Legitimate expectation, acquiescence and customary international law

It is a widely held view that States are only bound by rules of international law to which they have consented. For this reason consent might itself be considered a fundamental principle of international law. But though States consent explicitly to treaty rules through the act of signature or ratification, they usually do not consent explicitly to rules of customary international law. Instead, they are held to have consented to those customary rules to which they have acquiesced.

The word consent is not a particularly accurate description of the role of acquiescence in the customary process. Acquiescence often signifies ambivalence or even apathy to the rule in question rather than a conscious support for the rule on the part of the acquiescing State. Furthermore, the development of new rights or obligations based on acquiescence necessarily involves other States in addition to the acquiescing State. Rights and obligations in international law are never entirely the creation of a single State's will because they exist between and among States. Instead, it seems that all States consider that one State's acquiescence to a customary rule may give rise to rights or obligations having the potential to affect all States in some way, either as subjects of corresponding obligations, or as holders of corresponding rights.