Opinio Juris as a Constituent Element of Customary International Law: When Does Belief Become Law?

Studiu publicat în volumul In Honorem Flavius Antoniu Baias. Aparența în drept, tomul I, Ed. Hamangiu, București, 2021, p. 508-517.

Introduction

The notion and characteristics of customary international law are controversial, especially because few have been able to address exactly how custom formation works^[1]. The reason for this occurrence is that „unlike soft law and treaties, customary international law does not emerge from a specific process that marks the creation of a legal obligation”^[2].

Some scholars conclude that the content of customary international law and the manner in which it is identified has been scrutinized since international law was born^[3]. Others comment that „the demise of custom as a source of international law has been widely forecasted”^[4] or that „customary international law often seems like a riddle wrapped in a mystery inside an enigma”^[5]. For these reasons, international scholarship addressing the understanding of customary international law has proliferated^[6]. Further international institutions involved in the codification of international law, such as the International Law Commission of the United Nations have included the identification of customary international law on their agenda^[7]. 

The generally accepted interpretation of customary international law implies that a two-pronged test should be performed in order to determine the content of a rule^[8]. Even if a general practice among States must exist and, and it must be accepted by them as law^[9], this „orthodox view of how CIL [customary international law] works is deceptively simple”^[10]. It is not as clear cut as it seems at first glance, particularly because the interaction between practice (the objective element) and its acceptance as law (the subjective element) is not easily distilled, the two being interdependent^[11]. 

The salience of the subjective element, which has been assimilated to beliefs (or feelings) of States, is noticeable when looking towards the direction of the International Court of Justice or of the International Law Commission of the United Nations. Article 38 (b) of the Statute of the International Court of Justice prescribes that the Court decides the disputes that are submitted to it by applying „international custom, as evidence of general practice, accepted as law”^[12]. Further, the International Court of Justice referred to the beliefs and feelings of States, which crystallise opinio juris as a constituent element of customary international law, throughout its jurisprudence, in cases such as the North Sea Continental Shelf Cases^[13] or in the Asylum Case^[14], respectively. Nevertheless, the International Court of Justice has failed to clarify what constitutes opinio juris and how it interacts with the practice of states in order to determine the content of customary international law. 

The International Law Commission of the United Nations issued, in 2018, the Draft Conclusions on Identification of Customary International Law, through which the Commission endeavoured to clarify the meaning of opinio juris. In this sense, Conclusion 9 provides that a general practice accepted as law is not the same as „mere usage or habit”^[15]. Further, the Commentary of the Commission regarding the above-mentioned Conclusion clarifies the meaning of opinio juris and provides that states must „believe” or „feel” that they are legally compelled to act^[16].

§1. General considerations

Article 38 (1) of the Statute of the International Court of Justice prescribes the following:

„1. The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

a) international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;

b) international custom, as evidence of a general practice accepted as law;

c) the general principles of law recognized by civilized nations;

d) subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law”^[17].

The regulations of other international courts and arbitral tribunals contain similar provisions regarding the sources of international law applicable before them. Illustratively, the Rome Statute refers to violations of „customs”^[18] while the Report of the Executive Directors of the Convention on the Settlement of Investment Disputes between States and Nationals of other States contains the following statement regarding the applicable law:

„Under the Convention an Arbitral Tribunal is required to apply the law agreed by the parties. Failing such agreement, the Tribunal must apply the law of the State party to the dispute (unless that law calls for the application of some other law), as well as such rules of international law as may be applicable. The term „international law” as used in this context should be understood in the sense given to it by Article 38(1) of the Statute of the International Court of Justice”^[19].

As such, that customary international law is a source of law is settled. How this source of law is generally identified, not so much, especially because its interpretation, determination of content and application is governed by relevant dichotomies. 

§2. Dichotomies of creating customary international law

Several methods exist to identify customary international law. The doctrine speaks about declaratory of constitutive^[20], inductive and deductive^[21], traditional and modern^[22] or rulebook and progressive^[23]. Most of these categories are intertwined and some of them entail the same content. As such, what is referred to as inductive is also traditional while what is referred to as deductive is also modern. Further, what is labelled as being traditional is also labelled as rulebook. This is the reason for which the structure below does not refer to each of these categories. 

2.1. Declaratory or constitutive

Originally, the content of customary international law was understood as a manifestation of pre-existing natural law^[24]. In this sense, customary international law was purely declaratory, and the belief that a certain behaviour was mandatory by virtue of it being a norm was, thus, not constitutive of customary international law^[25]. 

Currently, this perspective is no longer accurate. The belief that an act is performed because it represents an international obligation, is a constitutive element of customary international law. The International Law Commission of the United Nations, through its Commentaries of the Draft Conclusions on identification of Customary International Law, concludes that „a general practice and acceptance of that practice as law (opinion juris) are the two constituent elements of customary international law: together they are the essential conditions for the existence of a rule of customary international law”^[26].

Further, the Commentary of the International Law Commission mentions that customary international law cannot exist in the absence of opinio juris, by arguing that „the presence of only one constituent element does not suffice for the identification of a rule of customary international law”^[27]. 

The answer to the question of which element takes the lead in the determination of the content of customary international law and which is secondary depends upon the application of two distinct approaches: induction and deduction. 

2.2. Induction: the prioritisation of state practice over opinio juris

The inductive approach implies that the rules of customary international law are determined by inferring a rule through the observance of the practice of States and opinion juris^[28]. As such, customary international law is induced from the „facts of international life” ^[29], concentrates on the freedom of action of the state,^[30] and, consequently serves their interests^[31]. In this sense, commentators have described the inductive method as a „process of observation and empirical generalisation” ^[32]. 

The Lotus Case is one instance in which the Permanent Court of International Justice concluded that custom existed because state practice and acquiescence existed.^[33] More clearly, the International Court of Justice addressed the same issue in the Gulf of Maine Case, in which it concluded as follows with respect to the inductive approach:

„A body of detailed rules is not to be looked for in customary international law which in fact comprises a limited set of norms for ensuring the co-existence and vital co-operation of the members of the international community, together with a set of customary rules whose presence in the opinio juris of States can be tested by induction based on the analysis of a sufficiently extensive and convincing practice, and not by deduction from preconceived ideas”^[34].

This conclusion seems to be in line with the finding of the International Court of Justice in the North Sea Continental Shelf Cases, in which it reached the conclusion that state practice and opinio juris must be present for the existence of customary international law^[35]. Even so, some commentators conclude that the Court, in this case, also endorsed the deductive approach, at least partially because the Court referred to the requirement of „pre-existing norm-creating content for a treaty provision to possibly crystalize into customary law”^[36], implying that at times the inductive approach will not be able, in all instances, to be decisive for the inclusion of certain behaviours within the scope of customary international law.^[37] While several authors have argued in favour of the deductive approach,^[38] Sir Michael Wood, in the First Report on Formation and Evidence of Customary International Law concluded as follows:

„In the course of the Commission’s work it will be necessary to address general questions of methodology in the identification of rules of customary international law, such as the relative weight to be accorded to empirical research into State practice, as against deductive reasoning”^[39].

Several commentators refer to the inductive methodology as being traditional^[40], positivist^[41], or voluntarist^[42]. In accordance with this approach, states cannot be bound by international law unless they express their consent and, further, in accordance with this approach, the focus is on practice rather than on opinio juris^[43]. However, this con­cep­tion has its shortcomings. One of these is represented by the absence of secondary rules^[44]. Further, the ILC Conclusions on identification of customary international law do not clarify the methodology through which „sufficiently widespread and representative, as well as consistent”^[45] practice should be identified^[46]. To take the point further, it is currently unclear which acts constitute state practice and which do not, especially because the activities of states take various and multiple forms^[47]. Authors like D’Amato have defined certain categories of acts that could be included in state practice^[48], but endeavours to provide lists of acts would overcomplicate the issue rather than resolve it. 

Conclusions 5-7 of the ILC Draft Conclusions on Identification of Customary International Law do not necessarily clarify the issue of determining state practice^[49]. Some scholars consider it unsurprising, as the ILC is a „fundamentally conservative organisation”^[50]. Perhaps this is the reason for which the states the commented on the Draft Conclusions found it as having the „potential to make a useful contribution to the development of international law”^[51]. What is clear is that the inductive methodology does not seem to provide sufficient elements in order to determine the content of customary international law. Further, throughout its jurisprudence, the International Court of Justice did not address the interpretation of state practice as an essential element of customary international law, with the exception of the North Sea Continental Shelf Cases^[52]. In this context, deduction may be relevant to fill the gaps generated by induction. 

2.3. Deduction: the prioritisation of opinio juris over state practice

The deductive approach entails that the rules of customary international law are determined by focusing on opinio juris firstly. Deduction is a process through which the truth of certain premises is tested and, if the test renders positive results, certain true conclusions are reached^[53]. As such, the deductive methodology implies that the norm is identified and, further, implies that if states believe (or accept) that norm to be legally binding, it is part of customary international law. 

To determine the existence of beliefs, statements and/or declarations of states, resolutions of international organisations, are more relevant for this methodology of determining the existence and content of customary international law. Anthea Roberts concludes in this sense that the deductive method also relies on „multilateral treaties and declarations by international fora such as the General Assembly, which can declare existing customs”^[54]. As such, the conclusion that deduction limits, to a certain degree, the freedom of states^[55] seems to have merit, because the deduction process goes from general norms to specific behaviours^[56].

One case in which the International Court of Justice referred to the deductive methodology was the Military and Paramilitary Activities in and Against Nicaragua, in which the Court concluded that customary international law can be deduced, in the following terms:

„In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a state acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule”^[57].

Commentators conclude that the deductive approach carries the flag of modernity^[58], that it is normative^[59] and intellectualist^[60]. Even so, the deductive methodology of determining the content of customary international law also manifests shortcomings. The main issue with the approach focusing on opinio juris is that the meaning and function of this concept is not easily defined. Several unsuccessful attempts have been made to determine its content^[61]. 

However, what is generally accepted is that this concept is one of the centrepieces of customary international law, but that it is also paradoxical. The opinio juris paradox has been described in the following terms:

„The so called opinio juris paradox has become a persistent problem in international legal scholarship. If it is the belief that something is already law that counts, then it can only be used to identify existent customary international law. The belief cannot be true with respect to conduct that had either hitherto not be covered by a norm or by a different norm, believing something to be law which is only becoming law”^[62].

Conclusion

Few conclusions exist regarding the reconciliation of induction and deduction as mechanisms of determining the content of customary international law. One notable argument is the so-called sliding scale. Kirgis argues, in this sense, that at one side of the sliding scale is practice (inductive methodology) while on the other side is opinion juris (the deductive methodology). As such, the inductive approach is applied, with the above-mentioned consequences, i.e., that it prioritises practice over opinion juris, as long as the consistent practice „is not negated by evidence of non-normative intent”^[63]. In accordance with this approach, the role of opinion juris increases as the existence of consistent practice fades. However, Kirgis concludes that how much state practice may substitute opinion juris „depends on the activity in question and on the reasonableness of the asserted customary rule”^[64].

In this sense, a generally accepted solution that represents a compromise between the mentioned approaches has not yet fully emerged in the doctrine, as most authors plead for the application of one or the other. Nevertheless, the International Law Commission seems to have reached certain relevant conclusions in this sense, codified within its Draft Conclusions on Identification of Customary International Law. 

The sliding scale approach is in line, to a certain degree, with the manner in which the International Law Commission codified the mechanisms used for the identification of customary international law. The Commission does not exclude the deductive approach per se, but mentions that it should be applied in a subsidiary manner. As such, the ILC concludes that it does not exclude deduction but that it should be employed with caution, „as an aid in the application of the two-element approach”^[65].

To conclude, it would appear that, presently, there is no clear indication as to when exactly is the belief of states, or opinion juris, the principal constituent element of customary international law. Today, practice is in the spotlight when addressing the content of the source of law provided by article 38 (1) (b) of the Statute of the International Court of Justice while opinion juris remains rather secondary.

Footnotes

^[1] J. Kammerhofer, Uncertainty in the Formal Sources of International Law: Customary International Law and Some of its Problema, European Journal of International Law, Volume 15, No. 3, 2004, p. 523.

^[2] A. Guzman, T. Meyer, Customary International Law in the 21st Century, in Russel Miller, Rebecca Bratspies, Progress in International Law, Brill Nijhoff, 2008, p. 197.

^[3] N. Banteka, A Theory of Constructive Interpretation for Customary International Law Identification, Michigan Journal of International Law, Volume 39, Issue 3, 2018, p. 301.

^[4] A.E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, American Journal of International Law, Vol. 95, 2001, p. 757.

^[5] D. Bodansky, Does Custom have a Source?, American Journal of International Law, Volume 108, 2017, p. 179.

^[6] J. D’Aspremont, The Discourse on Customary International Law, Oxford University Press, 2021; P.M. Dupuy (ed.), Customary International Law, Edward Elgar, 2021; Y. Tan, The Rome Statute as Evidence of Customary International Law, Brill Nijhoff, 2021; B. Lepard, Reexamining Customary International law, Cambridge University Press, 2018. 

^[7] Draft conclusions on identification of customary international law, with commentaries, available at: https://legal.un.org/ilc/texts/instruments/ english/commentaries/1_13_2018.pdf.

^[8] Ibid. 

^[9] C. Dahlman, The Function of Opinio Iuris in Customary International Law, Nordic Journal of International Law, Volume 81, 2012, p. 327.

^[10] D. Joyner, Why I Stopped Believing in Customary International Law, Asian Journal of International Law, Volume 9, Issue 1, p. 33. 

^[11] Draft conclusions on identification of customary international law, with commentaries, p. 125, available at: https://legal.un.org/ilc/texts/instruments/ english/commentaries/1_13_2018.pdf.

^[12] The Statute of the International Court of Justice, available at: https://www.icj-cij.org/en/statute (emphasis added).

^[13] North Sea Continental Shelf Cases (Federal Republic Of Germany/Denmark; Federal Republic Of Germany/Netherlands), Judgment, I.C.J. Reports 1969, p. 3.

^[14] Colombian Peruvian Asyum Case (Colombia v. Peru), Judgment, I.C.J. Reports 1950, p. 266.

^[15] Draft conclusions on identification of customary international law, with commentaries, p. 125, available at: https://legal.un.org/ilc/texts/instruments/ english/commentaries/1_13_2018.pdf.

^[16] Ibid.

^[17] The Statute of the International Court of Justice, available at: https://www.icj-cij.org/en/statute (emphasis added).

^[18] The Rome Statute of the International Criminal Court, p. 5, available at: https://www.icc-cpi.int/resource-library/documents/rs-eng.pdf.

^[19] Report of the Executive Directors on the Convention on the Settlement of Investment Disputes between States and Nationals of Other States, para. 40, p. 47, emphasis added, available at: https://icsid.worldbank.org/sites/default/files/documents/ICSID%20Convention%20English.pdf.

^[20] J. Kammerhofer, Uncertainty in the Formal Sources of International Law: Customary International Law and Some of its Problema, European Journal of International Law, Volume 15, No. 3, 2004, p. 536.

^[21] N. Banteka, A Theory of Constructive Interpretation for Customary International Law Identification, Michigan Journal of International Law, Volume 39, Issue 3, 2018, p. 310.

^[22] A.E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, American Journal of International Law, Vol. 95, 2001; W.T. Worster, The Inductive and Deductive Methods in Customary International Law Analysis: Traditional and Modern Approaches, op. cit. 

^[23] M. Hakimi, Making Sense of Customary International Law, Michigan Law Review, Volume 118, Issue 8, 2020, 1497.

^[24] J. Goldsmith, E. Posner, Understanding the Resemblence Between Modern and Customary International Law, Virginia Journal of International Law, Volume 40, 2000, p. 642.

^[25] J. Kammerhofer, Uncertainty in the Formal Sources of International Law: Customary International Law and Some of its Problem, European Journal of International Law, Volume 15, No. 3, 2004, p. 534.

^[26] Draft conclusions on identification of customary international law, with commentaries, p. 125, available at: https://legal.un.org/ilc/texts/ instruments/english/commentaries/1_13_2018.pdf (emphasis added).

^[27] Ibid. 

^[28] St. Talmon, Determining Customary International Law: The ICJ’s Methodology Between Induction, Deduction and Assertion, The European Journal of International Law, Vol. 26, No. 2, 2015, p. 420.

^[29] N. Banteka, A Theory of Constructive Interpretation for Customary International Law Identification, Michigan Journal of International Law, Volume 39, Issue 3, 2018, p. 310.

^[30] W.T. Worster, The Inductive and Deductive Methods in Customary International Law Analysis: Traditional and Modern Approaches, Georgetown Journal of International Law, Vol. 45, No. 2, p. 447.

^[31] J. Goldsmith, E. Posner, Understanding the Resemblance Between Modern and Customary International Law, Virginia Journal of International Law, Volume 40, 2000, p. 641.

^[32] St. Talmon, Determining Customary International Law: The ICJ’s Methodology Between Induction, Deduction and Assertion, The European Journal of International Law, Vol. 26, No. 2, 2015, p. 420.

^[33] Ibid.

^[34] Case Concerning Delimitation of Maritime Boundary in the Gulf of Maine Area (Canada/United States of America), Judgment, I.C.J. Reports 1984, p. 246, p. 57.

^[35] North Sea Continental Shelf Cases (Federal Republic of Germany/Denmark; Federal Republic of Germany/Netherlands), Judgment, I.C.J. Reports, 1969, pp. 3, 44. 

^[36] M. Meguro, Distinguishing the Legal Bindingness and Normative Content of Customary International Law, European Society of International Law Reflections, Volume 6, Issue 11, p. 6. 

^[37] P. Merkouris, Interpreting the Customary Rules of Interpretation, University of Groningen Faculty of Law Research Paper Series, No. 12, 2016, p. 7

^[38] Ibid.

^[39] Sir M. Wood, First report on formation and evidence of customary international law, Document A/CN.4/663, p. 121, available at https://legal.un.org/ilc/ documentation/english/a_cn4_663.pdf.

^[40] A.E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, American Journal of International Law, Vol. 95, 2001, p. 758.

^[41] J. Goldsmith, E. Posner, Understanding the Resemblance Between Modern and Customary International Law, Virginia Journal of International Law, Volume 40, 2000, p. 641.

^[42] O. Elias, The Nature of the Subjective Element in Customary International law, International and Comparative Law Quarterly, Volume 70, Part 3, 2021, p. 501.

^[43] A.E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, American Journal of International Law, Vol. 95, 2001, p. 758.

^[44] M. Hakimi, Making Sense of Customary International Law, Michigan Law Review, Volume 118, Issue 8, 2020, 1506.

^[45] Draft conclusions on identification of customary international law, with commentaries, p. 135, available at: https://legal.un.org/ilc/texts/instruments/ english/commentaries/1_13_2018.pdf.

^[46] M. Hakimi, Making Sense of Customary International Law, Michigan Law Review, Volume 118, Issue 8, 2020, 1506.

^[47] H. Charlesworth, Customary International Law and the Nicaragua Case, Australian Yearbook of International Law, 1984, p. 5.

^[48] A. D’Amato, The Concept of Custom in International Law, Cornell University Press, 1971, p. 172.

^[49] Draft conclusions on identification of customary international law, with commentaries, p. 132-138, available at: https://legal.un.org/ilc/texts/ instruments/english/commentaries/1_13_2018.pdf.

^[50] D. Bodansky, Does Custom have a Source?, American Journal of International Law, Volume 108, 2017, p. 179.

^[51] U.N. Doc. A/C.6/71/SR.22 (2016), para. 61, p. 11.

^[52] H. van der Wilt, State Practice as Element of Customary International Law: A White Knight in International Criminal Law?, International Criminal Law Review, Volume 20, Issue 5, p. 793.

^[53] W.T. Worster, The Inductive and Deductive Methods in Customary International Law Analysis: Traditional and Modern Approaches, Georgetown Journal of International Law, Vol. 45, No. 2, p. 455.

^[54] A.E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, American Journal of International Law, Vol. 95, 2001, p. 758.

^[55] W.T. Worster, The Inductive and Deductive Methods in Customary International Law Analysis: Traditional and Modern Approaches, Georgetown Journal of International Law, Vol. 45, No. 2, p. 447.

^[56] St. Talmon, Determining Customary International Law: The ICJ’s Methodology Between Induction, Deduction and Assertion, The European Journal of International Law, Vol. 26, No. 2, 2015, p. 420.

^[57] Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), Jurisdiction, Merits, Judgment, I.C.J. Reports, 1986, para 186, p. 88.

^[58] A.E. Roberts, Traditional and Modern Approaches to Customary International Law: A Reconciliation, American Journal of International Law, Vol. 95, 2001, p. 758.

^[59] J. Goldsmith, E. Posner, Understanding the Resemblance Between Modern and Customary International Law, Virginia Journal of International Law, Volume 40, 2000, p. 641.

^[60] O. Elias, The Nature of the Subjective Element in Customary International law, International and Comparative Law Quarterly, Volume 70, Part 3, 2021, p. 501.

^[61] Ch. Dahlman, The Function of Opinio Iuris in Customary International Law, Nordic Journal of International Law, Volume 81, 2012, p. 327.

^[62] J. Kammerhoffer, Uncertainty in the Formal Sources of International Law: Customary International Law and Some of its Problems, European Journal of International Law, Vol. 15, No. 3, p. 534.

^[63] Fr. Kirgis, Custom on a Sliding Scale, American Journal of International Law, Volume 81, 1987, p. 149.

^[64] Ibid. 

^[65] Draft conclusions on identification of customary international law, with commentaries, p. 132-138, available at: https://legal.un.org/ilc/texts/ instruments/english/commentaries/1_13_2018.pdf.