Reciprocity and Customary International Law

The contribution proposes to disconnect the reciprocity of the law of treaties, to bring it closer to the other main source of international law: customary international law. Such an approach may seem counter-intuitive, so much so that it seems to be understood: reciprocity would be mainly, if not exclusively, a matter for the world of international treaties. As for international custom, it would have a much more legislative character insofar as it is binding on all states. However, to take a closer look, reciprocity is by no means foreign to custom. It is just as likely to influence its formation—the ‘customary process’—as its application. From the point of view first of all of the formation of custom, one can certainly detect traces of reciprocity in the restraint, or more generally the motives, that govern states when they begin to reproduce the same behaviour. It would all be a matter of ‘calculation’, of anticipation, of legitimate expectations of treatment identical to that reserved for others. The same logic is equally valid at the stage of applying customary rules. The case of reprisals or ‘countermeasures’ is a topical example. The discipline introduced by reciprocity in inter-state relations remains, however, fundamentally precarious, since it is based solely on the mutual conditionality of rights and duties. The need to go through this type of incentive logic reveals above all, in the background, the ‘primitivity’ of international law, understood in the sense of its weak institutionalisation. The research also returns to the comparison of the different reciprocities at work in treaty law and outside it, to the autonomy of the principle of reciprocity and the differences between it and the principle of equality, in particular, and finally to the relationship it has with the principle of good faith. This will allow to respond to the fundamental question present in this volume whether reciprocity is a legal principle and may be regarded as a metalegal principle.