Growth and gaps: a meta-review of policy diffusion studies in the American states

Research on how policies diffuse throughout the American federal system is robust and vast, but there has yet to be a systematic accounting for its results. Using a systematic review and metaanalysis, this study presents the most comprehensive analysis to date of how policy innovation flows state-to-state and the average effects of commonly used variables in the study of policy diffusion (for example, neighbour adoptions, ideological distance between states and legislative professionalism). Additionally, heterogeneity in these effects is examined across regulatory, morality and governance policy types. The study not only estimates these weighted-average effects, but it also illuminates important biases in policy diffusion research. The article concludes by making recommendations for addressing those biases and increasing international collaboration on policy innovation research and results synthesis.