Erga Omnes Norms and the Enforcement of International Law

Abstract

Under traditional two-party norms of international law, an aggression by state X against state Y only gives state Y the right to retaliate. If, however, a matter of international law is governed by norms, all states are entitled to engage in countermeasures. This raises the question why some international legal norms have the status of erga omnes while others do not. Posner [2008] argues that erga omnes rules are likely to emerge in areas where, on average, externalities due to the original aggression are high, motives for taking norm violation as a pretext for engaging in predatory behaviour are weak, and incentives for free-riding are strong. Moreover, he predicts that erga omnes rules will reduce free-riding and hence increase the probability of enforcement. We shall argue herein that erga omnes rules can actually have the opposite effect of increasing free-riding and reducing the probability of enforcement. This might explain why genocide in Dafur is ongoing despite (or even because) being governed by an erga omnes norm.