Resumen de Brian D. Lepard. Customary International Law. A New Theory with Practical Applications

Niels Petersen

• Brian D. Lepard. Customary International Law. A New Theory with Practical Applications. Cambridge: Cambridge University Press, 2010. Pp. xx + 419. $55. ISBN: 9780521191364.

  The theory of customary international law is one of the big mysteries of international law scholarship. Every student of international law knows what customary law is. And yet, nobody knows what it actually is. Article 38(1)(b) of the Statute of the International Court of Justice defines custom as consistent state practice coupled with an opinio iuris. Although legal scholarship has filled whole libraries trying to come up with a set of rational criteria to identify these two elements, there remain many open questions. In his new book on Customary International Law, Brian Lepard intends to advance legal scholarship in the search for answers. He promises to �develop a new theory of customary international law that � helps to solve its theoretical and practical puzzles� (at 11�12).

  The core of Lepard's theory is the reduction of the two constitutive elements of customary law to one � opinio iuris: a �customary international law norm arises when states generally believe that it is desirable now or in the near future to have an authoritative legal principle or rule prescribing, permitting, or prohibiting certain conduct� (at 8, 97�98). State practice is not perceived as a mandatory requirement, but merely as evidence of this belief (at 98). With this focus on opinio iuris, Lepard follows a popular trend among international law scholars. There have been several recent attempts to diminish the importance of state practice for the identification of unwritten international norms.1 The rationale behind this tendency is the strengthening of norms with moral impact, such as international human rights law, as we often observe a divergence between official declarations of states and the actual practice in this area.

  For the identification of an opinio iuris, Lepard makes two presumptions. The first presumption is inspired by game theory: whether �