[citation needed] In international law, opinio juris is the subjective element used to judge whether the practice of a state is due to a belief that it is legally obligated to do a particular act.
Opinio juris essentially means that states must act in compliance with the norm not merely out of convenience, habit, coincidence, or political expediency, but rather out of a sense of legal obligation.
(the opinio juris or subjective component)[4] Thus, for example, while it may be observed that heads of state virtually always shake hands when they first meet, it is highly unlikely that they do so because they believe that a rule of international law requires it.
In the Paquete Habana case (decided by the United States Supreme Court in 1900 on the question of whether small coastal fishing boats are immune from capture during wartime under customary international law), evidence of opinio juris included medieval English royal ordinances, agreements between European nations, orders issued to the U.S. Navy in earlier conflicts, and the opinions of legal treatise writers.
At all events, to postulate that all States, even those which initiate a given practice, believe themselves to be acting under a legal obligation is to resort to a fiction, and in fact to deny the possibility of developing such rules.
For this reason, the necessity of demonstrating that a behavior was prompted by a sense of legal obligation makes it particularly difficult for customary international law to develop around the prohibition of a practice.
[8] The ICJ also declined to find evidence of customary international law in an opinion on the legality of the use or threat of nuclear weapons, despite what some argued to be uniform state practice.
Under one model, this process occurs in three stages: first, some States engage in a given practice for reasons other than a sense of legal obligation (e.g. political expediency, economic gain, courtesy, etc.