Selective Inclusion of International Organizations’ Rules in European Union Law and Policy: Which IOs Does the EU Draw On?

International organizations' (IOs) rules – frameworks, standards, guidelines, and definitions – are included in the European Union's (EU) laws and policies, yet the EU draws more heavily on some IOs than others. What explains this variation in which IOs become rule-providers for the EU? Existing research shows that the EU cooperates with, participates in, and diffuses its norms to other IOs. However, we know little about rule incorporation – the copying, modifying, upgrading of external IO rules within the EU legal and policy instruments. Focusing on the EU as a rule-taker and other IOs as rule-providers, this article examines whether EU rule incorporation is shaped by (1) EU–IO connectivity – dyadic cooperation, the EU’s status within an IO, and membership overlap, and (2) IO institutional design – delegated authority from member states and whether an IO is general-purpose or technical. The analysis draws on an original dataset of IO-rule incorporations in EU legal acts from 1993 to 2023. I test whether connectivity and institutional design condition the EU’s propensity to incorporate external rules. This study contributes to research on inter-organizational relations and diffusion across IOs, as well as scholarship on the EU’s external governance.