The Nationalization of American Lawmaking? Evidence from State Statutes

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This paper analyzes the adoption and diffusion of legislation enacted by U.S. States since 1800. We use a large language model to extract and structure the policy content of the universe of statutes enacted by state legislatures (N=2.5 million). We then construct sets of related policies across states using an unsupervised clustering algorithm applied to vector representations of the statutes' policy contents. We compute measures of similarity, diffusion, and innovation across states with legislative activity on these policy clusters. After validating this measurement procedure, we analyze the determinants of diffusion. We find that states with greater geographic, economic, and political similarity implement more similar policies. While polarization has risen significantly since the early 2000s, consistent with prior literature, we show that current levels of polarization are comparable to those observed in the pre-WWII period, following a U-shaped pattern over time. Polarization primarily reflects differences in policy choices within topics rather than in the topics on which states choose to legislate. Finally, we document an increasing nationalization of policy, with federal legislative texts exerting growing influence on state legislation since the post-war period.