2.1. The Identification of Customary Law as Law: Different Approaches

The method adopted to justify the identification of CIL as law is one of the most important theoretical questions addressed in the book. Indeed, since CIL does not hinge on any standardised procedure and follows a non-formal ascertainment pattern, there has been some reticence in qualifying customary law as a proper ‘source’ of international law.Footnote ³³

Lepard first made this point in his earlier book by affirming that many sceptics, particularly political scientists, doubt that customary norms are binding law and see them as mere ‘regimes’ based on political self-interest.Footnote ³⁴ They address them simply as ‘pragmatic conventions that states usually follow but that they are under no obligation to obey’.Footnote ³⁵ In Reexamining Customary International Law, Lepard re-introduces this argument in the first few pages, in holding that there is no particular reason to draw ‘legal inferences from any given practice’.Footnote ³⁶ After all, he claims, states like people ‘engage in a wide variety of practices for many reasons. Some reasons are found in self-interests; others may involve a sense of moral obligation, or even mere courtesy … More broadly, why should any regularity of behavior give rise to a legal obligation, permission, or prohibition?’.Footnote ³⁷ Interestingly, he suggests focusing on the concept of opinio juris. That is, regularity of behaviour becomes a legal rule only when states engaging in such behaviour believe that it does. However, why should mere state beliefs that their praxis is legally permitted or required create a legal rule? One of the great challenges, it seems, is determining when a norm followed by the international community has evolved from a moral rule into a legally authoritative provision.

This theoretical debate has been addressed by different courts and tribunals, identifying the status of CIL as law. For instance, in The Paquete Habana, decided in 1900, the US Supreme Court ruled that a customary international legal norm existed that exempted coastal fishing vessels from capture during wartime.Footnote ³⁸ The ICJ, in the North Sea Continental Shelf Cases, has also noted the challenge of distinguishing acts based on comity from those representing legal obligations under CIL. It stated in this sense that ‘[t]here are many international acts, e.g., in the field of ceremonial and protocol, which are performed almost invariably, but which are motivated only by considerations of courtesy, convenience or tradition, and not by any sense of legal duty’.Footnote ³⁹ Taking these cases into account, Lepard has argued that often courts struggle to differentiate mere custom from customary law.Footnote ⁴⁰

Several contributors in the book explore this important question. In a meticulous analysis, J Patrick Kelly (Chapter 2) addresses CIL from a historical perspective. He explains that a careful study of the history suggests that publicists and powerful nations have ignored inconvenient ‘state practice and generated customary international law norms based on prior assumed values or perceived self-interest irrespective of the general acceptance of a norm’.Footnote ⁴¹ This is why many customary rules are merely a selected collection of norms from European states reflecting their self-interest. This is far from a merely academic exercise, and Kelly validates his views through real-world examples – in particular, (i) the development of the law of state responsibility, with particular attention to the minimum standard for compensation for expropriation, which seems to have favoured the developed states; (ii) the customary international law of the territorial sea; and (iii) the history of the persistent objector principle.

Furthermore, Kelly observes that Vattel's ‘specific consent’ theory was replaced by the currently dominant view of ‘common consent’, which allows a majority of states to make binding law over a minority of nations without the consent of the latter. In order to overcome this challenge, he proposes the development of a more pluralistic version of international law, which would allow moving beyond the ethnocentric history of their development and imposition. Nations that were not represented in the original development of international law could now be ‘heavily represented at the ILC and at treaty conferences to increase the legitimacy of the codification process of norm formation’.Footnote ⁴² Accordingly, this active participation would serve to move towards some sort of ‘legislative model’, which would ensure fair representation and lead to greater levels of ‘democratic legitimacy’ in customary lawmaking.Footnote ⁴³ In Kelly's view, therefore, CIL should be law only when this democratic lawmaking process takes place.

The legal status and identification of CIL as law is also explained by Fernando Tesón (Chapter 3), who suggests that customary law is the most problematic concept in international law.Footnote ⁴⁴ He explores a specific phenomenon that has become increasingly important within the international sphere: the practice of ‘fake custom’, which will be described further below. Tesón criticises the legislative model, as he believes that CIL in its modern sense requires universal and specific consensus. In his words, ‘the modern theory of custom requires virtual unanimity for a rule to qualify. The salient characteristic of this modern law-making method is that universal, precise consensus is enough to create the rule’.Footnote ⁴⁵ This ‘virtual unanimity’ is precisely what would make CIL law.

Finally, Niels Petersen (Chapter 4) thoroughly explores one of the unsolved questions of CIL: how much state practice is needed for the formation of a customary norm?Footnote ⁴⁶ As he correctly points out, there are two competing views. While some scholars accept the practice requirement and see customary rules as ‘crystallisation points’ of state behaviour, others focus on opinio juris and perceive customary law as a type of tacit agreement, binding only upon those states that have consented to it.Footnote ⁴⁷ Taking this as a basis, Petersen interestingly relies on game theory to distinguish between different types of situation and argues that the role of consent ‘differs according to the structure of the social problem that a potential norm is supposed to address’.Footnote ⁴⁸ He suggests that specific consent should not be required for norms related to the protection of common goods. State consent in these cases, Petersen claims, is primarily an epistemological tool, and states can be bound under certain circumstances even if they object to the creation of such norms.Footnote ⁴⁹ There are also customary rules concerning cooperation issues, which would bind states only if they have not objected to them. In the case of norms dealing with ethical values, the author affirms that states should not be allowed to exempt themselves from such a norm even by persistently objecting to its emergence.Footnote ⁵⁰ Considering the views expressed by Tesón and Kelly on the ‘legislative model’, Petersen's argument is certainly important. Instead of focusing on the participation of states in this lawmaking process, he moves towards a more ‘thematic’ approach, which depends on the rules with which the international community would be involved.

As Lepard concludes, most of the contributors seem to agree that the legislative model does not insist on specific state consent for the formation of every norm of customary law.Footnote ⁵¹ Indeed, CIL does not require universal acceptance of any given rule by all subjects of international law.Footnote ⁵² While there are certain norms that are indeed universally accepted, others have less acceptance, but still form part of the international legal system.Footnote ⁵³ As will be seen, Tesón's theory of fake custom nonetheless cautions this if the rules do not have a robust and dense state praxis and opinio juris or a universal acceptance. This is an important point, as all authors seem to subject the legal status of CIL to the process of identification of rules: whether states participate in their development and how they do it determines that customary international law is in fact law.

2.2. The Persistent Objector Principle: Some Challenges and Difficulties

Another challenge addressed in the book is related to the persistent objector principle, which is explicitly rejected by Kelly but accepted by other authors. As a matter of legal theory, international law has been considered primarily as a system based on the consent of states. While treaties are usually consented to expressly, a state's consent to a customary rule is thought to be either tacit or implied. Klabbers has explained, however, that:Footnote ⁵⁴

Interestingly, Kelly believes that if the persistent objector theory were accepted, then customary law would be inconsistent and incoherent since ‘[t]he majority of states have had no opportunity to consent or indeed participate in the formation of norms to which they are bound, while other states may opt out of changes to those norms’.Footnote ⁵⁵ CIL, therefore, would be consensual for some and ‘a common consent theory for other, often less powerful states’.Footnote ⁵⁶ Interestingly, Kelly seems to continue a debate that began almost three decades ago between prestigious international law scholars.Footnote ⁵⁷ Back then, Weiler affirmed that the way in which this principle was perceived was a reflection of international politics, in which ‘Western states’ insisted on its application because they were in a minority.Footnote ⁵⁸ In contrast, Cassese explained that this principle played an important role for the socialist countries, which believed in the importance of their participation in the adoption of new customary international rules.Footnote ⁵⁹

Other commentators have identified three main critiques of this principle:Footnote ⁶⁰ (i) the judgments usually cited in its support do not actually support it; (ii) no state practice has ever supported it either; and (iii) the justification of the rule is merely a matter of theory and policy. As questioned by Mendelson, is it desirable, as a matter of policy, that the persistent objector principle should exist?Footnote ⁶¹

The possibility of relying on this principle reappears in other chapters of the book, as some contributors believe that it still plays a useful role. Petersen (Chapter 4), for instance, maintains that it could be applied with regard to some rules but not to others. In particular, he claims that it should not be permitted for norms that protect a common good and enjoy almost universal support, or where a norm is based ‘on certain compelling ethical principles, such as fundamental human rights’.Footnote ⁶² Henckaerts and Debuf (Chapter 6) also address this issue, indicating that the ICRC study may have caused some confusion by stating that it took no view regarding the applicability of the persistent objector principle in relation to CIHL norms. As they explain, ‘[a]lthough persistent objection may not be possible in the case of rules of jus cogens, presumably not all rules of customary IHL are jus cogens, and therefore persistent objection to at least some of them would be possible’.Footnote ⁶³

Contrary to Kelly's reasoning, Lepard has argued that this principle should continue to be recognised for different reasons. First, international law is supported by the fundamental ethical principles of state autonomy and unity in diversity, which is represented by the overall freedom of action of states. Therefore, if a state ‘so strongly believes that an emerging rule of customary law is not desirable to recognise that it repeatedly objects to it, the value of respecting that state's wishes should often – but not always – be considered to outweigh the community values that would be served by obligating the state to adhere to the rule’.Footnote ⁶⁴ Second, Lepard claims that despite having rarely been invoked, the persistent objector principle has become generally accepted as a secondary rule of CIL formation.Footnote ⁶⁵

Although this topic is well presented in the book, certain issues should be noted. Lepard is correct in affirming that this principle has rarely been invoked. Importantly, this may suggest either that states do not follow all lawmaking processes closely, or that they agree with new practices crystallising into customary international law.Footnote ⁶⁶ Moreover, if, as Kelly argues, the persistent objector principle ‘undermine[d] the legitimacy and efficacy of customary international law as a coherent body of norms applicable to all’,Footnote ⁶⁷ it remains unclear whether there would be an alternative mechanism for states that do not want to be bound by a specific customary norm without actually breaching it. Boyle, for example, has suggested focusing on the UN Security Council's lawmaking capacity to overcome the problems that this principle presents in customary law.Footnote ⁶⁸ Finally, although the application of the persistent objector principle is controversial, the ILC endorsed it in 2016 by declaring that ‘[w]here a state has objected to a rule of customary international law while that rule was in the process of formation, the rule is not opposable to the state concerned for so long as it maintains its objection’.Footnote ⁶⁹ Certainly, the discussion related to the application of this principle is one of the key aspects of the theory of CIL, and the variety of opinions shows that it is far from a well-settled topic.Footnote ⁷⁰ It will be important for international lawyers and bodies to continue to engage on this in order to shed some light on how it should be applied, and whether there are alternatives for states that do not want to follow a specific customary rule. In any case, the conclusions presented by the ILC will definitely have a practical impact on how those participating within the international realm relate to this principle in the near future.

2.3. Types of Custom: Fake, Genuine, and the Case of Human Rights Law

Another theoretical challenge is explained thoroughly by Tesón, who addresses a specific phenomenon that has become increasingly important within the international sphere: the practice of ‘fake custom’. As Tesón affirms, the currently prevalent paradigm tries to convince an audience that a rule is custom, when in reality it is not, since ‘fake customary rules are rules that are not law under any plausible theory of legitimate lawmaking’.Footnote ⁷¹ Instead, they are pieces of advocacy disguised as law that should be avoided. Thus, his chapter aims to address the legal methodology for the identification of CIL. It is, in the end, ‘a plea for transparency and intellectual honesty’.Footnote ⁷² This important phenomenon has been addressed elsewhere, in particular when dealing with the legal method for identifying and developing CIHL. Hakimi has suggested that if this method were more refined, states and other actors would have less discretion to characterise as customary law ‘their own, parochial policy references – and the uncertainty and susceptibility to abuse that are the hallmarks’ of this source might be reduced.Footnote ⁷³

After highlighting what Tesón refers to as ‘genuine custom’, he focuses on certain well-founded fake custom techniques, clearly organising them as follows:

1. 1. The ad nauseam fallacy – this consists of citing non-binding resolutions, speeches and memoranda, which would turn a principle into a norm of customary law.

2. 2. The treaty fallacy – this is defined as the practice of saying that a customary norm is binding on a given state by citing treaties to which it is not a party and which it has even expressly declined to join, or has repudiated.Footnote ⁷⁴

3. 3. The legislative fallacy – namely the practice of asserting that a customary rule is binding upon a state based on the fact that others have adopted that norm in their own legislation.

4. 4. The selectivity fallacy – this refers to citing only the soft law sources that support the desired norm, while omitting those that reject it.

5. 5. The corrupt pedigree fallacy is the consolidation and perpetuation of a legal mistake by repeated citation, which themselves become ‘precedents’ in support of a non-existent rule.

6. 6. The false-inferences-from-vague-generalities fallacy occurs when a court, lacking justification, affirms that a broad legal regime is part of general international law but without addressing the specific norms of that legal regime. In this sense, it would be virtually impossible to infer specific norms from this generality.Footnote ⁷⁵

According to Tesón, no other area of international law exemplifies fake custom better than IHRL.Footnote ⁷⁶ He puts forward two propositions: (i) most claims that human rights are customary are fake; and (ii) most human rights are binding because governments are morally required to respect them, and not as a result of a positivist assessment.Footnote ⁷⁷ While the former is linked to the lack of an authoritative determination of specific human rights provisions as customary law, the latter is based on a universal and sufficiently precise international consensus about the human right in question. Even if that consensus is not supported by state practice, Tesón claims, it can be argued that certain rights have acquired customary status, such as the prohibition of official torture and the prohibition of indefinite arbitrary detention.Footnote ⁷⁸ The difficulty, as Lepard identifies later (Chapter 8), is to determine when and how a human rights norm attains a customary character.Footnote ⁷⁹ The ICJ has repeatedly emphasised that at least some forms of human rights protection have attained this status, such as the prohibition of genocide.Footnote ⁸⁰

According to Tesón, this is the perfect example of ‘genuine’ custom. States, as he affirms, refrain from genocide and feel strong pressure to continue to respect the prohibition against such action.Footnote ⁸¹ Tesón's claims are at the core of the theory of CIL, under which the IHRL provision in question would be the result only of a ‘dense and robust practice by states that not only respect the right but feel normatively pressured to do so’.Footnote ⁸² Considering that states do not tend to publicly acknowledge the existence of customary rules, this activity tends to be left to courts, tribunals and other expert bodies.Footnote ⁸³ In many cases, however, this identification seems to lack ‘dense and robust’ practice by states.

Taking Tesón's perspectives into account, Lepard's chapter attempts to formulate a new approach both to the formation of CIL and to the determination of the customary character of specific IHRL rules.Footnote ⁸⁴ His theory, in fact, aims to provide practical guidance to international tribunals and courts on finding and applying these norms. In particular, he explains that the ICJ has frequently declared that certain human rights norms have attained this character without actually undertaking a rigorous analysis of whether they satisfy the requirements of consistent state practice and opinio juris.Footnote ⁸⁵ As mentioned above, this presupposes that courts and tribunals turn a blind eye to legal complexities and methods by not producing an accurate and robust database for the identification of CIL. This is not always the case, and nowadays there is a range of documents adopted within the international forums that can be taken into account to determine the content of actual legal rules, including those with a customary character.Footnote ⁸⁶

In any case, in order to resolve this and other difficulties identified throughout his chapter, Lepard makes an important argument by suggesting a new understanding of CIL, which emphasises opinio juris as opposed to state practice. This seems to follow recent scholars who have attempted to reduce the importance of state practice for the identification of customary rules.Footnote ⁸⁷ Lepard affirms that ‘[a] customary international norm arises when states generally believe that it is desirable now or in the near future to have an authoritative legal principle or rule prescribing, permitting, or prohibiting certain conduct’.Footnote ⁸⁸ Importantly, this perspective redefines opinio juris as a belief by states that a norm should be law, rather than a belief that it is already a legal provision, and highlights the role of ethics, normally left aside from a traditional point of view.Footnote ⁸⁹ According to Lepard, this conception best represents the use that states and international bodies have made of customary law. Since this theory has been challenged elsewhere,Footnote ⁹⁰ it suffices at this stage to say that it remains to be seen whether it will have a practical impact on the resolution of theoretical and everyday international law dilemmas and, in particular, whether it can be similarly applied to the identification of CIL beyond the realm of IHRL.

2.4. Re-examining Customary International Law: A Non-State Actor Perspective

The lack of reference in the book to the question of whether non-state actors may have a role in the creation and development of CIL should be noted.Footnote ⁹¹ Nowadays, the status of non-state actors has become an essential topic of analysis and discussion in order to better understand international dynamics. Although contemporary public international law still seems to be predominantly state-oriented – a scenario recognised by LepardFootnote ⁹² – it is undeniable that a variety of non-state actors play quite important roles in the international sphere.

In this sense, it can be argued that international lawmaking processes now involve an extraordinary number of non-state entities, such as transnational corporations, non-governmental organisations (NGOs), individuals, international organisations and non-state armed groups.Footnote ⁹³ The interactions between states and these entities (or exclusively between the latter) have become increasingly multi-layered. In the context of CIL, international organisations and international tribunals are relevant actors which can provide spaces for those processes to take place. Many of these entities are involved in the application, interpretation, disregard or reinforcement of customary rules, or even in the creation of new obligations.Footnote ⁹⁴ This is partially recognised by a number of contributors, but only with regard to the role of UN General Assembly resolutions as state practice or as evidence of opinio juris. While Tesón states that these instruments may provide some evidence of opinio juris under some conditions, Arajärvi affirms that ‘[t]he impact of international institutions and non-state actors on the formation of opinio juris is in fact important, and General Assembly resolutions can provide a well-articulated normative touchstone for the future evolution of state practice and opinio juris’.Footnote ⁹⁵

Lepard also suggests that UN General Assembly resolutions may be evidence of opinio juris, depending on a variety of factors, ‘necessitating a careful analysis of the text of and circumstances surrounding particular resolutions’.Footnote ⁹⁶ In particular, although affirming that states, and not interstate institutions, are the primary actors that formulate CIL, he claims that the acts of international organisations and their agents may provide indirect evidence of the forming of opinio juris by the attitudes of states.Footnote ⁹⁷ Interestingly, Lepard arguesFootnote ⁹⁸ that the Universal Declaration of Human RightsFootnote ⁹⁹ is:

When describing the methodology for the 2005 ICRC Study on CIHL, Henckaerts and Debuf assert that the study took into account only physical and verbal acts of states for the creation of CIL.Footnote ¹⁰⁰ Nonetheless, the practice of certain non-state actors has also been included – in particular, that of non-state armed groupsFootnote ¹⁰¹ and resolutions adopted by states in the framework of international organisations, such as the UN and regional organisations.Footnote ¹⁰² In the Tadić decision, the Appeals Chamber of the International Criminal Tribunal for the former Yugoslavia (ICTY) interestingly referred to the praxis and opinio juris of these non-state actors as part of the basis for identifying rules of CIHL.Footnote ¹⁰³ It affirmed that ‘[i]n addition to the behaviour of belligerent states, governments and insurgents, other factors have been instrumental in bringing about the formation of the customary’ IHL rules.Footnote ¹⁰⁴ Furthermore, it stated that the study by the ICRC on CIHL forms ‘an element of actual international practice; this is an element that has been conspicuously instrumental in the emergence or crystallisation of customary rules’.Footnote ¹⁰⁵ This has been interpreted by Arajärvi as if the court had paved the way for the formation of CIL beyond a state-dominated approach.Footnote ¹⁰⁶

Although the ICTY's position raises important elements, it cannot be denied that the lack of a full analysis of praxis and opinio juris, even when taking into account non-state actors such as the ICRC and non-state armed groups, weakens its customary law argument. One can easily refer to Tesón's fake custom techniques, and the Tadić ruling seems to be a clear example in which dense and robust practice would have been needed.

The book's position, in any case, follows the recent conclusions by Sir Michael Wood on the topic, which did not actually recognise the legal value of the praxis and opinio juris of non-state actors for the formation of CIL:Footnote ¹⁰⁷

Wood differentiates between states, international organisations and non-state actors. The acts of international organisations on which states have conferred authority, he claims, may contribute to the formation of a general practice ‘in the fields in which those organizations operate’.Footnote ¹⁰⁸ The actions of other actors, however, are not considered as ‘practice’ for the formation of CIL or evidence thereof.Footnote ¹⁰⁹

Even if this view is based on the ICJ's doctrine concerning implicit legal personality, as included in the Reparations case, some authors have claimed that this separation is actually artificial.Footnote ¹¹⁰ Instead, international law is made and shaped through interactive processes between members of the international community. Sivakumaran has argued that this is done, for instance, ‘through dialogue between states, courts, the ICRC and academics’.Footnote ¹¹¹ One could also add the UN, NGOs and non-state armed groups. This seems to follow the focus of the New Haven school on the notion of ‘participants’ in the lawmaking processes rather than the ‘subject-object’ dichotomy. In this context, although Sivakumaran affirms that the role of states in the making of international rules should not be downplayed, he concludes that non-state actors – in particular, the community of international lawyers – make international law through a process of dialogic interaction. Although it would exceed the scope of this review to provide an insightful analysis of the role of these entities in the formation of CIL, its inclusion and analysis in the book could have been useful considering their role within the international sphere.