The Spread of Policy Diffusion Studies: A Systematic Review and Meta-Analysis, 1990-2018

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 meta-analysis, 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 (e.g., neighbor adoptions, ideological distance between states, legislative professionalism, and more). Additionally, heterogeneity in these effects is examined across regulatory, morality, and governance policy types. The study not only reveals what we know about policy diffusion after 30 years of quantitative analysis, it identifies several correctable gaps in the literature. The results not only support the efforts to expand diffusion research to large-n analyses, but they also reveal additional benefits of small-n event history studies for the progressive advancement of this important research program.