Who Are Your Neighbors? The Role of Ideology and Decline of Geographic Proximity in the Diffusion of Policy Innovations

States adopt policy innovations within the confines of a dynamic American federal system, but our study of policy diffusion tends to be fairly static. Single‐policy studies incorporate temporal variation, but for only one innovation. Macro‐level analyses examine broad patterns, but often by completely pooling across policy and time. This makes it difficult to identify how diffusion patterns change over time, though Walker's early work explicitly identified such temporal instability. This study specifically examines how neighbor and ideological cues change in importance over time using a dataset of 556 policies adopted from 1960 to 2014. While the findings demonstrate the generality of many key internal, external, and policy‐level determinants of adoption, there is variation in these effects across time. Most important is the relative stability of ideological similarity between adopters and declining influence of contiguous neighbors. Further, political polarization plays a role in conditioning neighbor and ideological cues.