The role of causal mechanisms in policy diffusion

This study develops an interpretive approach to understanding complex causal mechanisms in policy diffusion, using it to examine how ideas and practices surrounding assisted voluntary return policies for migrants without the right to remain have spread and become institutionalised within policy systems. The article explains how diffusion occurs and why policies take root in particular political and institutional contexts, arguing that diffusion operates through both vertical and horizontal mechanisms. Vertical diffusion reflects the influence of actors such as the European Union in promoting policy harmonisation across Member States, while horizontal diffusion captures the spread of ideas and practices among organisations and actors within states, including implementation agencies and professional networks. Instead of treating diffusion mechanisms as fixed causal devices, they are conceptualised them as interpretive tools that illuminate how actors perceive problems, frame solutions and interact strategically within institutional settings. Mechanisms such as problem definition, institutionalisation, competition, incentives, framing, knowledge sharing, and processes of co-option and boundary setting are treated as sensitising concepts that guide analysis rather than rigid explanatory models. An interpretive approach to complex mechanisms enables a nuanced understanding of diffusion that recognises the role of meaning making and strategic action. This perspective reveals how diffusion processes may involve contradictory dynamics and adaptations, shaped by actors’ interpretations and institutional constraints, thereby offering a richer account of how policies spread and become embedded in federalised contexts such as Germany.