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The Nature of Customary (Traditional) Law and Customary Courts in ECOWAS Countries

• First Online: 28 August 2020

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• Elena P. Ermakova 5 ,
• Elena Inshakova 6 ,
• Evgenia E. Frolova 5 , 7 &
• Agnessa O. Inshakova 6  

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In this chapter, the authors examine issues related to the state regulation of the activities of African customary courts of the West African states that are members of ECOWAS (Benin, Burkina Faso, Gambia, Ghana, Guinea, Guinea-Bissau, Côte d’Ivoire, Cape Verde, Liberia, Mali, Niger, Nigeria, Senegal, Sierra Leone, Togo). It describes the characteristic features of African customary law in the pre-colonial and colonial era, as well as in the modern period. Filing a case in Nigeria’s customary courts has been used as an example of description. The authors identify and reveal the advantages and disadvantages of the administration of justice in customary courts. Customary courts have many advantages: (a) they are more accessible to an average person in African countries than formal courts; (b) they are inexpensive, compelling, and often geographically very close to court attendants; (c) hearings in customary courts are understood by litigants because they take place in the local dialect and avoid the use of complex legal terms; (d) they encourage mediation and reconciliation and make decisions that are restorative in nature. However, there are many disadvantages.

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This work was financially supported by the Grant of the President of the Russian Federation No. НШ-2668-2020.6 “National-Cultural and Digital Trends in the Socio-Economic, Political and Legal Development of the Russian Federation in the 21st Century.”

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RUDN University, Moscow, Russia

Elena P. Ermakova & Evgenia E. Frolova

Volgograd State University, Volgograd, Russia

Elena Inshakova & Agnessa O. Inshakova

Institute of Legislation and Comparative Law, Moscow, Russia

Evgenia E. Frolova

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Correspondence to Agnessa O. Inshakova .

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Moscow State Institute of International Relations (MGIMO), Moscow, Russia

Elena G. Popkova

Davis Center for Russian and Eurasian Studies, Harvard University, Cambridge, MA, USA

Bruno S. Sergi

Graduate School of Business, University of Zambia, Lusaka, Zambia

Lubinda Haabazoka

Chair of Compliance and Controlling, RUDN University, Moscow, Russia

Julia V. Ragulina

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Ermakova, E.P., Inshakova, E., Frolova, E.E., Inshakova, A.O. (2020). The Nature of Customary (Traditional) Law and Customary Courts in ECOWAS Countries. In: Popkova, E.G., Sergi, B.S., Haabazoka, L., Ragulina, J.V. (eds) Supporting Inclusive Growth and Sustainable Development in Africa - Volume I. Palgrave Macmillan, Cham. https://doi.org/10.1007/978-3-030-41979-0_11

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Accueil Numéros 34 The Quest for Opinio Juris: An An...

The Quest for Opinio Juris : An Analysis of Customary Law, from Hart’s Social Rules to Expectations and Everything in the Middle

The present essay addresses the conceptual structure of customary law, understood as a set of customary rules. More specifically, it deals with the core question of what opinio juris entails as a constituent element of customary law. The work will begin with an analysis of samples of common strategies in contemporary legal theory that deal with opinio juris when analyzing the structure of customary law. Subsequently, following Hart’s notion about what constitutes social rules, and introducing explanatory features from Game Theory applied to social philosophy and some insights from Behavioral Economics, the goal of the present study is to propose a different conceptual path for analyzing and identifying customary law with what could be understood as “ opinio juris ”, given the conceptual necessity of finding the normative element that lies within the usus , or social practices.

Le présent essai traite de la structure conceptuelle du droit coutumier, compris comme un ensemble de règles coutumières. Plus précisément, il traite de la question fondamentale de ce que l’opinio juris implique comme élément constitutif du droit coutumier. Nous commençons par une analyse d’exemples de stratégies typiques d’explication du rôle de l’opinio juris de la théorie juridique contemporaine. Par la suite, s’appuyant sur la conception des normes sociales de Hart, sur la théorie du jeu appliquée à la philosophie sociale et sur des notions d’économie comportementale, le but de la présente étude est de proposer une chemin conceptuel pour analyser et identifier ce qui peut être compris comme opinio juris dans le droit coutumier, puisqu’il est nécessaire de déceler l’élément normatif qui se trouve dans l’usus, ou les pratiques sociales.

Entrées d’index

Mots-clés : , keywords: , texte intégral, 1. accounting for customary law.

• 1 P. Chiassoni, “Tres buenos filósofos contra las malas costumbres”, Doxa Cuadernos de ­Filosofía d (...)

1 It seems uncontroversial to say that customary law is, generally, still an enigmatic subject for legal theory. There are many fascinating questions and little agreement over them. Legal philosophy has wondered about the authenticity or self-sufficiency of custom as a source of law or the conditions for judging the legality of customary rules. 1 However, the issue that legal philosophy –as well as legal doctrine– has addressed primarily and with more insistence regards the conditions for custom to be a source of law, that is: How come custom gets to become law? The query usually involves the following steps in its reasoning:

Custom is a property we use to name the fact that certain actions are repeatedly performed with some constancy over time.

When those actions are performed by the majority or a significant amount of the population within a society, we may say there is a social custom, at least as a shared practice.

Among these shared practices, some must have a certain relevant feature since they are socially required. Consequently, some customs might acquire a normative facet.

Moreover, from the socially required customs, some must have another peculiar trait since they can be legally enforceable. Therefore, some customs might be law.

2 R. Tamayo y Salmorán, Costumbre o eficacia , Mexico, Fontamara, 2005, p. 60-61.

• 3 One paradigmatic example of this is the 2015 “Third report on identification of customary Interna (...)

4 R. Tamayo y Salmorán, Costumbre o eficacia , op. cit ., p. 25-26.

2 This reasoning has incited legal scholars to dig into these peculiar customs that become law, looking for the special traits that make them be a part of the legal phenomena. Traditionally referred to as the question about the “structure” or the “elements” of custom, de larga data , legal doctrine has asserted that customary law is composed and therefore identifiable in a determined space and time, by two features: the usus regulae and opinio juris sive necessitates . The dualist scheme that composes the concept of a “customary law” is attributed to Bartolus of Saxoferrato, who tried to distinguish the legal consuetudo from mere facts of human conduct, and reached the conclusion that custom is law ( consuetudo est jus ) when one can identify the usus , i.   e ., a convergence of behavior from several persons in a certain community, and opinio juris , which would be the relevant legal data, consisting in a reflexive conviction from the members of the community that the behavior they are following is legally prescribed ( consuetudo est jus quoddam moribus institutum, quod pro lege suscipitur ). 2 Ever since, the dualist scheme and the explanation about its elements have endured, almost unquestioned by legal doctrine. 3 Likewise, this dualist scheme is the common starting point for the philosophical conceptualization of “customary law” inasmuch as analyzing legal doctrine for understanding how jurists have embraced and, somehow, used legal concepts, is necessary for the general understanding of the legal phenomena. 4

3 However, for legal philosophers that have searched for the elements that constitute customary law, the traditional dualist scheme composed of usus and opinio juris is not entirely sound. It is unquestioned that the usus is one of the constituent elements to identify when talking about customary law. It is also uncontroversial that the usus prominently consists in the convergence of behavior from the members of a certain community in repeatedly performing certain actions in certain situations. Meanwhile, the inquiry over opinio juris raises several conceptual difficulties and creates much controversy. Generally, analytical legal philosophers do not find satisfactory the subjective and imprecise statement that opinio juris is “the feeling or belief from the members of the group of acting according to law or as if the behavior referred was a law”. Several issues have been raised against the traditional dualist scheme and, more specifically, about the formulation of opinio juris and, hence, alternative solutions have been proposed.

4 Beside vagueness and imprecision, some common problems that opinio juris involves are:

• 5 N. Bobbio, La consuetudine come fatto normativo [1942], Turin, Giappichelli Editore, 2010, p. 57- (...)

How is it possible that opinio juris constitutes a customary rule if, at the same time, opinio juris entails an already constituted rule? 5

• 6 N. Bobbio, La consuetudine come fatto normativo , op. cit. , p. 60-61; P. Chiassoni, “Tres buenos f (...)

Mental states are only susceptible to conjectural knowledge through external hints. So how can we prove the existence of a collective mental state that one rule ought to be followed as if it was law? 6

• 7 G. J. Postema, “Custom, Normative Practice, and the Law”, Duke Law Journal , n o  62, 2012, p. 707-7 (...)

The reasons for abidance to a collective pattern of behavior might vastly vary among members of society. Consequently, if everybody has a different rule in mind when abiding by a pattern of behavior, can we say they are following the same rule? 7

5 To solve these problems, legal philosophers have proposed various strategies for giving a better account of customary legal rules . Those strategies can be sorted out into two big categories –to eradicate opinio juris and to rationalize opinio juris – of which I will present some of the main arguments.

1.1. Customary Law without Opinio Juris

8 N. Bobbio, La consuetudine come fatto normativo , op. cit. , p. 59.

9 Ibid . , p. 82-87.

6 Knowing that opinio juris entails important logical dilemmas and that subjective mental states are only susceptible to conjectural knowledge through external hints observable from behavior ( usus ), Bobbio proposes to get rid of opinio juris as a conceptual constituent element of customary law. 8 ­Instead, Bobbio suggests that we should identify “customary law” with the set of social rules that are essential for the constitution and preservation of any society. 9 Nevertheless, he does not get into much detail about criteria for identifying these rules or which would be the basis for considering them “legal”.

10 G. J. Postema, “Custom, Normative Practice, and the Law”, art. cit., p. 712.

11 Ibid . , p. 716-717.

7 More recently, Postema has also proposed a conceptual account of customary law that leaves aside opinio juris . Moreover, Postema regrets the marginal status that traditional accounts give to the usus by instead focusing too much on the opinio with the difficulties it involves. 10 Therefore, by stating the impossibility of determining the existence of a rule because of the convergence of the content of the internal attitudes of the members of a community, Postema suggests to eradicate the misleading presence of the opinio from the analysis of customary law to focus instead our attention on the usus , and to recognize what are the specificities that make it different from other types of regularities of behavior. Therefore, Postema suggests a discursive integrational account that considers that the convergent acts that create the rules are such because they are meaningful within a certain context of practice. Hence, the rule gets a determinate content because of the network of reasons for action and the network of accountability given shape by the practice. The former entails the realm of practical deliberation to determine if a certain action is justified, while the latter refers to the judgment each individual carries about others’ actions to determine if they are acting within the scope of the reasons of the practice. 11 Therefore, the proposal seeks to focus on the usus , understood as a deed rather than simple behavior. For Postema, to know if a regular activity in which a community engages in is a customary legal rule, we must look at how the conduct is “read” within the normative practice of a legal order.

• 12 F. Parisi, “The Formation of Customary Law”, art. cit., p. 18-19; F. Parisi and V. Fon, “Internat (...)

8 In a similar approach to Postema’s proposal, Parisi & Fon have put forward their “articulation theory”, focusing on custom in International Law. 12 For this, they assume that two important features of customary law are: a) that it is voluntary in nature, and b) that it is dynamic. Therefore, the proposal advises to look past opinio juris and stop trying to guess what it is and, instead, thoughtfully consider statements from states and their expressions of belief about the norms or course of action they intend to follow.

1.2. Rationalization of Opinio Juris

9 Most commonly, legal scholars still consider that an account of customary law cannot be properly constructed merely from the explanation of facts and that it is necessary to somehow explain how the reasoning of the agents involved in custom might create a sort of collective and legal abidance to a set of practices. In this line of thought, one common contemporary strategy is to assume the rational nature of human beings and, thus, attempt to reconstruct the traditional dualist scheme of customary law while introducing a more sophisticated explanation of what opinio juris must entail from the point of view of practical reasoning.

13 J. Finnis, Natural Law & Natural Rights , op. cit ., p. 239-240.

14 Ibid ., p. 242.

10 In this fashion, Finnis considers that opinio juris is a type of practical judgment whose paradoxical difficulties lie in being confused with other practical judgments involved in the formation phase of a customary rule. 13 Therefore, Finnis tries to recreate the stages of formation of the customary rule with a proposal that involves distinguishing two kinds of judgments in the formation phase of the rule, the empirical and the practical. Hence, the rule would begin with two interrelated judgments: a) that in a certain domain of human affairs it would be appropriate to have some determinate, common and stable pattern of conduct because an authoritative rule requiring that pattern of conduct would be more desirable than leaving conduct in that domain to the discretion of agents and; b) that the particular pattern of conduct φ would be appropriate as a common rule of conduct. Altogether, a) and b) conform to the initial practical judgment PJ 0 from which will follow two empirical judgments. One, about the widespread convergence and acquiescence by the agents in the pattern of conduct φ (EJ 1 ) and, the second, about the widespread subscription of PJ 0 by the agents (EJ 2 ). From the latter empirical judgments would follow two practical judgments: The first states that given the widespread subscription of PJ 0 and the prevalent concurrence in the pattern of conduct φ , there is a strong reason for judging that there is now an authoritative customary rule requiring φ (PJ 1 ) and, the second, that φ is required (or permitted) by virtue of an authoritative customary rule of law (PJ 2 ). 14 Accordingly, opinio juris would only be PJ 0 , and neither PJ 1 nor PJ 2 have to be confused with it, solving the common and paradoxical problem mentioned above in 1), since PJ 0 does not presuppose the existence of a rule like PJ 1 and PJ 2 , but acknowledges the need for it.

• 15 G. Lazzaro, “Reciprocità e consuetudine” [1983], in U. Scarpelli (ed.), La teoria generale del di (...)

16 P. Chiassoni, “Tres buenos filósofos…”, art. cit., p. 128-129.

11 Another effort of this sort is developed by Lazzaro who, using studies in sociology and ethnography, proposes to substitute the traditional conceptual explanation of opinio juris for a rational reconstruction based on reciprocity as the main feature to consider for a proper account of customary rules. 15 So, following Chiassoni’s reading of Lazzaro , the formation of a customary rule seems to be the result of the sum of five factors: 16

An auspicious context in which, it turns out, it is advantageous to establish ways of cooperation.

Adherence to an ethical-normative principle (a shared practical attitude) of synallagmatic justice, i. e . principle of reciprocity.

Availability of an effective sanctioning mechanism.

Reciprocal expectation.

Generalized repetition of the same model of behavior, i. e ., usus .

17 Ibid ., p. 130.

12 Factors 2 and 4 are prominently psychological so, to be precise, they do not seem to replace the opinio but rather to substitute the traditional doctrinal account for a more rationalized one based on adherence to the principle of reciprocity. Consequently, Lazzaro’s account works as a rational reconstruction in which the opinio would be better explained as the normative embrace of the principle of reciprocity based on the utility that a group of rational maximizers 17 expects from creating and preserving social rules.

1.3. Achievements and Pitfalls in Theorizing Customary Rules

13 The proposals sketched above, on both strategies, are illustrative of important aspects to consider when inquiring about custom and law, but they mostly display persistent difficulties in explaining customary law and the paradoxes and dilemmas embedded in the constitutive elements of the ­ opinio juris .

• 18 H. L. A. Hart, The Concept of Law [1962], 2 nd ed., New York, Oxford University Press, 1994, p. 19 (...)

14 The strategy of eradicating opinio juris as a conceptual element for explaining customary law leaves a void which cannot be satisfactorily fulfilled with a conception of customary rules as those that are “essential for the constitution and conservation of any society”. First of all, the proposal casts high controversy over which would be those rules. One could make a charitable interpretation of Bobbio’s proposal by considering his concept of customary law analogous to what Hart called “the minimum content of natural law”. 18 That is, to consider customary law as those rules which content fulfills the minimal requirement of survival in society. As Hart observes, without these rules no system of laws or codes of morals can be established. Nonetheless, when legal doctrine refers to custom, it does not seem to refer to the necessary conditions or to the foundations for the existence of a legal system. When referring to custom and the application of customary norms, legal doctrine already assumes the existence of a legal system that has to provide a legal resolution to legal disputes. Thus, to solve certain disputes, the system may apply norms that were created out of the spontaneous behavior of legal subjects in their respective fields of action.

• 19 Also, it is hard to imagine this belief being uniform. Several agents may hold different views ab (...)

15 Furthermore, eliminating the opinio to focus on the agents’ account about what they do competently within the frame of the practice, although suggestive, seems somehow vague and incomplete, as it does not provide a comprehensive explanation about customary rules and, instead, appears to reaffirm the existence of an opinio juris that we should look for in the agent’s explanation and justification of their courses of action. Something similar can be said about the “articulation theory”. Still, the latter account intro­duces some novel aspects that put forward conceptual difficulties and controversies. First, it is greatly debatable whether customary law is “voluntary”. Whose will are we referring to? Is it voluntary because, like in a contract, parties agree to abide by those rules? Voluntariness could only be understood as a metaphor for a doctrinal explanation about abidance to custom in International Law, but cannot be part of a satisfactory explanation since custom is supposed to arise and develop from spontaneous behavior, not from the agreement. Moreover, if we ought to look for the “expressions of belief” of the norms the states intend to follow, wouldn’t this expression of belief be described as a sort “ opinio juris ” since, as one might conjecture, states “intend” to follow those norms because they either believe them to be law or that they should be law. Finally, the custom is a matter of fact that transcends the beliefs of the agents. So, if we are willing to argue that norms arise from certain customs, the existence of those norms cannot depend on the beliefs of the agents about the concurrent practice, 19 but in the fact that there is a custom that is practiced.

16 On the other hand, the strategy of explaining opinio juris as the rational evaluation an agent would make about a course of action can be a plausible explanation for a wide range of customary rules, but not to all of them, especially those that arise from strictly spontaneous behavior and that are not designed with what intuitively looks like the practical reasoning employed for making contracts or crafting enacted law. In these cases, it is more plausible to imagine rational designers of the rule that are quite aware of the circumstances for making a rule out of a certain action and that consciously adhere to the principles of maximization of utility. Thus, in this account, the only regularities of behavior worth of being called “rules” would be those qualified as “efficient” because of their highly optimal outcome. Nonetheless, the idea of expectation should not be discarded, since it intuitively seems necessary to explain social rules as abidance to a convergent behavior in a society in which those who defect are somehow punished or called out and where that convergence can be used to justify one’ own course of action.

• 20 N. Bobbio, La consuetudine come fatto normativo , op. cit. , p. 54-56; J. Finnis, Natural Law & Nat (...)

17 More implicitly, theorizers of customary law also commonly face the problem of explaining the legal validity of customary rules while describing their constitutive elements. In this sense, one common methodological assumption seems to be that opinio juris is the relevant information for the identification of customary rules as law or legal . However, that assumption confuses the question of identifying rules with the one of their legal basis or validity. 20

18 Accordingly, giving a proper account of customary law necessarily seems to involve at least three aspects:

Explaining the peculiarities of group behaviour that creates custom and social rules.

To account for the special conceptual feature that involves that behaviour in order to create rules that belong to a sort of normative order, i. e. , the social order.

Giving an explanation about the legal validity of customary norms.

19 In what follows, firstly, I will begin by addressing some general and methodological aspects regarding the legal theory that mainly answers the third aspect. Consequently, this essay will develop an account of customary norms, starting from factual elements, to consider their formation.

2. Legal Theory and Methodological Concerns

21 F. J. Arena, “Embodied conventions”, Revus , n o  30, 2016, p. 59-67, p. 63.

20 There is a common notion –almost a platitude– that lies beneath the ideas of custom and customary law: namely that, no matter how we decide to conceptualize its constituent elements, we can agree that custom, and thus, customary norms, are conventional products. That is, as opposed to natural phenomena, custom is conventional because it could have been either one way or another. 21

21 Nonetheless, we must be careful with assessments about conventionality and the law. The quest to account for customary norms should not be confused with the philosophical inquiry that attempts to explain law as a conventional phenomenon. This is a subtle but relevant methodological concern to keep in mind not to lose focus on the scope of the inquiry about customary law.

• 22 See N. Southwood, “Laws as Conventional Norms”, in D. Plunkett, S. Shapiro and K. Toh (eds.), Dim (...)

23 J. Raz, The Authority of Law , Hong Kong, Oxford Clarendon Press, 1979, p. 85-86.

24 J. Raz, Practical Reasons and Norms , New York, Oxford University Press, 1999.

22 The strategy of legal theorists that assert that law is conventional often consists in showing that the main components of the law; i. e. laws, legal norms, rules, are the product of conventions. 22 Since these accounts dig into the nature of conventions and normativity, they definitely are helpful in the scope of understanding customary norms. Nevertheless, inquiring into the conventional nature of law and analysing the conceptual ­implications of customary norms are two different philosophical tasks. On the one hand, like Joseph Raz , 23 one could argue that customary norms that constitute customary law are conventional and, on the other hand, state that law as a whole is not conventional. 24

23 In order to understand where the conceptual differences are, two precisions are called for:

25 P. Chiassoni, “Tres buenos filósofos…”, art. cit., p. 106.

The role of rules or norms for inquiring into the nature of law is different from the one in conceptualizing customary law. In the former task, rules are elements studied as elements of the law. That is, they are important only in the light of the wider goal of explaining the law. In the latter, norms are objects of law. Namely, they are the protagonists of the inquiry, as a theoretical concept or nature of law is taken for granted –at least implicitly. For example, as ­Chiassoni points out, Hart considered that legal systems are composed of social rules, but social rules are only elements into the explanation of law, not the object of the explanation. 25

26 J. Gardner, Law as a Leap of Faith , Oxford, Oxford University Press, 2012, p. 59.

Conventions and custom and therefore customary norms are conceived to be the product of spontaneous convergent behaviour in a social group, but the law is also formed by enacted law. On the other hand, enacted law involves the deliberate act of an authority. 26 The validity of the actions of the legal authority lies in the existence of a previous legal norm that precedes them. If we take that line of reasoning backward until its last consequences, we will arrive at the question of the basis of existence and the validity of law. The basis for the existence and validity of enacted and customary norms could be a norm, but not necessarily a conventional norm –understood as the product of practice. For example, for Kelsen, the basis of legal systems was a hypothetical norm; i. e. the basic norm.

24 This digression about legal theory and methodological concerns is useful to this essay to introduce the question about what makes a customary norm legal .

2.1. The Legality of Norms

25 Recalling what was stated above, we should not fall into the trap of considering that there is something in opinio juris that inherently turns custom into law. As a matter of fact, there is nothing inherent in customary norms that makes them be a part of the law. In ontological terms, customary norms are a sort of social rules and are, in addition, the prototype of social rules given that they are the product of spontaneous social interaction. Therefore, what makes social rules be a part of the legal phenomenon and call them customary law is law itself. Specifically, the fact that those rules meet the criteria established by the legal system to be law.

• 27 R. Caracciolo, El sistema jurídico. Problemas actuales , Madrid, Centro de Estudios Constitucional (...)

28 J. Raz, The Concept of a Legal System , Hong Kong, Oxford Clarendon Press, 1980.

29 J. Raz, The Authority of Law , op. cit. , p. 86.

26 The present essay won’t go deeper into theoretical inquiries about the nature of law because it falls beyond its scope. However, it is relevant to mention that it is assumed here that law is essentially a set of rules, more precisely a system of rules, since it has a structure 27 and contains standards of membership or, to be precise, as it establishes which rules can be a part of the system. 28 In this regard, one can consider that, generally, customary laws are social rules that have met the criteria of the legal system. What these criteria are is a matter not exempt from controversy. Some could argue that a social rule can be considered customary law because it meets the criteria set by the system’s Rule of Recognition, for example. Which criteria make social rules become part of a legal system is not the main scope of the present analysis either. Here it suffices to say that the common systematic criteria lawyers use to identify a social rule as customary law is that a rule is either considered as customary law by a previous law in the system or because that rule has persistently been identified and applied by legal officials. 29

2.2. What Makes a Social Rule?

27 Now that we know how a social rule becomes part of a legal system to be customary law, we must face the question of how to identify them. To be precise, we must find the conceptual elements that make the existence of social rules possible and are useful to recognize them. This is of paramount importance since, as Gardner states:

30 J. Gardner, Law as a Leap of Faith , op. cit. , p. 67. […] if a custom is to form part of the law, it must be normative in the eyes of the law. It must constitute a legal norm. So there must be someone who, on behalf of the law, regards the custom as normative. 30

• 31 M. C. Redondo, Orden jurídico. Modelos y discusiones , Santiago, Ediciones Olejnik, 2017, p. 18-19 (...)

28 Therefore, identifying customary rules involves searching for normative elements in social practices. The task is by no means easy given that customary norms are born informally through spontaneous social interaction, so they lack authorship, which means they are not expressly crafted. Canonical expressions are lacking to inform us which are the norm and the specific conduct required. 31 All we have are human actions. Therefore, any analysis of customary rules must focus on the characteristics of actions.

32 R. Guastini, Interpretare e argomentare , Milan, Giuffrè Editore, 2011, p. 6-9.

29 Facts and actions do not say anything by themselves. To grasp anything like “normativity” from social interaction, we must engage in a conjectural interpretation of a causal relationship between the action of agents in a series of events. 32 This is to observe patterns and peculiar cues that might explain the agents’ actions.

30 At this point, it seems clear that, in order to grasp something like the “normativity” of social rules, we cannot neglect to mention a sort of “mental state” that relates the observance of the agents to a specific pattern of behavior. That “mental state” can just as well be called –because of doctrinal tradition– opinio juris . Nevertheless, the important thing is not how we call it, but being able to explain it and its features in a manner that can help anyone that wants to identify customary rules.

33 See, supra , note 11.

31 With this in mind, it appears adequate to endorse Finnis ’ methodological assumption that we cannot aim at grasping the concept and function of opinio juris without observing the formation phases of the rules were it lies. 33 It is in these phases that we can observe and assert something about the mental states of the agents that may perhaps entail what we understand by opinio juris. Hence, at this point, it is necessary to say something about how social rules are born and what features or “mental states” we can plausibly predicate that may explain what makes them, so to say, normative .

34 H. L. A. Hart, The Concept of Law , op. cit. , p. 89-92.

32 Possibly, the best and more familiar starting point to begin accounting for social rules that can be found in analytical legal philosophy is the framework developed by Hart. Trying to explain law as an ensemble of two different kinds of rules, primary and secondary, Hart developed a framework to explain how social rules differ from mere convergent behavior if we look at them from the internal point of view   34 of the participants. In this sense, he explains what accounting for social rules from the internal point of view entails, as opposed to the external point of view , where the observer would only perceive regularities of behavior that could not be distinguished from habits.

• 35 S. Shapiro, “What is the Internal Point of View?”, Fordham Law Review , n o  75, 2006, p. 1157-1170, (...)

36 H. L. A. Hart, The Concept of Law , op. cit. , p. 57.

37 Ibid ., p. 89, G. J. Postema, “Custom, Normative Practice, and the Law”, art. cit., p. 715-716.

38 Recalling supra , note 25.

33 Shapiro rightly notices that the internal point of view ought not to be understood as the insider’s point of view, which would mean considering the substantive reasons agents have for abiding by rules, variating from moral reasons to fear to punishment to mere non-reflexive action. Rather, the internal point of view is a synonymous of “internalized”, referring to the practical attitude of rule acceptance of the agents that could nevertheless have different opinions about the morality of the rule as well as diverse substantive reasons for abiding to it but who jointly consider the rule as a standard of social conduct by which it is legitimate to criticize defectors. 35 The latter is another relevant feature Hart introduces for distinguishing social rules, the reflective attitude of agents towards them. 36 Thus, Postema’s discursive integrational account makes a good point in emphasizing the need to look at what agents do competently within the context of the practice they carry on. After all, the emphasis of the internal point of view lies in distinguishing how agents are concerned with the rules. 37 However, as stated before, Postema’s account is too vague to tell us something more specific to identify social rules. Also, Hart’s internal point of view and the critical reflective attitude seem too broad, since he only looked for an explanation about social rules that was satisfactory for differentiating them from habits and that worked well enough in the scope of a (general) theory of law. 38

39 B. Celano, Dos estudios sobre la costumbre , Mexico, Fontamarap, 2009, p. 9-10.

34 In all probability, Hart’s account is a great framework of reference that can be complemented in order to give a more detailed explanation about social rules and the elements that compose them. More specifically, to clarify what opinio juris may entail if it is understood analogously to the internal point of view. In the latter sense, what I will develop can be understood as a supplement to Hart’s account with the purpose of giving an explanation of social rules as objects, i.   e. , sources of law. That means, going deeper into explaining social rules and specifying more their features so as to give a more detailed account that may work for someone trying to identify social rules –as well as the problematic opinio juris that composes them– in specific contexts. For this, as stated previously, the work will follow Finnis ’ strategy of explaining the phases of the formation of social rules and will introduce explanatory features form Game Theory and insights from Behavioral Economics that may hopefully shed light on the phenomena that social rules entail. Regarding the introduction of these peculiar conceptual apparatuses, Celano’s states that the development of an adequate theory of custom and customary law requires to look out for theoretical tools and paradigms that do not necessarily belong to the tradition of analytical legal philosophy but that are compatible with its methodological concerns. 39

2.2.1. The Birth of Behavioral Social Regularities

• 40 F. Parisi and V. Fon, The Economics of Lawmaking , New York, Oxford University Press, 2009, p. 135

35 How social rules come into existence is without a doubt an intriguing question. As we have seen, some legal philosophers believe that by reaching into the reasons that constitute a social practice as a common shared standard we may find the basis in which lies the “normativity” and, therefore, the roots of opinio juris . Accordingly, those legal philosophers embrace a sort of contractarian insight that, although it is plausible for explaining the motives of a few members of society to follow that rule, it does not manage to explain why that reason is widespread enough to be followed by other members of society. Such is the case of Finnis’s account. Certainly, it is highly possible that two or a few members of society reach a private spontaneous agreement about what to do in a certain present situation and, that freely, they start to act in the same manner for certain conceivable future instances in which similar conditions arise. It is also plausible to imagine the agreement involves the fact that they consider desirable to establish a specific rule for that kind of cases, being conscious that the solution it brings maximizes their expected payoffs, knowing the status quo at the time as well as the probabilities they will play different roles in the future. 40 Nevertheless, even though the contractarian insight is plausible as an explanatory input, it does not satisfactorily account for the widespread observance of a rule, considering that other members were not present in the negotiation and exposition of reasons for establishing that rule and that the payoffs such rule brings may not be the most favorable for them since their expectations were not considered when the rule was crafted. Consequently, it is highly plausible to consider that the reason ( opinio ) the latter have for abiding by such rule is not the same as those who crafted the rule. Also, as stated above, this explanation would leave aside spontaneous behavior that, even though it was not the product of a rational process in the beginning, works regularly to solve or rule a social situation.

41 S. Shapiro, “What is the Internal Point of View?”, art. cit., p. 1162.

36 Likewise, with Hart as a guide, we must remember that it is not a fructiferous path to search into the substantial reasons people may have for accepting and abiding by a rule, 41 and this premise should be observed for the explanation of the formation of social rules as well. Nonetheless, in order to explain social rules and their distinctive traits more acutely, it is necessary to say something about how they emerge.

• 42 R. Guibourg, “Las fuentes del derecho”, in E. Garzón Valdés and F. Laporta (eds.), El derecho y l (...)
• 43 The emphasis on the word “may” is to get clear that it is possible for a social habit to become a (...)

37 Guibourg suggests that the birth of even more spontaneous social rules can also be explainable through –but not confounded with– habits. 42 This seems to be a good way, to begin with, since a more plausible account of social rules should not exclude the possibility of explaining those that are born and spread in a non-contractarian manner, because of spontaneous and convergent social behavior that repeats itself so as to create a regularity that might make individuals form expectations. And how such regularities in behavior are born and spread may be better explained by starting with habits understood as the social regularities that may 43 precede social rules.

44 D. Ariely, Predictably Irrational , New York, Harper Collins Publishers, 2008.

45 Ibid ., p. 25-33.

• 46 A. Tversky and D. Kahneman, “Judgement under Uncertainty: Heuristics and Biases”, ­ Science , n o  41 (...)

38 It is commonly said that we, humans, are creatures of habits. One good explanation for the birth and persistence of long term habits in human behavior comes from Behavioral Economics. Ariely highlights that a combination of cognitive biases helps the formation of long term-habits we act upon in our everyday decision making. 44 For instance, the anchoring effect is the cognitive bias that explains that the initial value put on a fact sets the range of our future decisions, not only about that fact but at future instances of similar situations. 45 The value can be put by ourselves or by someone else, and even though we may adjust estimates for future instances according to different circumstances, the anchoring effect means that our perception about the fact is so nailed down in our mind that the adjusted value usually would be insufficient or not adequate to the new case. 46 For example, when buying a new plasma TV of 43”, if the first price we see at a department store for a TV of 43” is 300€, our next judgements about any another 43” plasma TVs, and whether we find them expensive or cheap, will revolve around that fist price we encountered, 300€. It does not matter much which different features other TVs could have and if the difference in value is adequate to such traits, the standard of 300€ is inevitably set in our mind.

47 D. Ariely, Predictably Irrational , op. cit. , p. 36-37.

48 Ibid ., p. 37-39.

39 Subsequent to anchoring , there is the cognitive bias of herding and self-­herding effects. When a judgment is made about others’ actions and we decide to follow it afterwards, then this is called herding . For example, when we see people queueing in a restaurant, there is a high probability that we will assume that the quality of the food in that restaurant is good and then join the queue or return on a different occasion. Self-herding happens when we judge something good, bad, adequate or inadequate based on our own previous behavior. 47 For instance, if, when facing problem Y 1 , I decide to do X 1 , and X 1 seems to me to solve the problem adequately, most likely during the future or similar instances Y n , I will perform X n , even though there may be another better solution for any Y. The key to breaking the cycle and engaging in a different behavior would be introducing a new factor or feature that prominently changes the experience of the subject, prompting her to change past decisions. 48 These cognitive biases have social effects as they produce public or social indicators that other agents might follow, and easily, can magnify into creating generally observable social regularities of behaviors.

40 However, something else must be added since these phenomena only give an explanation about the mental mechanisms by which social regularities most probably arise, but still cannot cope to fully explain “normativity” or social rules.

2.2.2. From Social Regularities to Social Coordination and Expectations

49 See above, page 6.

50 Let us remember that the notion of expectation introduced by Lazzaro. See above, note 15.

41 Recalling Hart, one relevant feature of social rules is the reflective critical attitude that the internal point of view entails. 49 The reflective critical attitude could be better explained as the product of social coordination and the expectation that lies in the reiteration of social practice. 50 From a social regularity a member of a group may form an expectation regarding further actions –not taking into account whether she regards it as good, fair, practical, or contrary to the precedent adjectives.

42 Imagine a society in which personal notifications are left under the door when the addressee is not present. Initially, it might be just a habit, but as time goes by and the practice consolidates, people will develop expectations, feeling frustrated when the practice is not followed by someone. Now, it is important to consider that although this practice may rule in a society, it is possible that no one ever explicitly expressed its existence. The practice is simply performed. It might be that someone thought about it as an ideal solution for a practical problem and performed it for the first time, but other people might have never known this. Even more so, others possibly consider that the current practice is deficient or unwise. Nonetheless, people follow the practice because they presume others do the same, so any personal judgment about it is irrelevant.

• 51 D. Hume, A Treatise of Human Nature [1739-1740], III, Of Morals , L. A. Selby-Bigge (ed.), Oxford, (...)

43 This idea was developed by Hume when he suggested a possible way to explain conventional agreements outside the institution of promise ( i. e ., a language artifact). 51 And it was Hume’s intuition that drove Lewis into exploring how is it possible that language is a convention if at the beginning we did not have a language to agree to it. That reasoning can be translated into the explanation of social norms when we take as a premise that they can arise spontaneously like habits and, possibly, evolve and spread more spontaneously, with nothing such as an explicit agreement, but eventually creating mutual social expectations.

52 D. Lewis, Convention , Oxford, Blackwell Publishers, 1969, p. 24.

53 Ibid ., p. 8.

44 To explain how conventions are created with no expressed agreement, Lewis benefits from the insights of Game Theory, focusing on pure coordination games to explain how social conventions could spontaneously arise and prevail. Hence, Lewis proposes to understand social conventions as referred to coordination problems in the face of situations of interdependent decisions by two or more agents, in which coincidence of interest predominates and in which there are two or more proper coordination equilibria. 52 The equilibria would be the combination of actions in which each agent has done as well as she can by considering the actions of the other agents. In the equilibria combination, no agent could have produced an outcome more to his liking by acting differently unless others’ actions had been different. It is important to notice that the equilibrium produced was not necessarily the best one and that it is not the case that there were no other equally good possible outcomes. 53 It just means that it was the finest possible outcome given the actual choices of agents in that situation.

45 One of the famous examples Lewis presents is the one where two persons are talking on the telephone and unexpectedly the connection cuts off. Both want the connection restored as soon as possible, which will happen only if one calls back while the other waits. It matters very little who calls back and who has to wait. There is a common interest, and two possible sets of actions that reach the desired coordination equilibrium.

46 As we can see, if both agents do the same actions {R1, C1}, {R2, C2} they do not fulfil their expectations and do not manage to resume their conversation, while by engaging in different actions, either {R2, C1} or {R1, C2}, they manage to reach their mutual goal no matter which one of the two possible coordination payoffs they chose.

• 54 The famous prisoner’s dilemma is a mixed motive game in which both agents are rational egoists an (...)

47 Different from the famous prisoners’ dilemma , 54 the case in hand is neither an opposed interest nor a one-shot game. The players may actually fail in their first attempts to coordinate, and once reached an equilibrium they would surely praise the payoff since they could easily foresee future similar situations of interaction. Hence, it does not matter how they reached either of the equilibria, what matters is that they reached it, and it will most probably become a salient precedent for future similar situations when a phone call is cut off. As Behavioral Economics explains, the latter is likely because of the anchoring , herding and self-herding effects. Consequently, we anchor value to the outcome of our coordinated actions and will herd and self-herd ourselves for future occasions in which a similar situation arises. Once more, this is not a one-shot game and the regularity may not come at once. The key is in the expectation that settles for the persistence of the regularity. A set of expectations for future occasions was most probably created from the first coordination equilibria.

55 D. Ariely, Predictably Irrational , op. cit. , p.  3-10.

48 Situations and actions assist us in constructing reference points that work as signs for our minds that help us to construct frameworks of ­actions for future occasions. That is how we are wired. It can be said that naturally inductive agents look out for patterns in the world for behaving on further occasions. 55 Once that set of expectations is yielded by commonly identified factors that stand for a determinate situation, those prominent signals drive us to frequently repeat the same pattern of behavior between us –and even between whoever else gets into our dynamic. Thus, we can say that we have a proper social rule because we have done so in past occasions when similar situations occurred, and it serves to coordinate our intentions because of the settled expectations it has established between us.

2.2.3. From Coordination to Social Rules

56 D. Lewis, Convention , op. cit. , p.  97-100.

49 Lewis considered his model to be appropriate to account for conventional phenomena such as social rules. Although his definition and analysis does not include normative terms like “should” or “ought”, he considers conventions to be species of rules, particularly considering regularities to which we think we ought to conform because of the presumptive reasons that another agent can have about the benefit we and others get by behaving that way. Moreover, for Lewis, conventions are social rules as they may be enforced by evoking unfavorable responses from others if failing to conform. 56 However, some specifications still seem to be missing to understand where something like the “normativity” embedded in the idea of opinio juris could be found.

• 57 J. Anomaly and G. Brennan, “Social norms, the invisible hand, and the law”, University of Queensl (...)

50 Traditional doctrinal accounts of customary law are misleading as they tend to represent any possible opinio juris that agents could hold as a positive or optimistic judgment towards the observance and endurance of that social practice. However, the account of social rules based on Lewis’ model, as well as the insight of cognitive bias exposed by Behavioral Economics, gives us the possibility to consider as “customary norms” social practices that are sub-optimal equilibria ( Pareto inferior ). Moreover, since the outcome could be far from being optimal (with low to null payoffs) and even harmful somehow, it is likely that a vast number of the members of society have a negative perception of the persistence the practice and would prefer ­changes. 57 This raises the questions: if a large part of the population, ­perhaps even the majority, considers a practice as bad, can this be considered a social rule susceptible of being used for adjudication? Can there be something like an opinio juris in this so-called “social rule”?

• 58 The expression “standard Law and Economics insight” refers mostly to the Chicago school that unde (...)
• 59 “A particular situation is said to be Pareto or allocatively efficient if it is impossible to cha (...)

60 F. Parisi, “The Formation of Customary Law”, art. cit., p. 13-15.

51 Prima facie , any traditional contractarian insight would answer negatively to the questions above. It is contrary to rationality to follow a practice that is considered negatively and that yields low payoffs. No conceptual account could justify a hypothetical agreement of this sort between rational agents. For this reason, from a standard Law and Economics perspective, 58 it would be advisable to consider as customary law –that is, enforceable norms– only those rules that pass the test of Pareto efficiency. 59 The standard Law and Economics approach would argue the latter to be an adequate stance regarding customary law, given that rational agents would only regard as convenient and enforceable those rules that enhance welfare for themselves. Furthermore, from a practical point of view, rules that are Pareto efficient would foster stability of practice, since these rules are useful to avoid defection strategies because they tend to be considered as genuine rules of reciprocity for a game that presents constant socially desirable payoffs. 60

52 Thus, social rules enforceable by law ought to be weighted as solutions to cooperation problems such as prisoner’s dilemma games of more than one shot. It can only be considered as a customary legal rule when its outcome reaches the Pareto optimal payoff {R1, C1}. The Law and Economics approach is an interesting reconstruction for analyzing outcomes in social rules, but it is not the result of a conceptual enterprise and, consequently, we must not confuse normative observations with the theoretical explanation of reality and conceptual objects. Under this approach, several rules that do work in reality and that are factually enforced by courts –by imperfectly rational human beings– would be expelled from theoretical consideration, even though they do coordinate expectations and resolve a social situation –although not necessarily in an optimal manner. Such would be the case of the payoff {R2, C2} that, although it is a suboptimal Nash equilibrium in relation to {R1, C1}, is still an equilibrium and may effectively be a social rule. It can be argued that social rules ought not to be perfect to exist and to provide solutions –not necessarily good ones– to social situations.

• 61 E. Fittipaldi and E. Timoshina, “Theory of Custom, Dogmatics of Custom, Policy of custom: On the (...)
• 62 E. Bulygin, “El concepto de eficacia” [1965], in H. Kelsen, E. Bylygin and R. Walter (eds.), Vali (...)

53 It is important not to confuse the enterprise of accounting for customary law with the point of view of legal politics. 61 There can be a wide consensus that a certain rule is despicable, but that does not make it stop being a rule. An enacted law can be deemed unfair by the majority of the population, but it is still a law, likely to be either generally obeyed by the population or enforced by tribunals. Therefore, the legal norm can be used by someone to justify his own behavior –even though it is considered unfair– or by a judge as a basis to punish those who do not observe the norm. The same holds true for customary norms. They are norms inasmuch they are useful either to justify one’s behavior or to criticize or punish defectors. 62 Therefore, any account about a mental state or element analogous to what could be called opinio juris must be capable of explaining that the social practice entails a rule inasmuch it can be rightfully used to justify or criticize behavior independently of other considerations like morality or efficiency.

63 C. Bicchieri, The Grammar of Society , New York, Cambridge University Press, 2006, p. 38-40.

64 Ibid. , p. 25-26.

54 Consequently, social rules do not necessarily coincide with our self-­interest as the pure coordination game model presupposes. Also, coordination games always go against our own self-interest, while that cannot necessarily be said about social rules. Finally, one can say that a social rule exists without it being followed. Indeed, a social rule, as such, is avoidable if one avoids the situation in which it rules, 63 while conventions à la Lewis do not account for this possibility. Bicchieri observes these considerations to be relevant distinctions between Lewis ’ conventions and social rules. For that reason, Bicchieri considers her account of social norms to be distinct from Lewis ’ . Nonetheless, Bicchieri’s account does not contradict much of the scheme of conventions sketched by Lewis. Rather, it could be seen as a supplement for a more accurate analysis of social rules as conventions, where mental states can be sustained as an opinio juris for participants to consider any social practice to be a rule. First, Bicchieri is right to point out that many social rules do not arise from self-interest, but that does not exclude that many others do, as in Lewis’s account. Second, even Bicchieri states that, eventually, social rules that arise from mixed-motive games ( prisoner’s dilemma ) evolve into coordination games through a Bayesian game . The latter case comes up in a prisoner’s’ dilemma situation when the agents figure out that their best option is to cooperate, thus transforming the ranking of the criteria selection into a cooperation game. 64 Accordingly, realizing that the other agent has the same options at hand and that by both choosing the same selfish option, both would lose and get the worst outcome, they better come into terms by choosing the Nash equilibrium option in which none gets the best payoff possible for themselves but do no end up losing everything.

65 B. Celano, Fatti istituzionali, consuetudini, convenzioni , Rome, Aracne, 2010, p. 194-195.

66 C. Bicchieri, The Grammar of Society , op. cit ., p. 28.

67 Ibid ., p. 11.

68 G. J. Postema, “Implicit Law”, Law and Philosophy , vol. 13, n o  3, 1994, p. 361-387, p. 364.

55 Consequently, we might say that in a mixed-motive game situation, for a social rule to exist there has to be a conditional follower with a Bayesian game that changes his preferences if the normative and empirical expectations are fulfilled. The empirical expectation is the belief of the agents that a sufficiently large subset of the population conforms to the regularity of a certain type of situation 65 so as to choose that option and not the selfish one. 66 The normative expectation –that could be called opinio juris – is the sum of the empirical expectations plus the foreseeable possible “sanction” in case of defection. 67 The former account is concordant with the well-­established notion that “implicit rules arise and draw their practical force from the interdependence of expectations and aims”. 68

69 C. Bicchieri, The Grammar of Society , op. cit. , p. 39-40.

56 Regarding not following social rules, but still affirming their existence, Bicchieri refers to the avoidance of the situation in which the rule is in force. The example Bicchieri elucidates is one of the stag hunters where whoever does not cooperate is sanctioned by not eating. The alternative for avoiding the rule would be to go and hunt rabbits individually. 69 Thus, the regularity of people preferring to hunt rabbits by themselves would mean that the social rule in action for stag hunters is not regularly observed. Still, one could argue that another permissive social rule is in force since it justifies an alternative course of action, the rule being: whenever hunters do not meet for hunting stags or whenever one does not meet with those who hunt stags, it is permissible to eat what each one hunts individually.

57 Although Bicchieri’s observation is opportune to prove what a consolidated social rule would be, and even how it may come to an end, it doesn’t seem like her accounts differs much from Lewis ’ , since the only thing it would mean is that whenever the situation for collective stag hunting arises, the convention for cooperating is in force. The situation is avoidable, but once the situation stands, the social rule is in force. In Lewisian terms, in the phone call example, the next occasion in which my counterpart and I call each other again could be over a long period of time. I could have avoided or changed my phone number. Still, when for any circumstance we call each other again, the same practice for the situation of phone calls between us is in place, that is, the convention over who should call back if the connection fails still rules. However, Bicchieri’s observations are opportune to emphasize important traits of social rules:

in the beginning, the interest of the agents could be opposed because of self-interest;

customary social rules are composed of kinds of expectations: empirical and normative;

70 And the way for avoiding them may give place for the birth of another customary social rule.

that although they are customary, there is a way of avoiding social rules, 70 by evading the situation in which they are in force; and

how social rules embed mutual expectation is referred to as correlated interests that left aside the selfish option for the one(s) that reach cooperation.

3. Conclusion. Finding Opinio Juris

• 71 Here has been embraced the doctrinal explanation that usus is convergence in the behavior in the (...)

58 The present work has tried to describe the constituent elements of customary law, understanding it as social rules that are integrated into a legal system. For this, we have accepted the common doctrinal assumption that one of the constituent elements of customary law is the usus , 71 and more attention has been given to the analysis of opinio juris instead. To solve the problem of giving a proper account of customary law and, more specifically, of its constituent element, opinio juris , our work used Hart’s account of social rules, assuming the opinio as analogous to the internal point of view .

• 72 At this point, a vast majority of the population might not like the ruling practice, so in this s (...)

59 Following general directives, Hart establishes to understand what the internal point of view is, this analysis has deepened into an inquiry of how social rules emerge in different phases, in order to discover some more specific features that may help someone to identify social rules as practices that include a normative content. To summarize, we can agree with Hart in not trying to say that social rule arise because of substantive reasons (being morals, calculation or fear), but instead, from studies in Behavioral Economics, we can explain regularities of behavior because of the anchoring, herd, and self-herding effects to which humans are prone to when they –not necessarily consciously– assign a random value to a certain social fact and begin to act regularly in accordance. With the aid of social philosophy and Game Theory, we can say that a frequent behavior that at first might be simply called a habit could become the solution to a coordination problem when the agent’s frequent behavior creates a pattern that meets the expectations that others have created because of that frequent behavior. It is important to notice that the equilibrium reached in the process of coordination does not have to be the best possible one, but that it is the one in force because it has proven to be the one that coordinates that situation. 72 To say we have a social rule, we have to keep in mind that it does not necessarily arise from self-interest (selfish motivations). Instead, a rule is consolidated when a social practice generally surpasses the agents’ self-reasons in favor of coordination considering others’ expectations (social expectations), and the agents internalize the practice as the required action for the given situation, to the point they even take it as a standard for justifying their own actions and criticizing others’ conduct when not abiding by the established standard. Finally, all things considered, it seems like this internalization process can provide a plausible account of what opinio juris entails, that is, the agents’ internalized notion that a certain course of action is deemed adequate and required, according to existing social expectations.

1 P. Chiassoni, “Tres buenos filósofos contra las malas costumbres”, Doxa Cuadernos de ­Filosofía del Derecho , n o  31, 2008, p. 105-142, p. 108-110.

3 One paradigmatic example of this is the 2015 “Third report on identification of customary International law” of the International Law Commission from the United Nations General Assembly, which continues to assert that in International Law custom is formed by usus plus opinio juris , and that the latter is understood to be “acceptance as law” of the practice.

5 N. Bobbio, La consuetudine come fatto normativo [1942], Turin, Giappichelli Editore, 2010, p. 57-58; J. Finnis, Natural Law & Natural Rights , 2 nd  ed., New York, Oxford University Press, 1980, p. 239; F. Parisi, “The Formation of Customary Law”, George Mason Law & Economics Research Papers Series , n o  1-6, 2001, p. 6.

6 N. Bobbio, La consuetudine come fatto normativo , op. cit. , p. 60-61; P. Chiassoni, “Tres buenos filósofos…”, art. cit., p. 115.

7 G. J. Postema, “Custom, Normative Practice, and the Law”, Duke Law Journal , n o  62, 2012, p. 707-738, p. 716-718. Another way of formulating this issue is presented by Dworkin referring to the disagreement that may exist about the scope of the rule for particular cases. Dworkin’s famous example refers to the disagreement that may arise between churchgoers when determining if the social rule of not wearing hats in church applies to the case of babies wearing bonnets. R. Dworkin, Taking Rights Seriously , Cambridge, MA, Harvard University Press, 1978, p. 54-55.

12 F. Parisi, “The Formation of Customary Law”, art. cit., p. 18-19; F. Parisi and V. Fon, “International Customary Law and Articulation Theories: An Economic Analysis”, Brigham Young University International Law & Management Review , vol. 2, n o  2, 2006, p. 201-232, p. 212-213.

15 G. Lazzaro, “Reciprocità e consuetudine” [1983], in U. Scarpelli (ed.), La teoria generale del diritto. Problemi e tendenze attuali , Milan, Edizioni di Communità, 2003, p. 233.

18 H. L. A. Hart, The Concept of Law [1962], 2 nd ed., New York, Oxford University Press, 1994, p. 193-200.

19 Also, it is hard to imagine this belief being uniform. Several agents may hold different views about the customary practice and the reasons for abiding to it. So, this line of reasoning would bring back one of the main questions for accounting customary norms and the opinio juris : if everybody has a different rule in mind when abiding to a pattern of behavior, can we say they are following the same rule?

20 N. Bobbio, La consuetudine come fatto normativo , op. cit. , p. 54-56; J. Finnis, Natural Law & Natural Rights , op. cit ., p. 240.

22 See N. Southwood, “Laws as Conventional Norms”, in D. Plunkett, S. Shapiro and K. Toh (eds.), Dimensions of Normativity , New York, Oxford University Press, 2009.

27 R. Caracciolo, El sistema jurídico. Problemas actuales , Madrid, Centro de Estudios Constitucionales, 1988.

30 J. Gardner, Law as a Leap of Faith , op. cit. , p. 67.

31 M. C. Redondo, Orden jurídico. Modelos y discusiones , Santiago, Ediciones Olejnik, 2017, p. 18-19.

35 S. Shapiro, “What is the Internal Point of View?”, Fordham Law Review , n o  75, 2006, p. 1157-1170, p. 1159.

40 F. Parisi and V. Fon, The Economics of Lawmaking , New York, Oxford University Press, 2009, p. 135.

42 R. Guibourg, “Las fuentes del derecho”, in E. Garzón Valdés and F. Laporta (eds.), El derecho y la justicia , 2 nd  ed., Madrid, Trotta, 2000, p. 181.

43 The emphasis on the word “may” is to get clear that it is possible for a social habit to become a rule, but is not necessary. As a matter of fact, it is considered that from the wide range of habits that can exist in a society, comparatively, very few get to transform into social rules.

46 A. Tversky and D. Kahneman, “Judgement under Uncertainty: Heuristics and Biases”, ­ Science , n o  4157, 1974, p. 1124-1131, p. 1128-1130.

51 D. Hume, A Treatise of Human Nature [1739-1740], III, Of Morals , L. A. Selby-Bigge (ed.), Oxford, Oxford Clarendon Press, 1960, p. 490.

54 The famous prisoner’s dilemma is a mixed motive game in which both agents are rational egoists and do not care for the others’ payoff, looking individually for the best outcome for themselves without wondering how much this may affect their counterpart’s expectations.

57 J. Anomaly and G. Brennan, “Social norms, the invisible hand, and the law”, University of Queensland Law Journal , vol. 33, n o  2, 2014, p. 263-283, p . 263.

58 The expression “standard Law and Economics insight” refers mostly to the Chicago school that understands law ( Common Law ) as the result of an effort to lead to efficient outcomes. Nonetheless, the Yale school, with its normative stance towards policy intervention in the face of correcting unfair outcomes, can be considered as belonging to this category as well. See F. Parisi, “Positive, Normative and Functional Schools in Law and Economics”, European Journal of Law and Economics , n o  18, 2004, p. 259-272.

59 “A particular situation is said to be Pareto or allocatively efficient if it is impossible to change it so as to make at least one person better off (in his own estimation) without making another person worse off (again, in his own estimation)”. R. Cooter and T. Ulen, Law & Economics , 6 th  ed., Boston, Addison-Wesley, 2012, p. 14.

61 E. Fittipaldi and E. Timoshina, “Theory of Custom, Dogmatics of Custom, Policy of custom: On the Threefold Approach of Polish-Russian Legal Realism”, Ratio Juris , vol. 30, n o  1, 2017, p. 105-22, p. 107-108.

62 E. Bulygin, “El concepto de eficacia” [1965], in H. Kelsen, E. Bylygin and R. Walter (eds.), Validez y eficacia del derecho , Buenos Aires, Astrea-UNAM, 2005, p. 32.

71 Here has been embraced the doctrinal explanation that usus is convergence in the behavior in the members of the society. B. Celano, Fatti istituzionali… , op. cit ., p. 198.

72 At this point, a vast majority of the population might not like the ruling practice, so in this sense, they value it negatively. Nonetheless, the majority still follows the practice because however dislikable the practice is, it still is the only existing manner to coordinate and/or solve that social situation, and behaving differently could mean unbearable expenses for agents. As Parisi rightly notices “Less efficient rules may persist if the transition to more efficient alternatives is costly”. F. Parisi, “The Formation of Customary Law”, art. cit., p. 21-22.

Table des illustrations

Pour citer cet article, référence papier.

Piero Mattei-Gentili , «  The Quest for Opinio Juris : An Analysis of Customary Law, from Hart’s Social Rules to Expectations and Everything in the Middle  » ,  Noesis , 34 | 2020, 89-114.

Référence électronique

Piero Mattei-Gentili , «  The Quest for Opinio Juris : An Analysis of Customary Law, from Hart’s Social Rules to Expectations and Everything in the Middle  » ,  Noesis [En ligne], 34 | 2020, mis en ligne le 15 juin 2022 , consulté le 02 mai 2024 . URL  : http://journals.openedition.org/noesis/5154 ; DOI  : https://doi.org/10.4000/noesis.5154

Piero Mattei-Gentili

Piero Mattei-Gentili is a fellow of the research group Càtedra de Cultura Jurídica from the University of Girona, and a member of the Barcelona Institute of Analytic Philosophy (BIAP). He holds an LL.M from the National Autonomous University of Mexico (UNAM) with Magna cum laude and is currently a Ph.D. candidate at the University of Girona, developing a dissertation in Jurisprudence about the conceptual implications and problems of Customary Law.

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Pritzker Legal Research Center

• International Legal Research Guide
• Journal Articles
• Encyclopedias
• Governmental, IGO, and NGO Publications
• News Articles
• Sources of International Law
• Locate the Full-Text Treaty
• Check the Status of the Treaty
• Find Background and Interpretation Information
• Customary International Law
• General Principles of Law
• Full-Text Sources for Judicial Decisions
• Teachings of Publicists
• Intergovernmental Organizations (IGOs)
• Additional Resources

A rule of customary international law is demonstrated by showing evidence of two elements: (1) state practice (objective element); and (2) opinio juris (subjective element).

Evidence of State Practice in the United States

Digests provide insight into the "view and practice of the Government of the United States in the arena of public and private international law."  ​

• U.S. State Department: Digest of United States Practice in International Law Contains annual volumes from 1989-1990, 1991-1999, and 2000-2016 (note that there was a hiatus in publication from 1989-1999).
• U.S. State Department: Foreign Relations of the United States This series is the official documentary historical record of major U.S. foreign policy decisions and significant diplomatic activity.
• HeinOnline: U.S. Law Digests Contains Cumulative Digest of United States Practice in International Law, 1981-1988, Digest of United States Practice in International Law, and other digests.

Evidence of State Practice in Other Jurisdictions

Check Sources of State Practice in International Law to locate compiled state papers. Governmental websites, particularly those of foreign ministries, can be useful sources of information on the current international practice of the given country. Additionally, international law yearbooks  often contain information on current state practice. 

• HeinOnline: International Yearbooks and Periodicals
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• Last Updated: Nov 23, 2021 7:46 AM
• URL: https://library.law.northwestern.edu/InternationalResearch

Public International Law Research - A Brief Guide

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Advice on Researching Customary Law

United states practice - digests, united states practice - other sources, state practice of foreign nations, domestic/municipal law of foreign nations, customary humanitarian law.

• Courts/Decisions
• United Nations/IGOs
• Learning More

Researching customary law is as complex as it is essential. Some useful (but by no means all) sources include: digests of state practice (for those nations that keep them); domestic/municipal law; and international law yearbooks. Actions by international governmental organizations (such as the U.N.) may also serve as evidence of customary law. For these actions, see the United Nations/IGOs section of this guide.

These are some good online and print guides on researching customary law. Where good digital sources for customary law research are available, the online guides will take you to them.

• Researching Customary International Law, State Practice and the Pronouncements of States regarding International Law By Silke Sahl of Columbia and Catherine Deane of Shearman & Sterling. Part of the GlobaLex collection of foreign and international research guides. (Updated in 2018.)
• Other Sources of International Law [primarily Customary International Law] A very thorough guide from the Boston University's Pappas Law Library, "covering the sources of customary international law."
• Customary International Law: Research Guides & Background Information A section of the helpful International Legal Research Tutorial . (A collaboration between Duke University School of Law and University of California, Berkeley, School of Law.)
• Researching Public International Law: Custom and State Practice By Kent McKeever of Columbia University's Arthur W. Diamond Law Library. (Published in 2006.)
• Research Guide to Customs, General Principles, & the Teachings of Highly Qualified Publicists A 14-page PDF guide by Lee Peoples of the Oklahoma City University Law Library. (Published in 2005.)

• Digest of United States Practice in International Law, 1989- A "historical record of the views and practice of the Government of the United States in public and private international law." Supplemented, in many cases, with the full-text documents referenced in the Digest . Also in print: KZ 21 .R68 Law Stacks.
• Cumulative Digest of United States Practice in International Law, 1981-1988 From the U.S. Department of State. (Provided here by HeinOnline — for current USF students, faculty, and staff.) Also in print: KZ 21 .R68 Law Stacks. [ On-campus link for Cum. Digest of United States Practice in Int'l Law, 1981-1988 .]
• Digest of United States Practice in International Law, 1973-1980 From the U.S. Department of State. (Provided here by HeinOnline — for current USF students, faculty, and staff.) [ On-campus link for Digest of United States Practice in Int'l Law, 1973-1980 .]
• Digest of International Law (Whiteman), 1940-1960 From the U.S. Department of State & compiled by Marjorie M. Whiteman. (Provided here by HeinOnline — for current USF students, faculty, and staff.) In print: JX 237 .W5 Law Compact Storage. (Dates of coverage vary by topic.) [ On-campus link for Digest of International Law (Whiteman), 1940-1960 .]
• A Digest of International Law, 1776-1906 Compiled by John Bassett Moore. A revision of Wharton's Digest of the International Law of the United States . (Provided here by HeinOnline — for current USF students, faculty, and staff.) Also in print: JX 237 .M66 1906 Law Compact Storage.[ On-campus link for A Digest of International Law, 1776-1906 .]
• A Digest of the International Law of the United States Compiled by Francis Wharton and published in 1886. (Provided here by HeinOnline — for current USF students, faculty, and staff.) Also in print: JX 237 .W5 1886 Law Compact Storage. [ On-campus link for Digest of the International Law of the United States (Wharton) .]
• Digest of Published Opinions of the Attorneys-General and of the Leading Decisions of the Federal Courts, with Reference to International Law, Treaties, and Kindred Subjects Compiled by John L. Cadwalader. Published in 1877. (Provided here by HeinOnline — for current USF students, faculty, and staff.) [ On-campus link for Digest of Published Opinions of the Attorneys-General and of the Leading Decisions of the Federal Courts .]
• History and Digest of the International Arbitrations to Which the United States Has Been a Party Compiled by John Bassett Moore. Published in 1898. (Provided here by HeinOnline — for current USF students, faculty, and staff.) [ On-campus link for History and Digest of Int'l Arbitrations (U.S. as Party) .]

These are just a few of the many other useful sources for researching United States state practice in international law. For more options, see "Advice on Researching Customary Law," above.

• Foreign Relations of the United States (FRUS) - HeinOnline This link opens in a new window This HeinOnline version covers 1861 to the present. The U.S. Dep't of State has some 20th century portions of the Foreign Relations of the U.S. The Univ. of Wisconsin has a version of Foreign Relations of the United States covering 1861 - 1960 .
• Contemporary Practice of the United States Relating to International Law - American Journal of International Law (HeinOnline) Each issue of the American Journal of International Law includes a section on the "Contemporary Practice of the United States Relating to International Law." The journal is also in print at: K1 .M456 Law Stacks. [ On-campus link for Contemporary Practice of the United States Relating to Int'l Law - Am. J. Int'l Law .]
• United States Practice in International Law (2002- ) By Sean D. Murphy. In print: KF 4581 .M87 2002 Law Stacks. (USF currently has the first of two volumes.)

Here are some good places to begin your search for state practice of non-U.S. nations. For more options, see "Advice on Researching Customary Law," above.

• International Yearbooks - HeinOnline Yearbooks on international law often reproduce key documents relating to state practice. This HeinOnline collection includes 16 national yearbooks and several topical yearbooks. (For current USF students, faculty, and staff.) [In print the Zief Library has the Asian, British, Canadian, and South African yearbooks, and early volumes of the Australian and Italian yearbooks.] [ On-campus link for International Yearbooks .]
• Commentaries on the Draft Articles on State Responsibility [PDF] These Commentaries (from the International Law Commission ) can be a rich source of citations for building arguments about state practice and customary law.

Domestic laws (including constitutions) can provide evidence of customary law, as well as of "general principles." Here are a few places to start.

• Researching Foreign Legal Systems - Basic Resources An introductory guide to researching the laws of non-United-States nations.
• Guide to Law Online: Nations of the World - Library of Congress Useful for finding constitutions and selected other legal sources.
• Oxford Constitutional Law - Constitutions of the World This link opens in a new window "Fully-translated English-language versions of all the world's constitutions (both national and sub-national), accompanied by individual jurisdictional commentaries, and supplementary materials, including foundation documents, historical versions of constitutions, and amendment Acts/Laws." Includes the U.S. Federal constitution, plus access to Oxford's "U.S. Constitutional Law" module (for U.S. state constitutions). Oxford Constitutional Law - Constitutions of the World link for USF on-campus access .
• World Constitutions Illustrated - HeinOnline This link opens in a new window Current and historical constitutions; books; articles; selected web sites. (For current USF students, faculty, and staff.)
• International Humanitarian Law - Treaties & Documents - ICRC "The database contains a hundred or so treaties and instruments of international humanitarian law from 1856 to the present." Organized by topic and date.
• Customary International Humanitarian Law Database - ICRC For international humanitarian law issues, this database, from the International Committee of the Red Cross, is a good first stop. It sets forth the 161 rules of international humanitarian law humanitarian law, and the practice underlying those rules.
• National Implementation - Humanitarian Law - ICRC Database "Documentation and commentaries concerning the implementation of international humanitarian law at the national level." Includes laws, regulations, and cases. Can be searched by nation or key word.
• << Previous: Treaties
• Next: Courts/Decisions >>
• Last Updated: Oct 10, 2023 9:24 AM
• URL: https://legalresearch.usfca.edu/InternationalLaw
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Custom as a Source of International Law.pdf

Related Papers

Armin Klica

The doctrine of sources has served international law well over the past century, providing structure and coherence during a time when international law was expanding rapidly and dramatically. But the doctrine's explanatory power is increasingly being challenged. Current doctrine tells us that treaties are international law; empirical evidence, however, suggests that treaties are poor predictors of state practice. The expansion of the international community, the rise of human rights, developments in international legal theory, and the international system's need to adapt to changing circumstances have also put pressure on the reified role of " treaty " in identifying rules of international law. Drawing from a number of theories developed to explain why states comply with international law, this Article proposes a new doctrine of sources focused on opinio juris and how norms come to be accepted as international law. Rather than taking for granted that a treaty reflects international law, the rules laid out in a treaty would themselves be judged by the internalized norms supporting them, either (a) in the strength and legitimacy of the process that led to the adoption of those rules or (b) in the customary acceptance of the rule itself. This Article argues that such a revised doctrine of sources will better capture which rules are actually treated as law in the international system, blunting skepticism about international law and placing international law on firmer

Akash Shrivastava

Georgetown Journal of International Law

William Worster

This paper will criticize the current characterization of customary international law analysis as falling into two competing analytical methods, specifically, the “traditional” and “modern” methods. Following Koskennieni, Roberts, Wouters, Ryngaert, and others, contemporary customary international legal analysis is said to have divided into two schools: the traditional and modern. These schools are supposed to reflect the use of the inductive and deductive methods, respectively, and, in turn, to either apologize for the freedom of state action or establish a utopia where the freedom of the state is limited. This image of struggle between competing methods, however, does not fully capture the ways in which the inductive and deductive methods are actually intertwined in customary international law analysis. The methods are not two opposing monolithic techniques. Instead, in practice, the methods are intermixed, combining a variety of choices. Deductive steps are taken regarding the use of induction, inductive steps are taken in reaching patterns from practice, deductive steps are taken in identifying sample pools, and so on. This paper concludes that induction and deduction are both used in parallel in most analyses of customary international law in a delicate, yet valuable, balance of corrective tension. Furthermore, this situation is neither traditional, nor modern – in a chronological sense – but is consistent throughout the history of customary international law analysis. Following an introductory section reviewing the background to the debate over approaches, the paper will consider deduction and induction in more detail, attempting to understand these two methods of logical reasoning. Then the author will examine the way customary international law is analyzed by international tribunals by deconstructing the analytical process into smaller logical steps. For each step, an analysis will be undertaken of whether the conclusion is reached through induction or deduction, and whether this approach is consistent across tribunals and historical eras. Through this approach, the paper will take a first step to suggest the complex ways inductive and deductive analyses are layered in the assessment of customary international law.

Jelena Obradovic

The wording of Article 38(1) of the Statute of the International Court of Justice might imply that no other customs apart from the “universal” ones could be considered as sources of international law. However, the International Court of Justice took a proactive role and recognised the existence of “special” (i.e. local or regional) customary rules and perhaps employed a more creative approach and actually “read them” into Article 38(1) of the Statute. Moreover, the Court also seems to have created the “rules on how to ascertain the rules” of special custom, when it set a higher threshold for proving its establishment through its case-law in Anglo-Norwegian Fisheries, Asylum, Nationals in Morocco and Right of Passage cases. The 2009 Navigational and Related Rights case seems out of sync with the previous cases on special custom, and it remains to be seen whether it can be considered as more than anomaly in the Court’s jurisprudence. Even though the International Court of Justice seemingly had quite a proactive role with regards to special custom, it appears that it avoided to pronounce explicitly on some issues such as how to ascertain opinio juris or relation between general and special customary rules.

Ezequiel Heffes

This review explores certain challenges related to the notion of customary international law. It seems that it was a long time ago when international law academics and practitioners ever thought that the nature of this source was a well-settled topic. Nowadays international lawmaking processes involve an extraordinary number of interactions, taking place both formally and informally. Such complex features are reflected by an exponential increase in the scholarly study of international legal sources. The legal nature, its applicability and principles regulating customary international law are addressed in the book under review (Brian D Lepard (ed), Reexamining Customary International Law (Cambridge University Press 2017)) through several topical essays. The chapters offer a comprehensive analysis of these lawmaking processes and the challenges they portray from various perspectives and in various fields, such as: What is customary international law and why is it law? Is it law because it reflects a ‘global legislative’ model? What is the current value of the persistent objector theory? Is the two-element definition of customary international law still applicable? By meticulously addressing these and other inquiries, the book presents novel arguments and represents a stimulating addition to the literature on sources of international law

This paper discusses the relationship between the sources of international law; specifically discussing the complex entanglement of the international law of treaties and custom.

Sanjay Singh

Am. J. Int'l L.

Sos Avetisyan

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