The Identification of Customary Rules in International Criminal Law

Author:Yudan Tan
Position:PhD Candidate, Grotius Centre for International Legal Studies, Leiden University, NL
Pages:92-110
Yudan Tan, ‘The Identification of Customary Rules in International
Criminal Law’ (2018) 34(2) Utrecht Journal of International and
European Law pp. 92–110. DOI: https://doi.org/10.5334/ujiel.434
UTRECHT JOURNAL OF
INTERNATIONAL AND EUROPEAN LAW
RESEARCH ARTICLE
The Identification of Customary Rules in
International Criminal Law
Yudan Tan*
This paper aims to examine whether a different methodology has emerged to identify customary
rules in the field of international criminal law. For this purpose, this paper briefly touches upon
debates regarding customary law as a source and an interpretative aid of international criminal
law. It then critically studies the identification methodology of customary law to seek whether
a new approach deviating from the classic two-element (State practice and
opinio juris
) approach
is emerging in academia. It also recapitulates some cases of international criminal tribunals to
ascertain whether these tribunals have formed a distinct method for custom identification.
Finally, it explores the unique characteristics and difficulties in identifying customary rules
in international criminal law. It concludes that a different method has not been developed in
academia or adopted by tribunals in practice to identify customary rules in international criminal
law. The two-element approach still serves as guidance for custom-identification in general, but
a flexible application of it is acceptable in specific cases. International practitioners should be
cautious in the identification of customary rules in international criminal law, so as to prosecute
and punish suspects of international crimes without endangering the principle of legality.
Keywords: State practice; opinio juris; custom; identification; international criminal law
I. Introduction
Customary international law is important for practitioners in international and national courts. Parallel
with the movement for the codification of international law, custom and treaties may co-exist on the same
subject matter.1 In this circumstance, a rule that exists in each of these two sources may overlap or conflict
with each other or may have identical content, whereas the two sources do not supplant each other and have
separate methods of application.2 In addition, customary law continues to govern the area not stipulated
by treaty law.3
This paper aims to examine the method for ascertainment of customary rules in the field of international
criminal law. When we ask the question of how to identify customary law, we refer to a method to ascertain
the existence of a customary rule rather than the substance of that rule. In other words, the former deals with
the process of identifying whether a customary rule has formed; while the latter concerns the content of a
customary rule.4 The classic approach to identify ing custom is to seek sufficient evidence of State practice
and opinio juris (two-element approach). The recent work of the International Law Commission (ILC) has
also adopted the two-element approach, namely ‘a general practice’ and ‘accepted as law’ in the identif ica-
tion of customary international law.5 It should be noted that, due to an inherent dilemma concerning the
* PhD Candidate, Grotius Centre for International Legal Studies, Leiden University, NL. Contact: tina18tan8@gmail.com.
1 Sir Robert Jennings and Sir Arthur Watts (eds), Oppenheim’s International Law, vol 1 (9th edn, Longman 1996) §§24–32.
2 Military and paramilitary Activities in and against Nicaragua (Nicaragua v USA) (Merits) [1986] ICJ Rep 14, 93–96 paras 175–79
(Military and paramilitary Activities).
3 Vienna Convention on the Law of Treaties (adopted 23 May 1969, entered into force 27 January 1980) 1155 UNTS 331, preamble.
4 Maurice Mendelson, ‘The Formation of Customary International Law’ (1998) 272 Recueil des Cours de l’Académie de Droit 155,
284; Herbert LA Hart, The Concept of Law (2nd edn, CUP 1994); Noora Arajärvi, ‘Between Lex Lata and Lex Ferenda? Customary
International (Criminal) Law and the Principle of Legality’ (2011) 15 Tilburg Law Review 181, 170–72; Christian Tams, ‘Meta-Custom
and the Court: A Study in Judicial Law-making’ (2015) 14 The Law and Practice of International Courts and Tribunals 51.
5 UNGA ‘Text of the draft conclusions on identification of customary international law’ in ‘Report of the International Law
Commission’ UN GAOR 73rd session Supp No 10, UN Doc A/73/10 (Advance) (2018) para 65, Conclusion 2.
Tan 93
evidence of practice and opinio juris,6 some commentators have proposed other theories, including vari-
ations of the two-element approach,7 and one-element approach.8 As to customary rules in international
criminal law, scholars have also proposed a ’core right’ approach, which is a form of one-element approach.9
International criminal law indeed presents some peculiarities as opposed to other branches of international
law. Its objects are individuals, criminal law principles play a role, and it is a regime inspired by both civil and
common law criminal systems.10 An issue arises here as to whether a different custom-identification method
has emerged in this field, departing from the two-element approach.
To ascertain the custom-identification method, Article 38 of the Statute of the International Court of Justice
(ICJ) remains a good starting point.11 Concerning the identification method of customary law, there exists
no treaty, customary rule or general principle. According to Article 38, ‘judicial decisions and the teachings
of the most highly qualified publicists of the various nations’ are subsidiary sources.12 Academic writings
and judicial decisions, in this context, would be the main sources to analyse the method used to identify
the existence of a customary rule. This paper looks into the theories and case law of international and
internationalised criminal tribunals to answer the question of whether a distinct methodology has emerged
for the identification of customary rules in international criminal law.
This paper consists of six sections including this introduction and conclusion. Section II comments on
the role of customary law in international criminal law. Section III briefly discusses the classic theory and
critically evaluates the ‘core right’ approach. The jurisprudence of international and internationalised
criminal tribunals is analysed in Section IV to observe the custom-identification approach employed by
these tribunals.13 Section V attempts to explain that the custom-identification of international criminal rules
is unique in various aspects, leading the assessment of evidence of the two elements to be complicated. The
paper concludes that in identifying customary rules in international criminal law, a different methodology
that deviates from the two-element approach has not come into existence, whereas a flexible formula of the
two-element approach is acceptable.
II. The Role of Customary Law in International Criminal Law
Before examining the method of custom-identification, it is necessary to comment on the role of customary
law in international criminal law. The idea of customary law as a source of international criminal law has
been contested. Rules derived from customary law are quite imprecise and vague.14 Its ambiguity seems to
be inconsistent with the principle of legality requiring specificity and legal certainty.15
6 Generally, a State’s practices are accompanied with its intent, while the intent is difficult to know. If no corresponding
pronouncement of that State is available, it seems that the only evidence of opinio juris is inferred from State practice; whereas if
no action but merely abstract statements exist, it seems that the evidence of State practice is also deducted from opinio juris.
7 Michael Byers, Custom, Power and the Power of Rules: International Relations and Customary International Law (CUP 1999).
8 For example, the practice-based approach, Akehurst, Müllerson, and Mendelson are advocates of this view. Mendelson (n 4);
Michael Akehurst, ‘Custom as a Source of International Law’ (1976) 47 British Yearbook of International Law 53; Rein Müllerson,
‘On the Nature and Scope of Customary International Law’ (1997) 2 Austrian Review of International and European Law 341;
Committee on Formation of Customary (General) International Law, ‘Final Report of the Committee, Statement of Principles
Applicable to the Formation of General Customary International Law’ in International Law Association Report of the Sixty-Ninth
Conference (London 2000) (International Law Association, London 2000) (Formation of General Customary International Law)
741–42. Mendelson claims that subjective and objective elements of State practice are two sides of a coin. Opinio juris is not an
element for the formation of customary international law, but a part of subjective element of State practice. The International
Law Association adopted this standpoint in its 2000 resolution. An alternative is the opinio juris-based approach, see Roberto Ago,
‘Legal Science and International Law’ (1956) 90 Recueil des Cours de l’Académie de Droit 85; Brian Lepard, Customary International
Law: A New Theory with Practical Applications (CUP 2010) 98–100.
9 Theodor Meron, Human Rights and Humanitarian Norms as Customary Law (Clarendon Press 1991) 9, 94; Theodor Meron,
‘International Law in the Age of Human Rights’ (2003) 301 Recueil des cours 9, 378, 384–86.
10
Yeghishe Kirakosyan, ‘Finding Custom: The ICJ and the International Criminal Courts and Tribunals Compared’ in Carsten Stahn and
Larissa van den Herik (eds), The Diversification and Fragmentation of International Criminal Law (Martinus Nijhoff 2012) 149–61.
11
Statute of the International Court of Justice (signed 26 June 1945, adopted 1 December 1949) 33 UNTS 993 (the Statute of the ICJ);
Humphrey Waldock, ‘General Course on Public International Law’ (1962) 106 Recueil des Cours de l’Académie de Droit 41; Tams (n 4).
12 The Statute of the ICJ, arts 38 (1)(d) and 59.
13 William Bishop, ‘General Course of Public International Law’ (1965) 115 Recueil des Cours de l’Académie de Droit 147, 152–53;
Hersch Lauterpacht, The Development of International Law by the International Court (CUP 1982); Christian Tams and James Sloan,
The Development of International Law by the International Court of Justice (CUP 2013).
14 Judge Abdul Koroma, ‘Foreword’ in Jean-Marie Henckaerts and Louise Doswald-beck (eds), Customary International Humanitarian
Law, vol 1, Rules (ICRC and CUP 2005) xiii.
15 For discussions about the principle of legality, see Mohamed Shahabuddeen, ‘Does the Principle of Legality Stand in the Way of
Progressive Development of Law?’ (2004) 2 Journal of International Criminal Justice 1007–10; Shahram Dana, ‘Beyond Retroactiv-
ity to Realizing Justice: A Theory on the Principle of Legality in International Criminal Law Sentencing’ (2009) 99 Journal of Crimi-

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