain), Judgment
of 5 February 1970, ICJ Reports (1970) 3, para. 74; Fisheries Jurisdiction (U.K. v. Iceland), Judgment of
25 July 1974, ICJ Reports (1974) 3, para. 52; Delimitation of the Maritime Boundary in the Gulf of Maine
(Can. v. U.S.), Judgment of 12 October 1984, ICJ Reports (1984) 246, para. 94; Continental Shelf (Libya
v. Malta), Judgment of 3 June 1985, ICJ Reports (1985) 13, para. 45; Military and Paramilitary Activities
in and against Nicaragua (Nicaragua v. U.S.), Judgment of 27 June 1986, ICJ Reports (1986) 14, paras 184,
187–189, 198, 203–204, 218; Frontier Dispute (Burkina Faso v. Mali), Judgment of 22 December 1986,
ICJ Reports (1986) 554, para. 20; Land, Island and Maritime Frontier Dispute, supra note 55, para. 40;
Gabčíkovo-Nagymaros (Hungary v. Slovakia), Judgment of 25 September 1997, ICJ Reports (1997) 7, paras
42–43, 50–51, 99, 109, 123; Maritime Delimitation and Territorial Questions between Qatar and Bahrain
(Qatar v. Bahrain), Judgment of 16 March 2001, ICJ Reports (2001) 40, paras 175, 184–185; Israeli
Wall, supra note 55, paras 89, 100; Pulp Mills, supra note 55, para. 203; Jurisdictional Immunities of the
State (Germany v. Italy), Judgment of 3 February 2012, ICJ Reports (2012) 99, paras 59–61; Obligation
to Prosecute or Extradite (Belgium v. Senegal), Judgment of 20 July 2012, ICJ Reports (2012) 422, para.
97; Territorial and Maritime Dispute (Nicaragua v. Colombia), Judgment of 19 November 2012, ICJ Reports
(2012) 624, paras 114, 138; Certain Activities Carried Out by Nicaragua, supra note 53, para. 106.
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370 EJIL 28 (2017), 357–385
question.58
The most prominent example of using consent as a basis for customary
norms is the Nicaragua judgment.59
In its analysis of the applicable rules of customary
international law, the Court leaned heavily on the consent of the parties.60
The Court
starts out its examination by observing ‘that there is in fact evidence ... of a considerable
degree of agreement between the Parties as to the content of the customary
international law relating to the non-use of force and non-intervention’.61
In the following, it refers to the assertion of the USA in its counter-memorial on
jurisdiction and admissibility that ‘Article 2 (4) of the Charter is customary and general
international law’62
and to the ‘attitude of the Parties’ to UNGA Resolution 2625
(XXV)63
when analysing the scope of the prohibition of the use of force.64
With regard
to the principle of non-intervention, it referred to ‘numerous declarations adopted by
international organizations and conferences in which the United States and Nicaragua
have participated’.65
Finally, the Court also relied on principles of humanitarian law
that it derived from the Geneva Conventions66
that the USA had ratified.67
Consequently, the ICJ used consent in order to overcome jurisdictional issues. As a
result of a reservation of the USA, the Court could not apply the multilateral treaties
governing the issue at hand. For this reason, it relied on customary law. The reference
to consent can be seen as an expression of the principle of good faith. The USA
should not be able to deny the applicability of rules to which it had agreed in general.
However, even if the Court relies on consent, this does not mean that the parties necessarily
agree on the application of the rule in the concrete case. Instead, what is important
is that the parties have accepted the rule in the abstract, while the Court claims
autonomy to interpret and apply the rule. In the Corfu Channel case, the ICJ had to
deal with the question whether the damaging of British war ships through Albanian
mines in the Corfu Channel violated international law.68
The Court noted that it was
‘generally recognized and in accordance with international custom that States in time
of peace have a right to send their warships through straits used for international
navigation between two parts of the high seas without the previous authorization of
a coastal State’.69
58
	 See Venzke, ‘Understanding the Authority of International Courts and Tribunals: On Delegation and
Discursive Construction’, 14 Theoretical Inquiries in Law (2013) 381, at 392.
59
	 Military and Paramilitary Activities, supra note 57, paras 184, 187–189, 198, 203–204, 218.
60
	 See Simma and Alston, ‘The Sources of Human Rights Law: Custom, Jus Cogens, and General Principles’,
12 Australian Yearbook of International Law (1992) 82, at 97 (calling this tendency ‘astonishing’);
Tomuschat, ‘International Law: Ensuring the Survival of Mankind on the Eve of a New Century’, 281
Recueil des Cours (1999) 13, at 326–327.
61
	 Military and Paramilitary Activities, supra note 57, para. 184.
62
	 Ibid., para. 187 (emphasis in the original).
63
	 GA Res. 2625 (XXV), 24 Oct. 1970.
64
	 Military and Paramilitary Activities, supra note 57, at para. 188.
65
	 Ibid., para. 203.
66
	 Geneva Conventions 1949, 1125 UNTS 3.
67
	 Military and Paramilitary Activities, supra note 57, at para. 218.
68
	 Corfu Channel, supra note 53.
69
	 Ibid., at 28.
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The ICJ and the Identification of Customary International Law 371
Albania did not challenge the existence of such a right of passage.70
However, it
denied that the Corfu Channel belonged to the group of international straights. There
was thus no disagreement on the existence of the customary norm but, rather, disagreement
on its application. The Court argued that the Corfu Channel had always
been used by international maritime traffic and was thus to be considered as an international
waterway.71
In contrast, the Court usually does not assert the existence of
a customary norm if one of the parties is explicitly opposed to the norm in question.
The most prominent example is the North Sea Continental Shelf case.72
For the delimitation
of the continental shelf, Denmark and the Netherlands asked the Court to apply
the equidistance principle contained in Article 6 of the 1958 Geneva Continental
Shelf Convention.73
While Denmark and the Netherlands had ratified the convention,
Germany had only signed, but not ratified, and objected to the application in its argument
before the ICJ. The Court concluded that Article 6 of the Geneva Continental
Shelf Convention74
did not reflect customary international law.75
This pattern can also be found in further judgments. In Territorial and Maritime
Dispute, the parties did not agree on whether paragraphs 4–9 of Article 76 of the
United Nations Convention on the Law of the Sea (UNCLOS)76
formed part of customary
international law.77
Colombia had explicitly opposed the provisions to be considered
as rules of customary international law.78
The Court sidestepped the issue by
declaring that, for the present case, it did not have ‘to decide whether other provisions
of Article 76 of UNCLOS form part of customary international law’.79
In the 2015
Genocide Convention case, the parties disagreed on whether Article 10(2) of the ILC
Articles on State Responsibility80
reflected customary law.81
Again, the Court avoided
deciding on the issue and argued the provision did not apply to the case.
The importance of consent as a basis for customary norms underlines two characteristics
of the ICJ’s style of reasoning. On the one hand, the Court has a considerable
interest in its judgments being accepted by both parties. Certainly, it is rarely possible
to avoid that a decision has both a winning and a losing party. However, by basing its
arguments on principles to which both parties have explicitly consented, the Court
makes the acceptance by the losing party more likely. On the other hand, the Court is in
many cases rather more interested in settling the dispute at issue than in proclaiming
70
	 Ibid.
71
	 Ibid., at 28–29.
72
	 North Sea Continental Shelf (Germany v. Denmark; Germany v. the Netherlands), Judgment of 20 February
1969, ICJ Reports (1969) 3.
73
	See ibid., para. 37.
74
	 Convention on the Continental Shelf, 499 UNTS 312.
75
	 North Sea Continental Shelf, supra note 72, paras 37–81.
76
	 UN Convention on the Law of the Sea 1982, 1833 UNTS 3.
77
	 Territorial and Maritime Dispute, supra note 57, para. 117.
78
	 Ibid..
79
	 Ibid., para. 118.
80
	 International Law Commission, Articles on the Responsibility of International Organizations, Doc.
A/66/10 (2011).
81
	 Croatia v. Serbia, supra note 55, paras 102–104.
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372 EJIL 28 (2017), 357–385
general and abstract principles of international law. Consent is a very narrow basis for
a judgment. However, in order to achieve acceptance, the Court is willing to concede
the generalizability of its legal reasoning and to refrain from an active development of
international law.The reliance on consent is thus a strong indication that the ICJ takes
into account institutional constraints in its reasoning and decision-making.
2  Customary Law Derived from Treaties and UNGA Resolutions
A second important means to identify norms of customary international law is the
reference to international treaties,82
UNGA resolutions83
and ILC documents.84
In this
respect, the ICJ usually relies on so-called traités lois or law-making treaties.85
The treaties
and documents to which it refers most frequently are the VCLT, UNCLOS and the
ILC Articles on State Responsibility.86
However, the ICJ does not resort to international
treaties to extend their scope to states that have not ratified the particular treaty and
82
	See Nottebohm (Liechtenstein v.  Guatemala), Judgment of 6 April 1955, ICJ Reports (1955) 4, at 23;
Legal Consequences for States of the Continued Presence of South Africa in Namibia [South West Africa]
Notwithstanding Resolution 276 [1970], Advisory Opinion, 21 June 1971, ICJ Reports (1971) 16,
para. 94; Fisheries Jurisdiction (U.K. v. Iceland), Jurisdiction of the Court, 2 February 1973, ICJ Reports
(1973) 3, para. 36; Western Sahara, Advisory Opinion, 16 October 1975, ICJ Reports (1975) 12, paras
52–59; United States Diplomatic and Consular Staff in Tehran (U.S. v. Iran), Judgment of 24 May 1980, ICJ
Reports (1980) 3, para. 62; Interpretation of the Agreement of 25 March 1951 between the WHO and Egypt,
Advisory Opinion, 20 December 1980, ICJ Reports (1980) 73, paras 46–47; Continental Shelf (Tunisia
v. Libya), Judgment of 24 February 1982, ICJ Reports (1982) 18, paras 45, 101, 111; Gulf of Maine,
supra note 57, para. 94; Continental Shelf (Libya v. Malta), supra note 57, paras 34, 77; Military and
Paramilitary Activities, supra note 57, paras 183–192, 198, 212, 218; Arbitral Award, supra note 55,
para. 48; Maritime Delimitation in the Area between Greenland and Jan Mayen (Denmark v. Norway), 14 June
1993, ICJ Reports (1993) 38, paras 48, 54–58; Legality of the Threat or Use of Nuclear Weapons, Advisory
Opinion, 8 July 1996, ICJ Reports (1996) 226, paras 79, 82; Gabčíkovo-Nagymaros, supra note 57, paras
46, 109, 142; Qatar v. Bahrain, supra note 57, paras 167, 175, 184, 185, 195, 201, 207, 208, 214;
LaGrand, supra note 55, para. 101; Arrest Warrant of 11 April 2000 (D.R.C. v. Belgium), Judgment of 14
February 2002, ICJ Reports (2002) 3, para. 52; Land and Maritime Boundary between Cameroon and Nigeria
(Cameroon v. Nigeria), Judgment of 10 October 2002, ICJ Reports (2002) 303, paras 263, 264; Israeli
Wall, supra note 55, paras 78, 87–89; Armed Activities on the Territory of the Congo (D.R.C. v. Uganda),
Judgment of 19 December 2005, ICJ Reports (2005) 168, paras 172, 214, 217; Armed Activities on the
Territory of the Congo (D.R.C. v. Rwanda), Judgment of 3 February 2006, ICJ Reports (2006) 6, paras 41,
46; Certain Questions of Mutual Assistance, supra note 55, paras 124, 174; Accordance with International
Law of the Unilateral Declaration of Independence in Respect of Kosovo, Advisory Opinion, 22 July 2010, ICJ
Reports (2010) 403, para. 80; Obligation to Prosecute or Extradite, supra note 57, paras 99, 113; Territorial
and Maritime Dispute, supra note 57, paras 114, 117, 138–139, 177, 182.
83
	See Reservations to the Genocide Convention, supra note 57, at 23; Western Sahara, supra note 82, paras
52–59; Military and Paramilitary Activities, supra note 57, paras 188, 202–204; 228; Armed Activities
(D.R.C. v. Uganda), supra note 82, paras 162, 244; Kosovo, supra note 82, para. 80.
84
	See Gabčíkovo-Nagymaros, supra note 57, paras 50, 83, 123; Immunity from Legal Process of a Special
Rapporteur of the Commission on Human Rights, Advisory Opinion, 29 April 1999, ICJ Reports (1999) 62,
para. 62; Bosnia v. Serbia, supra note 55, paras 385, 388, 398, 420; Jurisdictional Immunities, supra note
57, para. 58; Croatia v. Serbia, supra note 55, para. 128.
85
	 On the concept of law-making treaties, see P. Daillier, M. Forteau and A. Pellet, Droit international public
(8th edn, 2009), para. 65.
86
	 GA Res. 56/83, 12 December 2001.
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The ICJ and the Identification of Customary International Law 373
are explicitly opposed to it. Instead, the reference to treaties and resolutions is in many
cases closely connected to the reliance on consent, discussed in the previous section.
We can distinguish several constellations in which the ICJ relies on treaties as a means
of identifying customary international law. In a first group of cases, the reference to
treaties is an expression of the consent principle discussed in the previous section. For
example, the Court relies on treaties when both parties have ratified a particular treaty,
even if the treaty does not directly govern the concrete case. In Nicaragua, the Court relied
on the UN Charter as a reflection of customary law.87
The UN Charter had been ratified
by both parties, but the Court did not have jurisdiction to apply the Charter. In GabčíkovoNagymaros,
the Court relied on the VCLT as evidence for customary international law.88
Again, both parties had ratified the VCLT, but it was inapplicable ratione temporis since the
treaty in question had been concluded before the entry into force of the VCLT.89
In other cases, the ICJ relied on a treaty if only one party had ratified the treaty but
if the other had acknowledged that the treaty reflected to a large extent customary
law. On this basis, the court applied UNCLOS in Qatar v. Bahrain and in the Territorial
and Maritime Dispute between Nicaragua and Colombia, even though one of the parties
was not directly bound by UNCLOS in both disputes.90
Finally, there are a few cases
in which certain documents are not binding for either of the parties, but in which the
parties agree on the status as customary law.91
In the Israeli Wall advisory opinion, for
example, the Court held that the Hague Regulations, annexed to the Fourth Hague
Convention from 1907,92
formed part of customary law.93
It noted that the customary
status of the Hague Regulations had been recognized by all participants in the
proceedings before the Court.94
In contrast, as we have already seen, the ICJ is very reluctant to extend the scope of
a treaty if one party has not ratified the respective treaty and has explicitly objected
to its application.95
In the North Sea Continental Shelf judgment, the Court refused to
accept Article 6 of the Geneva Continental Shelf Convention as customary law against
the opposition of Germany.96
Equally, in the Territorial and Maritime Dispute between
Nicaragua and Colombia, the Court avoided applying a specific provision of UNCLOS
when Colombia explicitly objected to its application.97
87
	 Military and Paramilitary Activities, supra note 57, paras 187–190.
88
	 Gabčíkovo-Nagymaros, supra note 57, paras 46, 99, 109, 142.
89
	See ibid., paras 42, 43, 99.
90
	 Qatar v. Bahrain, supra note 57, para. 167; Territorial and Maritime Dispute, supra note 57, paras 114, 138.
91
	See Gabčíkovo-Nagymaros, supra note 57, paras 50, 123 (relating to Art. 33 of the Articles on State
Responsibility and to Art. 12 of the Draft Articles on the Succession of States in respect of Treaties,
reprinted in 2 Yearbook of the International Law Commission (1974) 174.
92
	 Hague Regulation Concerning the Laws and Customs of War on Land 1899, 187 CTS 227; Hague
Convention IV on Respecting the Laws and Customs of War on Land and Its Annex: Regulation concerning
the Laws and Customs of War on Land 1907, 187 CTS 227.
93
	 Israeli Wall, supra note 55, para. 89.
94
	 Ibid.
95
	 See section II.C.1 above
96
	 North Sea Continental Shelf, supra note 72.
97
	 Territorial and Maritime Dispute, supra note 57, paras 117–118. See also notes 77–79 above and the
accompanying text.
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374 EJIL 28 (2017), 357–385
In a second group of cases, the ICJ relies on treaties as a means to identify customary
law if the principles expressed by the treaty concern abstract meta-rules that
seem to form part of almost any legal system so that no state could reasonably object
to them. Most prominently, the Court referred to the principles of treaty interpretation
expressed in Article 31 and 32 of the VCLT in numerous cases.98
But the Court
also referred to other principles of the VCLT that were unlikely to be disputed. These
comprise the principle of pacta sunt servanda (Article 26 of the VCLT),99
the prohibition
to invoke provisions of domestic law as a justification for the failure to perform
a treaty (Article 27 of the VCLT)100
and the principle of non-retroactivity of treaties
(Article 28 of the VCLT).101
Similarly, the ICJ often derives several hardly contested
rules from the ILC Articles on State Responsibility.102
These include the principles
that counter-measures have to be proportionate,103
that the conduct of an organ of a
state is attributable to the respective state,104
that what constitutes an internationally
wrongful act is governed by international law105
and that whether a wrongful act has
been committed has to be determined according to the law applicable at the time of
committal.106
The final category includes cases in which the ICJ relies on treaties to afford the status
of customary law to widely accepted principles of high moral value. In Nicaragua,
the Court argued that the common Article 3 of the four Geneva Conventions formed
part of customary international law:
There is no doubt that, in the event of international armed conflicts, these rules also constitute
a minimum yardstick, in addition to the more elaborate rules which are also to apply to international
conflicts; and they are rules which, in the Court’s opinion, reflect what the Court in
1949 called ‘elementary considerations of humanity’.107
On several occasions, the Court confirmed the customary nature of the fundamental
principles of humanitarian law.108
Moreover, the Court asserted that the principle of
self-determination, as reflected by UNGA Resolution 2625 (XXV), was part of customary
international law. Other customary norms of high moral value, which, according
98
	 See cases in note 55 above.
99
	 Pulp Mills, supra note 55, para. 145.
100
	 Ibid., para. 121; Obligation to Prosecute or Extradite, supra note 57, para. 113.
101
	 Obligation to Prosecute or Extradite, supra note 57, para. 100.
102
	 For an account on the influence of the ILC Articles on State Responsibility on the court’s jurisprudence,
see Crawford, ‘The International Court of Justice and the Law of State Responsibility’, in C.J. Tams and
J.  Sloan (eds), The Development of International Law by the International Court of Justice (2013) 71, at
81–85.
103
	 Gabčíkovo-Nagymaros, supra note 57, para. 83.
104
	 Special Rapporteur of the Commission on Human Rights, supra note 84, para. 62. See also Bosnia v. Serbia,
supra note 55, paras 385, 388, 395, 398, where the ICJ refers to the Articles on State Responsibility as
evidence for rules of attribution.
105
	 Croatia v. Serbia, supra note 55, para. 128.
106
	 Jurisdictional Immunities, supra note 57, para. 58.
107
	 Military and Paramilitary Activities, supra note 57, para. 218.
108
	 See, e.g., Nuclear Weapons, supra note 82, paras 79, 82; Armed Activities (D.R.C. v. Uganda), supra note 82,
para. 214.
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The ICJ and the Identification of Customary International Law 375
to the Court, are reflected by treaties or UNGA resolutions, include the prohibition of
genocide109
and torture.110
Allof theseprinciplesarewidelyacceptedbytheinternationalcommunityandbelong
to the ethical cornerstones of the international legal order. Certainly, some of these principles
are still often violated in practice today, so that it might be doubtful whether they
would meet the traditional requirements of customary international law.111
However,
states will rarely ever deny their general validity.112
They will rather deny accusations
of violating international law on factual grounds or try to carve out specific exceptions.
For this reason, the ICJ does not jeopardize its legitimacy by proclaiming these principles.
It would rather endanger its credibility if it denied legal status to such widely accepted
principles, even if the supporting practice may not always be uniform.113
Unlike consent, the reliance on treaties could, in principle, also be an expression of
a doctrinal approach or of a progressive development of the international legal order.
Doctrinally, treaties or resolutions of international organizations are often seen as
indications of opinio juris or even ‘paper’ practice.114
However, there may sometimes be
conflicts between this paper practice and actual practice.115
If the Court relies on different
human rights instruments to justify why torture is prohibited under customary
international law,116
it should have at least discussed the relevance of a considerable
practice of torture that still exists in the world.
This observation might suggest that the ICJ is using treaties as a means to develop
international law progressively. However, in most cases in which the Court bases the
identification of a customary norm on treaties or resolutions of international institutions,
it takes into account whether the treaty norm is accepted by the parties of the
case. If one party refuses to recognize the customary status of a norm, the Court avoids
proclaiming a corresponding customary rule. This suggests that treaties or resolutions
are primarily an instrument to signal impartiality and that the Court is rather motivated
by its institutional position than by doctrinal concerns or political preferences.
3  Analysis of Individual State Practice
Detailed analyses of state practice are rare in the ICJ’s jurisprudence if we exclude
multilateral (or ‘paper’) practice expressed through treaties or resolutions, as analysed
109
	 Armed Activities (D.R.C. v. Rwanda), supra note 82, para. 64.
110
	 Obligation to Prosecute or Extradite, supra note 57, para. 99.
111
	 Simma and Alston, supra note 60, at 90.
112
	 See Petersen, ‘International Law, Cultural Diversity and Democratic Rule – Beyond the Divide Between
Universalism and Relativism’, 1 Asian Journal of International Law (2011) 149, at 152.
113
	 See A. von Bogdandy and I. Venzke, In Whose Name? – A Public Law Theory of International Adjudication
(2014), at 57–59 (describing the legitimacy crisis of the ICJ in the 1960s when the court declined jurisdiction
in several cases concerning the South African Apartheid policy).
114
	 See Virally, ‘Le rôle des “principes” dans le développement du droit international’, in Recueil d’études de
droit international en hommage à Paul Guggenheim (1968) 531, 550; Akehurst, ‘Custom as a Source of
International Law’, 47 British Yearbook of International Law (BYIL) (1974) 1, at 4; Treves, ‘Customary
International Law’, in R. Wolfrum (ed.), Max Planck Encyclopedia of Public International Law (2006) para.
47, available at http://opil.ouplaw.com/home/EPIL (last visited 2 February 2017).
115
	 See Simma and Alston, supra note 60, at 90–92.
116
	See Obligation to Prosecute or Extradite, supra note 57, para. 99.
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376 EJIL 28 (2017), 357–385
in the previous section. If the Court examines state practice thoroughly, it usually
comes to the conclusion that a principle of customary international law does not
exist.117
One recent example is the Jurisdictional Immunities case.118
In this judgment,
the Court had to analyse the extent of the jurisdictional immunity of states before
foreign courts. The Court first observed that states were generally entitled to immunity
with regard to acta jure imperii.119
Subsequently, it examined whether there was
a customary exception to this principle with regard to war crimes. For this purpose,
it made a detailed analysis of the relevant state practice, which consisted primarily of
decisions of domestic courts.120
It observed that most domestic courts had granted
immunity to foreign states even for acts classified as war crimes.121
The only other
country in which national courts had issued judgments consistent with the practice
of the Italian courts had been Greece.122
In Greece, the Hellenic Supreme Court had
denied immunity to Germany for acts of war crimes committed during World War II.
But, even there, the Greek Special Supreme Court had overturned the judgment of
the Hellenic Supreme Court in its Distomo judgment123
, so that the Greek practice had
only limited precedential value.124
In judgments in which the ICJ confirmed the positive existence of a customary
rule, practice was a mere auxiliary instrument. It usually confirmed a result that
had already been found through other means. And, even here, the Court predominantly
refers to practice only in the abstract without analysing the specific practice in
detail.125
For example, in the dispute between Argentina and Uruguay concerning the
Pulp Mills on the River Uruguay, the Court observed:
[A] practice, which in recent years has gained so much acceptance among States that it may
now be considered a requirement under general international law to undertake an environmental
impact assessment where there is a risk that the proposed industrial activity may have
a significant adverse impact in a transboundary context, in particular, on a shared resource.126
117
	See Corfu Channel, supra note 53, at 35; Asylum Case (Colombia v. Peru), Judgment of 20 November 1950,
ICJ Reports (1950) 266, at 276–278; Fisheries Case, supra note 57, at 131; North Sea Continental Shelf,
supra note 72, paras 70–81; NuclearWeapons, supra note 82, para. 71; ArrestWarrant, supra note 82, para.
58; Ahmadou Sadio Diallo (Republic of Guinea v. D.R.C.), Preliminary Objections, 24 May 2007, ICJ Reports
(2007) 582, paras 86–93; Jurisdictional Immunities, supra note 57, paras 73–76, 83–85.
118
	 Jurisdictional Immunities, supra note 57, paras 73–76, 83–85.
119
	 Ibid., para. 61.
120
	 Ibid., paras 73–76.
121
	 Ibid., paras 73–75.
122
	 Ibid., para. 76.
123
	 Areios Pagos, Case no. 11/2000, Prefecture of Voiotia v. Federal Republic of Germany, Judgment of 4 May
2000, 129 ILR 513.
124
	 Jurisdictional Immunities, supra note 57, para. 76.
125
	See Fisheries Case, supra note 57, at 128; Nottebohm, supra note 82, at 22; Barcelona Traction, supra note
57, para. 70; Fisheries Jurisdiction, supra note 57, paras 23, 26; Western Sahara, supra note 82, para. 80;
Continental Shelf (Tunisia v. Libya), supra note 82, para. 111; Qatar v. Bahrain, supra note 57, para. 173;
Cameroon v. Nigeria, supra note 82, paras 263, 264; Kosovo, supra note 82, para. 79; Pulp Mills, supra note
55, para. 204; Obligation to Prosecute or Extradite, supra note 57, para. 97. See also Wood, First Report,
supra note 1, para. 62 (arguing that the court refers to practice without a detailed analysis when it considers
the existence of a customary norm to be obvious).
126
	 Pulp Mills, supra note 55, para. 204.
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The ICJ and the Identification of Customary International Law 377
The only judgments where the Court has analysed state practice in detail are cases
that concern the bilateral practice of the parties.127
In its decision on Navigational
Rights, the Court held that Costa Rican fishermen had a customary right to subsistence
fishing in the San Juan River.128
Nicaragua had not denied such a practice but
only claimed a lack of opinio juris – a claim that was rejected by the Court.129
In Right of
Passage, the Court determined that Portugal had a general right of passage over Indian
territory, based on a long continued practice between the two states.130
However, these
cases do not concern generalizable norms that are applicable outside the context of
the specific case.
Consequently, even though state practice is seen as a constitutive element of customary
international law in most international law textbooks and treatises,131
it only
plays a marginal role in the case law of the ICJ. The fact that the Court pays only lip
service to the traditional definition of custom is a strong sign that doctrinal constraints
play a minor role in the identification process of customary law. At the same
time, it is also an indication that the Court is motivated by institutional concerns. State
practice is often difficult to observe and rarely homogenous. Therefore, courts necessarily
have to be selective if they want to confirm the existence of a norm of customary
international law based on state practice.132
This selectivity in identifying the relevant
state practice could give the impression of partiality – for example, that the Court
favours the practice of one party over the practice of another; Northern practice over
Southern practice or the practice of influential states over that of less influential ones.
In order to preserve their own legitimacy, courts have to avoid such an impression.
4  Functional Arguments, Equity and Assertion
There is a further group of cases in which the ICJ neither refers to consent, treaties
or state practice in order to establish a customary norm. When rendering a decision,
courts do not usually have the option of a non liquet.133
They cannot simply refrain
127
	 See, e.g, Right of Passage over Indian Territory (Portugal v. India), Judgment of 12 April 1960, ICJ Reports
(1960) 6, at 39; Navigational Rights, supra note 55, paras 140–141.
128
	 Navigational Rights, supra note 55, paras 140–141.
129
	 Ibid., para. 141.
130
	 Right of Passage, supra note 127, at 39.
131
	 Daillier, Forteau and Pellet, supra note 85, para. 207; A. Cassese, International Law (2nd edn, 2005), at
156; J. Crawford, Brownlie’s Principles of Public International Law (8th edn, 2012), at 23; Treves, supra
note 114, para. 8; H. Thirlway, The Sources of International Law (2014), at 56–57; Wood, Third Report,
supra note 1, para. 13; Tams, ‘Die Identifikation des Völkergewohnheitsrechts’, 47 Berichte der deutschen
Gesellschaft für Internationales Recht (2016) 323, at 333.
132
	Talmon, supra note 4, at 432. See, e.g., Pavoni, ‘An American Anomaly? On the ICJ’s Selective Reading
of United States Practice in Jurisdictional Immunities of States’, 21 Italian Yearbook of International Law
(2011) 143 (accusing the ICJ of neglecting important parts the US practice in Jurisdictional Immunities).
133
	 See Lauterpacht, ‘Some Observations on the Prohibition of “Non Liquet” and the Completeness of the
Law’, in F.M. van Asbeck et al. (eds), Symbolae Verzijl (1958) 196; Weil, ‘“The Court Cannot Conclude
Definitively...” Non Liquet Revisited’, 36 Columbia Journal of Transnational Law (1998) 109. But see also
J. Stone, ‘Non Liquet and the Function of Law in the International Community’, 35 BYIL (1959) 124
(arguing that courts are neither prohibited, nor obliged to declare a non liquet); U. Fastenrath, Lücken im
Völkerrecht(1990),at272–284(arguingthatthereisnoprohibitionof anonliquetininternationallaw);
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378 EJIL 28 (2017), 357–385
from deciding because there is no legal norm governing the case. In such situations,
the ICJ primarily relies on three strategies. First, it resorts to functional arguments.
These arguments try to show that certain norms necessarily follow from the characteristics
of a specific institution. Second, the Court relies on considerations of equity.
In some cases, finally, the Court simply asserts the existence of a legal norm without
making an attempt to justify its finding.
(a)  Functional arguments
There are a few cases in which the ICJ resorted to functional arguments.134
In Barcelona
Traction, it analysed the nature of legal personality and argued that Belgium could not
exercise diplomatic protection for the shareholders of a Canadian company because
individual shareholders were generally not allowed to exercise rights on behalf of the
company.135
In the ArrestWarrant case, the Court used functional considerations when
determining the extent of the immunity of foreign ministers.136
In this case, Belgium
had issued an arrest warrant against the acting foreign minister of the Democratic
Republic of Congo (DRC). The DRC challenged this arrest warrant before the ICJ. It
argued that the warrant had violated the foreign minister’s immunity. Belgium countered
that the immunity of foreign ministers did not extend to war crimes or crimes
against humanity.
There were two ways of framing the inquiry. First, the ICJ could have framed it as
a question of the extent of the principle of immunity. Does the immunity of foreign
ministers also extend to acts of war crimes or crimes against humanity? Under this
framing, the DRC would have had to prove a uniform practice of applying the principle
of immunity even in cases where the concerned person had committed war crimes
or crimes against humanity. The second possibility was to frame the problem as a
question of principle and exception. The Court would then have imposed a burden on
Belgium to prove a uniform practice that there is an exception to the immunity principle
in cases of war crimes and crimes against humanity.
The ICJ chose the second avenue. It did not analyse state practice and opinio iuris to
establish the general principle of immunity. Instead, it made a functional argument:
In the performance of these functions, [the foreign minister] is frequently required to travel
internationally, and thus must be in a position freely to do so whenever the need should
arise. He or she must also be in constant communication with the Government, and with its
von Bernstorff, ‘Hans Kelsen on Judicial Law-Making by International Courts and Tribunals: A Theory
of Global Judicial Imperialism?’, 14 Law and Practice of International Courts and Tribunals (2015) 35,
at 49–50 (advocating that sectorial courts and tribunals should hand down sectorial non liquets if
they deal with questions of general international law in order not to promote the fragmentation of
international law).
134
	See Reparations for Injuries Suffered in the Service of the United Nations, Advisory Opinion, 11 April 1949,
ICJ Reports (1949) 174, at 178–184; Reservations to the Genocide Convention, supra note 57, at 22;
Barcelona Traction, supra note 57, paras 39–49; Nuclear Tests, supra note 43, para. 49; Arrest Warrant,
supra note 82, paras 53–54.
135
	 Barcelona Traction, supra note 57, paras 41, 42.
136
	 Arrest Warrant, supra note 82, paras 53–54.
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The ICJ and the Identification of Customary International Law 379
diplomatic missions around the world, and be capable at any time of communicating with representatives
of other States. ...The Court accordingly concludes that the functions of a Minister
for Foreign Affairs are such that, throughout the duration of his or her office, he or she when
abroad enjoys full immunity from criminal jurisdiction and inviolability.137
The Court thus established a normative preconception of a full immunity principle
on functional grounds. It then went on to analyse the state practice on whether there
was an exception for the case of war crimes and crimes against humanity. However, it
was unable to find a uniform practice in this respect.138
For this reason, it held that the
Belgian arrest warrant violated the immunity principle.
(b)  Equity
In cases concerning the delimitation of continental shelf areas and maritime boundaries,
the ICJ often relied on considerations of equity when it could not come up with
a different solution.139
The seminal case is the North Sea Continental Shelf judgment.140
In North Sea Continental Shelf, the Court found that there was no customary rule governing
the case. The predominant equidistance principle had not been sufficiently
supported by state practice and opinio iuris.141
In order to resolve the case, the Court
referred to equitable considerations.142
It imposed a procedural obligation to negotiate
on the parties and established some factors to be considered in these negotiations.
However, the ICJ quickly realized that the reference to equity was too vague to provide
a solution for many conflicts.143
For this reason, it refined its approach in subsequent
cases. In Gulf of Maine, the Court held that the delimitation of maritime
boundaries had to be effected by the agreement of the parties.144
In the absence of such
an agreement, ‘delimitation is to be effected by the application of equitable criteria and
by the use of practical methods capable of ensuring, with regard to the geographic
configuration of the area and other relevant circumstances, an equitable result’.145
A similar wording can be found in the ICJ’s judgment on the delimitation of the
Continental Shelf between Libya and Malta. Here, the Court argued that ‘the delimitation
of a continental shelf boundary must be effected by the application of equitable
137
	 Ibid., paras 53–54.
138
	 Ibid., para. 58.
139
	See NorthSeaContinentalShelf, supranote72,para.85;Gulf of Maine, supranote57,para.112;Continental
Shelf (Libya v. Malta), supra note 57, para. 60; Maritime Delimitation (Denmark v. Norway), supra note 82,
para. 48. See also Fisheries Jurisdiction, supra note 57, paras 63–68 (where the Court established the concept
of preferential fishing rights based on equity considerations); Interpretation of the Agreement between
the WHO and Egypt, supra note 82, paras 43–49 (where the Court derives a duty to negotiate and inform
the other party from the principle of good faith).
140
	 North Sea Continental Shelf, supra note 72.
141
	 Ibid., paras 70–82.
142
	 Ibid., paras 83–99.
143
	 Lowe and Tzanakopoulos, ‘The Development of the Law of the Sea by the International Court of Justice’,
in C.J. Tams and J. Sloan (eds), The Development of International Law by the International Court of Justice
(2013) 177, at 189.
144
	 Gulf of Maine, supra note 57, para. 112.
145
	 Ibid.
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380 EJIL 28 (2017), 357–385
principles in all the relevant circumstances in order to achieve an equitable result’.146
On this basis, the Court introduced a two-step procedure to determine the boundary
of the continental shelf. First, it effected a provisional delimitation based on a criterion
of distance from the coast, before considering corrections of the initial result based on
requirements derived from other criteria.147
In particular, it considered the disproportionate
length of opposing coastlines as a primary criterion for correcting the provisional
result.148
It later refined this approach by making the disproportionality test a
separate third stage in its determination of maritime boundaries.149
Over the course of several judgments and decades, the ICJ thus developed a refined
approach for delimiting maritime boundaries. The three-stage test applied by the
court is neither based on specific treaty provisions nor on state practice. Rather, the
court justified its approach referring to considerations of equity.150
If cases of boundary
limitations come to court, the parties usually have a strong interest in resolving
the dispute at hand.151
While they have preferences regarding the exact shape of the
boundary, they deem the resolution of the dispute to be more important than the
exact outcome of the ruling.152
What is important for the resolution of the dispute,
however, is the impartiality of the court. If the court seems to side with one party, the
other will most likely not accept the result.153
For this reason, courts have strong incentives to make compromises in such situa-
tions.154
In order to signal their impartiality, they will usually give each party a share
of the cake. By developing a flexible approach to maritime boundary delimitation, the
ICJ has put itself into a position to strike such compromises.The equidistance principle
may be easy to administer, but it often favours one party. For this reason, the court
only uses it as a starting point but applies certain corrections later on in order to grant
concessions to the party that is disadvantaged by the application of the mere equidistance
criterion. The reliance on equitable principles is thus an instrument to ensure
compliance in an area where clear rules do not exist and where they would also not be
appropriate to resolve most disputes in a satisfactory way for both parties.
(c) Assertion
Finally, the ICJ sometimes just proclaims the existence of specific customary norms
without justifying how it derived them.155
The most famous example is probably the
146
	 Continental Shelf (Libya v. Malta), supra note 57, para. 45.
147
	 Ibid., paras 60–61.
148
	 Ibid., para. 57.
149
	 Territorial and Maritime Dispute, supra note 57, paras 190–193.
150
	 See T. Cottier, Equitable Principles of Maritime Boundary Delimitation: The Quest for Distributive Justice in
International Law (2015), at 352–353.
151
	 Simmons, ‘Capacity, Commitment, and Compliance’, 46 Journal of Conflict Resolution (2002) 829, at 832.
152
	 Petersen, ‘The Role of Consent and Uncertainty in the Formation of Customary International Law’, in
B. Lepard (ed.), Reexamining Customary International Law (2016) 111, at 126–127.
153
	Kingsbury, ‘International Courts: Uneven Judicialization in Global Order’, in J.  Crawford and
M. Koskenniemi (eds), The Cambridge Companion to International Law (2012) 203, at 216.
154
	 See Stone Sweet, Governing with Judges, supra note 15, at 200.
155
	See Reparations for Injuries, supra note 134, at 185; Reservations to the Genocide Convention, supra note 57,
at 21; Fisheries Case, supra note 57, at 129; Monetary Gold Removed from Rome in 1943 (Italy v. France,
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The ICJ and the Identification of Customary International Law 381
birth of the concept of erga omnes principles in Barcelona Traction.156
When determining
whether Belgium had standing to exercise diplomatic protection on behalf of
Belgian shareholders of a Canadian company, the court argued that:
[i]n particular, an essential distinction should be drawn between the obligations of a State
towards the international community as a whole, and those arising vis-à-vis another State in
the field of diplomatic protection. By their very nature the former are the concern of all States.
In view of the importance of the rights involved, all States can be held to have a legal interest
in their protection; they are obligations erga omnes.157
The court then continued to specify these obligations:
Such obligations derive, for example, in contemporary international law, from the outlawing of
acts of aggression, and of genocide, as also from the principles and rules concerning the basic
rights of the human person, including protection from slavery and racial discrimination. Some
of the corresponding rights of protection have entered into the body of general international
law.158
The court did not make any effort to justify how it derived the concept of erga omnes
or the principles to which it attributed this status. The court probably deemed such a
justification to be unnecessary for two reasons. First, the cited passages are only obiter
dicta. After having established the category of erga omnes rights, the court quickly
noted that Belgium could not rely on the violation of an erga omnes principle in the
present case.159
Second, the court could assume that the mentioned norms had such a
high moral importance that it was unnecessary to justify their existence in customary
international law.
The importance of this passage lies in its precedential value. The concept of erga
omnes obligations influenced the later development of international law to a significant
extent.160
In particular, the ICJ referred to the concept of erga omnes developed in
Barcelona Traction several times in later judgments.161
In these subsequent decisions,
the reference to erga omnes obligations was not a mere obiter dictum anymore. Instead,
the court derived concrete consequences from its classification. In its Israeli Wall
opinion, the court argued that the construction of the wall by the Israeli authorities
U.K., and U.S.), Judgment of 15 June 1954, ICJ Reports (1954) 19, at 32; Temple of PreahVihear (Cambodia
v. Thailand), Judgment of 15 June 1962, ICJ Reports (1962) 6, at 26; Barcelona Traction, supra note 57,
paras 33–34; Military and Paramilitary Activities, supra note 57, para. 115; Nuclear Weapons, supra note
82, para. 29; Gabčíkovo-Nagymaros, supra note 57, para. 152; Qatar v. Bahrain, supra note 57, para. 223.
See also Talmon, supra note 4, at 434–440, who even argues that assertion is the main method of identification
employed by the ICJ. However, his category of assertion is broader than the one employed in thus
study, as Talmon also includes reference to treaties, resolutions, or the work of the ILC.
156
	 Barcelona Traction, supra note 57, paras 33–34.
157
	 Ibid., para. 33 (emphasis in the original).
158
	 Ibid., para. 34.
159
	 Ibid., para. 35.
160
	 See N. Petersen, ‘Lawmaking by the International Court of Justice’, 12 German Law Journal (2011) 1295,
at 1307–1310.
161
	See East Timor (Portugal v. Australia), Judgment of 30 June 1995, ICJ Reports (1995) 90, para. 29; Israeli
Wall, supra note 55, para. 155; Obligation to Prosecute or Extradite, supra note 57, para. 68.
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382 EJIL 28 (2017), 357–385
violated obligations erga omnes.162
For this reason, the court held that ‘all States are
under an obligation not to recognize the illegal situation resulting from the construction
of the wall in the Occupied Palestinian Territory’.163
In Obligation to Prosecute
or Extradite, the court derived Belgium’s standing to invoke Senegal’s responsibility
under the Convention against Torture164
from the erga omnes nature of the convention’s
obligations.165
It is not a rare phenomenon for courts to introduce important and possibly controversial
legal concepts in contexts where they will rarely face opposition.166
When
the US Supreme Court introduced the principle of constitutional review in Marbury
v. Madison,167
it did so in order to reinforce the position of the executive, even though
the concept as such is primarily directed against the political branches. It was only
later that the court resorted to its power to review the constitutional compatibility
of statutes and turned it against the government. Similarly, when the ICJ introduced
the concept of erga omnes in Barcelona Traction, it did so in an obiter dictum without
any consequences for the result of the decision. It took the court more than 20 years
to refer to the concept of erga omnes obligations for a second time. By then, however,
international law scholarship and the majority of states had predominantly accepted
the concept.168
Consequently, the court could base its decision on the concept without
having to fear strong opposition against its reasoning.
(d) Summary
These three gap-filling argumentation patterns are more difficult to qualify than the
reliance on consent or treaties. While all three are difficult to reconcile with a doctrinal
explanation, they could be regarded as an expression of political preferences of
the judges. In particular, if the ICJ merely asserts certain legal principles, as it did in
the Barcelona Traction case, it is likely that the judges had the intention to engage in
judicial law-making. However, even though assertion is a significant argumentation
pattern, it is much less important then reliance on consent or treaties. Furthermore,
as the Barcelona Traction case shows, even in these situations, the court is mindful of
its institutional position. It introduces the principle through an obiter dictum and only
picked it up two decades later and attached legal consequences to it once the principle
had received a positive reception by states and in international law scholarship.
In contrast, the main purpose of the equity argument in the law of the sea is to
signal impartiality. The doctrinal framework developed by the ICJ is so flexible that
it does not impose significant constraints for future decisions. Rather, it is supposed
162
	 Israeli Wall, supra note 55, paras 155–156.
163
	 Ibid., at 159.
164
	 Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment 1984,
1465 UNTS 85.
165
	 Obligation to Prosecute or Extradite, supra note 57, para. 70.
166
	 See Friedman and Delaney, ‘Becoming Supreme: The Federal Foundation of Judicial Supremacy’, 111
Columbia Law Review (2011) 1137, at 1152–1159.
167
	 Marbury v. Madison, 5 US 137 (1803).
168
	Petersen, supra note 160, at 1308–1310.
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The ICJ and the Identification of Customary International Law 383
to give the court flexibility to decide individual boundary delimitations in a way that
they are acceptable to both parties of the dispute. Finally, functional considerations
could, at the same time, be a sign of institutional constraints or of political preferences.
However, even if the latter interpretation was pertinent, it would not change
the overall picture, given that the argumentation pattern is of only minor importance
in quantitative terms.
5  Customary Law and Precedent
In some judgments, finally, the ICJ relies on precedents when identifying a customary
norm. In the vast majority of cases, the court refers to its own former decisions or to
decisions of its predecessor, the Permanent Court of International Justice.169
In such
situations, the court disburdens itself from justifying a customary principle that it had
already justified in an earlier decision. Furthermore, it reinforces its own position and
signals coherence. However, the court sometimes merely evokes the impression that
it is only applying already established legal norms, while it is in fact developing the
law. For example, in its East Timor decision, the court argued that the principle of selfdetermination
had erga omnes character.170
It asserted that:
Portugal’s assertion that the right of peoples to self-determination, as it evolved from the
Charter and from United Nations practice, has an erga omnes character, is irreproachable. The
principle of self-determination of peoples has been recognized by the United Nations Charter
and in the jurisprudence of the Court.171
But the decisions the ICJ referred to – the SouthWest Africa172
and the Western Sahara173
advisory opinions – dealt with the interpretation of the principle of self-determination
governed by treaty instruments, while the court in East Timor referred to the principle
of self-determination contained in customary law.
The law of the sea is another field where the ICJ relies on precedents without revealing
that it is, in fact, developing the law. In its 2012 judgment regarding the Territorial
and Maritime Dispute between Nicaragua and Colombia, the court applied a three-step
analysis for the delimitation of overlapping continental shelf and exclusive economic
169
	See Nottebohm, supra note 82, at 22; Application for Review of Judgment No. 158 of the United Nations
Administrative Tribunal, Advisory Opinion, 12 July 1973, ICJ Reports (1973) 166, 222; Continental Shelf
(Tunisia v. Libya), supra note 82, paras 45, 101; Continental Shelf (Libya v. Malta), supra note 57, paras
55–58; Military and Paramilitary Activities, supra note 57, para. 202; Applicability of the Obligation to
Arbitrate under Section 21 of the United Nations Headquarters Agreement of 26 June 1947, Advisory Opinion,
26 April 1988, ICJ Reports (1988) 12, para. 57; Land, Island and Maritime Frontier Dispute, supra note 55,
para. 73; Maritime Delimitation between Greenland and Jan Mayen, supra note 57, paras 50–53; East Timor,
supra note 161, para. 29; Legality of the Use by a State of Nuclear Weapons in Armed Conflict, Advisory
Opinion, 8 July 1996, ICJ Reports (1996) 66, para. 25; Gabčíkovo-Nagymaros, supra note 57, para. 83;
Armed Activities (D.R.C. v. Uganda), supra note 82, paras 172, 217; Armed Activities (D.R.C. v. Rwanda),
supra note 82, para. 46; Ahmadou Sadio Diallo, supra note 117, paras 86–93; Pulp Mills, supra note 55,
para. 101; Territorial and Maritime Dispute, supra note 57, paras 138–139, 182, 190–193.
170
	 East Timor, supra note 161, para. 29.
171
	 Ibid.
172
	 South West Africa, supra note 82, paras 52–53.
173
	 Western Sahara, supra note 82, paras 54–59.
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384 EJIL 28 (2017), 357–385
zone entitlements.174
It justified this three-step analysis with a reference to earlier
judgments.175
The first judgment to which the court referred was the 1985 Continental
Shelf judgment regarding a dispute between Libya and Malta. However, in the 1985
decision, the test of the court had only consisted of two explicit stages.176
The two last
steps of the 2012 test had still been part of the same step in 1985.
In other cases, it is doubtful whether the argument used in the precedent can really
be transferred to subsequent cases. This is particularly the case if the court based the
identification of a customary rule on the consent of the parties. In its Nicaragua judgment,
the court observed that: ‘[t]he Parties also agree in holding that whether the
response to the attack is lawful depends on observance of the criteria of the necessity
and the proportionality of the measures taken in self-defence’.177
It called this necessity
and proportionality requirement ‘a rule well established in customary international
law’.178
Later, it referred to the Nicaragua judgment in its Nuclear Weapons and
Oil Platforms decisions to justify necessity and proportionality as preconditions to
self-defence.179
These observations show that resorting to precedents is not necessarily a confirmation
of a doctrinal approach of the ICJ. Instead, the court sometimes uses precedents
for a progressive development of international law, extending the scope of concepts
beyond the decision in the precedent to which it refers. Nevertheless, the reasoning
shows that the judges are mindful of their institutional constraints. By relying on precedents,
they want to highlight the coherence of the court’s case law. At the same time,
they signal impartiality as the decision appears to be based on already firmly established
legal principles so that the court is only ‘applying’, not ‘developing’, the law.
5 Conclusion
Robert Jennings once famously quipped that ‘[m]ost of what we perversely persist in
calling customary international law is not only not customary law: it does not even
faintly resemble a customary law’.180
This article has sought to explain this phenomenon.Three
potential explanations for the court’s decision-making have been offered: a
doctrinalapproachaccordingtowhichthedecisionsareprimarilydeterminedthrough
legal norms and legal doctrine, a policy approach arguing that judges develop international
law according to their political preferences and an institutional approach that
focuses on the institutional constraints that judges face. In line with previous findings,
it has been argued that the doctrinal approach cannot explain the ICJ’s jurisprudence
on customary international law. Unlike the definition of customary law that Article
174
	 Territorial and Maritime Dispute, supra note 57, paras 189–199.
175
	 Ibid., para. 189.
176
	 Continental Shelf (Libya v. Malta), supra note 57, para. 60.
177
	 Military and Paramilitary Activities, supra note 57, para. 194.
178
	 Ibid., para. 176.
179
	 Nuclear Weapons, supra note 82, para. 41; Oil Platforms, supra note 169, paras 74, 76.
180
	 Jennings, ‘The Identification of International Law’, in Bin Cheng (ed.), International Law: Teaching and
Practice (1982) 3, at 5.
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The ICJ and the Identification of Customary International Law 385
38 (1) lit. b of the ICJ Statute suggests, the analysis of individual state practice only
plays a marginal role in the court’s argumentation. In contrast, the court prominently
relies on the consent of the parties to a particular customary norm even though the
consent of the parties of a case only has a very minor significance in the doctrine of
customary law.
However, even if legal doctrine has no important constraining function for the
identification of customary norms, this does not mean that judges automatically follow
their political preferences. Instead, the ICJ’s reasoning predominantly resorts to
consent, treaties or resolutions of international institutions since the court primarily
wants to signal impartiality in order to enhance its own legitimacy. This does not
exclude the idea that the court, in certain cases, also tries to develop international law
progressively. Examples are the establishment of the principle of erga omnes norms
in Barcelona Traction or the extension of legal concepts when relying on precedents.
However, in quantitative terms, these instances are rather the exception. Furthermore,
even in such situations, the court takes its institutional constraints into account and
tries to preserve its authority by referring to precedents or by relegating sweeping
statements to obiter dicta.
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