Customary International Law – A Brief Account

Customary International Law – A Brief Account

The recognized sources of International Law are envisaged under the Article 38 of the Statute of International Court of Justice. Those sources include International Conventions, International Customs, General Principles of Law, Judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law. Focus of this Article however would be on the source of International Custom, more commonly referred to as Customary International Law (CIL). The concept of CIL is probably as diverse and detailed as the remainder of the sources combined. It implies that there are certain norms, referred to as international customs, which are a binding obligation upon every State, irrespective of the fact whether that particular State recognizes the probative force of law of those norms or not.

However, alike other laws, this is also a general rule and exceptions to this rule do exist and have also been recognized by the International Court of Justice (ICJ) as well, for instance the “Persistent Objector Rule” and the “Subsequent Objector Rule” recognized by the ICJ in the “Asylum case (Colombia v. Peru), 1950” and elsewhere. To understand this concept, we must initially study its definition. Simply put by Rosenne, in his book Practice and Methods of International Law, “Customary International Law consists of rules of law derived from the consistent conduct of States, acting out of the belief that the law required them to act that way”. This definition is considerably eloquent as it points towards the two requisite elements of customary international law, namely “State Practice” and “Opinio Juris”. These elements of CIL have also been affirmed by the International Court of Justice in the Advisory Opinion on “Legality of the Threat or Use of Nuclear Weapons, 1996”and various other adjudications , which will be further discussed in this Article.

State practice is the first requisite element of CIL. For a norm to reach the status of CIL, firstly, it must be exercised or applied by States in their conduct. For instance, the protection of fundamental right to life has a customary status. This however could never have been CIL unless States didn’t practice it through domestic legislation, domestic legal precedents etcetera. However, it must be kept in mind that an evidence of State Practice is not confined to domestic legislation or legal precedents, as other conducts such as ratification, accession, State Paper and various other methods and conducts also evidence the satisfaction of the first element of CIL, which is State Practice. The best available source for observing the Practice of States are the Year Books, which are available in global, continental, regional and country-wise variants.

As far as the general rule for State Practice goes, the Practice of States can be further dissected into three requisite elements. Firstly, that it should be “long term”, meaning thereby, for a norm to be considered State Practice, its usage by States must be from a long period of time, however, it has certain exceptions as observed by the ICJ. Secondly, that it should be “widespread”, meaning thereby a significant number of States must show their compliance to that norm for it to be recognized as a State Practice, however, this too has certain exceptions as observed by the ICJ. Thirdly, there must be “uniformity” in the practice as a norm cannot be labeled custom if some States follow it and others do not, or follow something inverse, or if there is an inconsistency in its practice. The first two elements automatically give rise to the most common questions, which, the concept of State Practice is often confronted with.

The first question is with respect to the characteristic of “widespread” nature of State Practice, which is; “How many States (in numbers) must show compliance with a norm for it to be considered as State Practice evidencing CIL?” The reply to this question is twofold. Firstly, the general rule for international norms to be considered customary is that the Practice of States with respect to that norm ought to be considerably widespread, encompassing many States. Meaning thereby, a considerably large number of States must follow and exercise that norm, for it to be considered as a widespread State Practice. Secondly, however, the ICJ in its “North Sea Continental Shelf Cases (F.R.G. v. Denmark; F.R.G v. Netherlands), 1969” judgment observed that a norm can also be customary if only the States most affected by that norm show their compliance with it. Meaning thereby, in case of those norms which specifically affect only a certain amount of States, the practice of those States, irrespective of not being widespread, can evidence Customary International Law. Moreover, scholars also point to an approach that an uncontested and unopposed State Practice of a smaller number of States also evidences a recognized CIL. This is the reason whereby certain regional treaties relating to religious nature or biological diversity or certain practices have a status of CIL, irrespective of being practiced merely by a handful of States.

The second question is with respect to the characteristic of “long-term” nature of State Practice, which is; “For how long should a norm be complied with for it to be considered as State Practice evidencing CIL?” The reply to this question is also twofold. Firstly, the general rule is that the norm in question must be acted upon or applied for a considerably long term, or in some cases, for time immemorial but no time period can be specified as it varies contextually. Secondly however, the ICJ in its “North Sea Continental Shelf Cases (F.R.G. v. Denmark; F.R.G v. Netherlands), 1969” judgment also observed that “even without the passage of any considerable period of time, a very widespread and representative participation in the convention might suffice of itself, provided it included that of States whose interests were specially affected”, which yet again serves as an exception to the general rule of the longevity of term of State Practice.

Providing a basic account for State Practice, we will now proceed to the second requisite element for formation of CIL, namely “Opinio Juris”. I apologize for bursting the bubble of many amongst you but NO! OPINIO JURIS DOES NOT MEAN “OPINION OF A JURIST” OR “JUDICIAL OPINION ON FORMATION OF CIL” and has nothing to do with jurists even by the farthest stretch of imagination. This fairly common misconception emanates from an individual’s attempt to translate the Latin term into English so let’s try not to and let’s understand its etymology and meaning.

The Latin term “Opinio juris sive necessitates” means “an opinion of law or necessity” which has been shortened to “Opinio Juris” meaning simply “an opinion of law”. The simplest way to understand this concept is that Opinio Juris is the belief that an action was carried out under a sense of a legal obligation. An act which even reaches the threshold for being considered a legitimate State Practice does not formulate CIL unless it is coupled with Opinio Juris. This implies that until and unless the practice is performed with a belief of the practicing State(s) that its performance is a legal obligation incumbent upon it/them, it cannot be considered CIL, irrespective of how many States perform it.

Seems confusing? Let’s understand it further with an illustration. It is a widespread State Practice which is performed by almost every country across the globe that when the President or the Prime Minister or any Head of a State visits another State, he/she is greeted warmly by his/her counterpart or any high official of the receiving State with bouquets and/or rolled out carpets upon his/her arrival. So let us imply, for the sake of illustration, that the President of Pakistan arrives at the Narita International Airport, Tokyo for a summit. Upon his arrival, he witnesses that not even a single person, let alone his counterpart, is present to greet him and there are no carpets or bouquets to welcome him into Japan. Now bearing in mind the fact that greeting Heads of States, by the receiving State, is a wide spread State Practice performed by almost every Country across the globe, can Pakistan take the matter before the ICJ claiming that the customary obligation to “greet” has been breached by Japan by disregarding this widespread State Practice making it liable to compensatory or any other relief? The reply, obviously, is No! This is simply because of the fact that not even a single country across the globe indulges in this practice of greeting the Heads of other States considering it to be their legal obligation to do so. This absence of a sense of legal obligation, or in other words, the absence of Opinio Juris inhibits the State Practice of greeting Heads of States to be considered as Customary International Law.

 However, presence of “Opinio Juris” is a condition precedent which is a general rule, followed by certain exceptions. The ICJ in its “Military and Paramilitary Activities in and Against Nicargua (Nicaragua. vs U.S.)1986” judgment recognized General Assembly resolutions to evidence CIL, subject to certain condition precedents. Other non-traditional sources of CIL include, but not limited to, international treaties, findings of the ICJ, basic human rights having universality (Barcelona Traction, Light and Power Company, Limited (Belgium v. Spain), 1962), Findings of Regional courts and tribunals, etcetera.

Lastly, the common question which the element of Opinio Juris is confronted with is; “How Opinio Juris (a sense of a legal obligation of a State) is to be ascertained?” The reply to this question emanates from the observation of the ICJ in the Barcelona Traction, Light and Power Company, Limited (Belgium v. Spain), 1962 judgment whereby it observed that the sense of a legal obligation, behind any practice of a State, is construed from the State’s conduct and approach. Meaning thereby, what a State performs, on domestic as well as international forums, evidences the satisfaction of the element of Opinio Juris. For that matter, the principles of international law ratified or acceded to by a State, the domestic legislation of a State, the legal precedents within a State, foreign policy of a State and State Paper are few of the key instruments for understanding the State’s Opinio Juris behind its Practice.

To conclude, one can say that Customary International Law, accompanied by parallel concepts of Jus Cogens and Obligation Erga Omnes/ Erga Omnes Partes, is essentially the core of various international disputes which are referred to the International Court of Justice and other International tribunals/forums. This ever-developing concept encompasses various factors, limitations and aspects which undeniably makes it the most complex one, amongst the other recognized source of International Law, envisaged in the Statute of the International Court of Justice.

Momin Lodhi


Momin Lodhi

Author Since:  March 23, 2020