Opinio Juris

What is opinio juris?

1. Article 38 (1) (b) of the Statute of the International Court of Justice explains customary international law as comprising of “(1) a general practice (2) accepted as law”. The general practice or state practice was discussed in an earlier post. The ICJ, in its jurisprudence, has relied on, and interpreted, Article 38 (1) (b) to include two elements that assist the Court to determines the existence of an alleged customary international law – state practice and opinio juris (also known as opinio juris sive necessitates). The ICJ explained opinio juris, in the Nicaragua case, as follows:

“[…] for a new customary rule to be formed, not only must the acts concerned ‘amount to a settled practice’, but they must be accompanied by opinio juris sive neccessitatis.  Either the States taking such action or other States in a position to react to it, must have behaved so that their conduct is evidence of a belief that the practice is rendered obligatory by the existence of a rule of law requiring it. The need for such belief..the subjective element, is implicit in the very notion of opinio juris sive neccessitatis. ” 

2. In the North Sea Continental Shelf Cases, the Court examined 15 cases where States had delimited their boundaries using the equidistance method, after the Convention came into force. The court concluded, even if there were some State practice in favour of the equidistance principle the court could not deduct the necessary opinio juris. The North Sea Continental Shelf Cases confirmed that both State practice (the objective element) and opinio juris (the subjective element) are essential pre-requisites for the formation of a customary law rule.

3. State practice is often seen as a reflection of opinio juris. In the Asylum case, the court held that the relevant practice must be consistent and uniform to show an expression of a right belonging to one state and a duty incumbent on another. The court held that Columbia did not prove the existence of a regional custom because it failed to prove consistent and uniform usage of the alleged custom by relevant States. The fluctuations and contradictions in State practice did not allow for the uniform usage ( see also Nicaragua case, p. 98). The court held in the Asylum case:

“The Party which relies on a custom of this kind must prove that this custom is established in such a manner that it has become binding on the other Party… (that) it is in accordance with a (1) constant and uniform usage (2) practiced by the States in question, and that this usage is (3) the expression of a right appertaining to the State granting asylum (Columbia) and (4) a duty incumbent on the territorial State (in this case, Peru). This follows from Article 38 of the Statute of the Court, which refers to international custom “as evidence of a general practice accepted as law(text in brackets added).”

4. The fact that a state undertakes a particular because of political expediency and not because of a belief that the said practice is binding on the State by way of a legal obligation (opinio juris) is detrimental to the formation of a customary law. In the Asylum case the Court said:  “considerations of convenience or political expediency seemed to have prompted the territorial State to recognise asylum without such a decision being dictated by any feeling of legal obligation”.(see also North Sea Continental Shelf Cases and Lotus Case).

5. Opinio juris is reflected in acts of states (Nicaragua Case) or in omissions (Lotus case) in so far as those acts or omissions are done following a belief that the said State is obligated by law to act or refrain from acting in a particular way. In the Lotus case, France alleged that jurisdictional questions on collision cases are rarely heard in criminal cases because States tend to prosecute only before the flag State. France argued that this absence of prosecutions points to a positive rule in customary law on collisions. The Court held that this, 

“…would merely show that States had often, in practice, abstained from instituting criminal proceedings, and not that they recognized themselves as being obliged to do so; for only if such abstention were based on their being conscious of having a duty to abstain would it be possible to speak of an international custom. The alleged fact does not allow one to infer that States have been conscious of having such a duty; on the other hand, as will presently be seen, there are other circumstances calculated to show that the contrary is true.”

6.The Court in the Nuclear Weapons case (1995) held that General Assembly resolutions, even if they are non binding, can be important (1) to establish the existence of a rule of customary law or  (2) to establish the emergence of an opinio juris.

“To establish whether this is true of a given General Assembly resolution, it is necessary to look at its content and the conditions of adoption; it is also necessary to see whether an opinio juris exists as to its normative character. Or a series of resolutions may show the gradual evolution of the opinio juris required for the establishment of a new rule…

….several of the resolutions under consideration in the present case (which discusses the prohibition of the use of nuclear weapons) have been adopted with substantial numbers of negative votes and abstentions; thus, although those resolutions are a clear sign of deep concern regarding the problem of nuclear weapons, they still fall short of establishing the existence of an opinio juris on the illegality of the use of such weapons.”  

7. In the North Sea Continental Shelf Cases, the ICJ explained the difference between customs (i.e. habits) and customary law:

“Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e, the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation. The frequency, or even habitual character of the acts is not in itself enough. There 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.”

8. If a state acts in a particular way because it is using its discretion, then, too, the relevant opinio juris is lacking. In the Rights of Passage case, the ICJ held:

“It would thus appear that, during the British and post-British periods, Portuguese armed forces and armed police did not pass between Daman and the enclaves as of right and that, after 1878, such passage could only take place with previous authorization by the British and later by India, accorded either under a reciprocal arrangement already agreed to, or in individual cases. Having regard to the special circumstances of the case, this necessity for authorization before passage could take place constitutes, in the view of the Court, a negation of passage as of right. The practice predicates that the territorial sovereign had the discretionary power to withdraw or to refuse permission. It is argued that permission was always granted, but this does not, in the opinion of the Court, affect the legal position. There is nothing in the record to show that grant of permission was incumbent on the British or on India as an obligation.” 


Recommended Reading

  • ILA Report on the Formation of CIL: Comments 14-18 (Click here for the Report) 
  • Asylum Case, ICJ, 1949 – pages 14 and 15 (Click here for the judgement and here for a summary).This case deals with “local customs”.
  • North Sea Continental Shelf Cases, ICJ – paras 76- 77 (Click here for the Judgement and here for a summary)
  • Nicaragua vs USA, ICJ, 1986, p.108 -109, 188 – 190 (Click here for the judgement)
  • Continental Shelf (Libya/Malta) (EEZ had become part of customary international law), ICJ (1985), para 34
  • Lotus CasePCIJ (1927), paras 76 and 79 (Click here for the judgement and here for a summary)
  • Rights of Passage Case, ICJ, (1960), p. 42 and 43. This case deals with “local customs”. (Click here for the Judgement)

Additional Reading

  • Shaw, International Law, “Sources”
  • Dixon, Textbook on International Law, “The Sources of International Law”
  • Brownlie, Public International Law, “Sources of the Law”
  • M.Akehurst, “Custom as a Source of International Law”, (1974-75) BYIL 53
  • Oberg, “Legal Effects of the Resolutions in the SC and the GA in the Jurisprudence of the ICJ”, EJIL, (click here for the article)
  • Kammerhofe, “Uncertainty in the Formal Sources of International Law: CIL and Some of its problems” EJIL, (click here for the article)
  • Baker, “Customary International Law in the 21st Century: Old Challenges and New Debates” EJIL, 2010 (click here for the article)

© Ruwanthika Gunaratne and Public International Law at https://ruwanthikagunaratne.wordpress.com, 2008 – present. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Ruwanthika Gunaratne and Public International Law with appropriate and specific direction to the original content.



  1. this is and other lessons have been very useful. the content is substantive- and simplified for clarity and precision .thank you

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