Liability In Solidum in the Law of International Responsibility: A Comment on Guiding Principle 7

The Guiding Principles on Shared Responsibility in International Law seek to address an issue hitherto unresolved in the law of international responsibility: if two or more states or organizations together cause a single harm to a victim, what are the consequences for suit and reparation? Commentators generally counsel against the use of domestic concepts such as 'solidary liability' or 'joint and several liability' in international law. This comment highlights the role of domestic analogies in the formulation of the Guiding Principles, focusing on two elements: the application of liability in solidum as the key consequence of multiple responsibility (Principle 10), and 'concerted action' (Principle 7) as a condition for multiple responsibility. Both of these concepts can be found in many domestic legal systems, but the Principles place differential weight on domestic analogies in the elaboration of Principles 10 and 7: Principle 10 draws useful analogies with the rationale behind liability in solidum in domestic law, while Principle 7 on concerted action does not rely on related domestic concepts. That is likely for good reason. However, responsibility based on concerted action is a novel basis for responsibility in international law, and therefore its justification is all the more important. The justification provided for Principle 7 is not fully convincing, and its scope of application is uncertain. I query whether the exploration of cognate concepts in domestic legal systems may have helped to justify the rationale for, or the scope of, responsibility based on concerted action.