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. 各州在充满活力的美国联邦制度下进行着政策创新，但我们对政策扩散的研究往往是静态的。尽管各政策研究包含了时间维度的变化，但仅适用于某单一项的政策创新。宏观层面的分析考察了政策扩散的大致模式，但这通常需要进行跨越政策和时间的大规模整合。这使得我们难以确定扩散模式是如何随时间而发生变化的；对此，沃克的早期工作明确说明了这种时间不稳定性。本研究使用的数据集涵盖了1960年至2014年间被采用的556项政策，专门研究了邻州和意识形态因素的重要性随时间而发生的变化。虽然我们的研究结果证明，许多重要的内部、外部以及政策层面的决定因素具有普遍性，但是我们发现这些因素会因时间不同而存在差异。最重要的是，政策采用者之间意识形态的相似性是相对稳定的，而相邻州的影响则随时间而减小。此外，政治两极分化在调节邻州和意识形态因素的影响上也发挥着作用。