Shifting focus: interactions between the norms of the Chemical and Biological Weapons Conventions and UN Security Council Resolution 1540

The Biological and Toxin Weapons Convention (BWC) and Chemical Weapons Convention (CWC) create obligations for states parties to pass national legislation penalizing several offenses by all actors. UN Security Council Resolution 1540 encompasses a similar scope of offenses but mandates penal legislation pertaining to acts of nonstate actors only. These regimes establish similar but not identical norms on legislative implementation, which may result in interactions. This article first identifies implementation obligations across all regimes, situates potential interactions within established norm-interaction frameworks, and highlights the potential, even among synergistic norms, to lead to resource competition. The article then offers an analysis of domestic penal legislation, demonstrating that a significant number of states have passed legislation covering only the narrow scope of Resolution 1540 while neglecting implementation obligations in relation to the BWC and the CWC. This divergence is explained by the factors of resource allocation, least-contentious-measure preference, and institutional support. The article provides an empirical overview of implementation practices and highlights potential negative effects of norm interactions among synergistic norms. It thereby contributes to research on implementation practices of multilateral disarmament regimes and provides further empirical evidence of concepts of norm interactions, particularly of norm competition and hierarchization.