Whereas the rational choice approach to international law has been widely accepted in legal scholarship and international relations theory, challenges to the rational choice paradigm in economic analysis of international law have hitherto not been systematically explored. Nevertheless, behavioral law and economics and psychology have been successfully applied to national law constellations. Behavioral economic insights have furthermore been used in international relations scholarship under the heading of political psychology but international norms are neglected. Building on all those insights, the article explores the potential and challenges of extending the behavioral law and economics approach to public international law and thus to further refine our understanding of international law. It looks specifically at treaty design problems and compliance questions. This ties in with increased use of empirical research in international law: a clear desideratum for evidence-based international law.