Reciprocity
The expectation that states will extend to others what they claim for themselves — the principle that holds international cooperation together, and occasionally apart.
Reciprocity is the principle that states conduct their international relations on the basis of mutual expectation: what a state extends to others, it may expect in return, and what it claims for itself, it must be prepared to grant. It is not a rule with a single codified source but a structural norm embedded throughout international law — in treaty design, diplomatic practice, and the logic of customary international legal obligation.
In the context of mutual legal assistance (MLA), reciprocity operates as both an incentive and a constraint. States are more likely to cooperate with MLA requests from jurisdictions that have cooperated with their own. Conversely, a state that routinely delays, refuses, or conditions its responses may find that its own requests receive equivalent treatment. The principle creates informal accountability where formal enforcement mechanisms are absent.
Reciprocity has limits. It cannot justify a state violating a binding obligation on the grounds that another state has done the same. But as a driver of cooperation in a system with no enforcement authority, it does significant work — and its absence is conspicuous.
See also: Mutual legal assistance. Sovereignty.
Sources: ICJ, Case Concerning Military and Paramilitary Activities in and against Nicaragua (Nicaragua v United States) [1986] ICJ Rep 14; Declaration on Principles of International Law concerning Friendly Relations among States, UN GA Res 2625 (XXV) (24 October 1970).