American Political Science Review

Research Article

The Ideological Mapping of American Legislatures


a1 University of Chicago

a2 Princeton University


The development and elaboration of the spatial theory of voting has contributed greatly to the study of legislative decision making and elections. Statistical models that estimate the spatial locations of individual decision-makers have made a key contribution to this success. Spatial models have been estimated for the U.S. Congress, the Supreme Court, U.S. presidents, a large number of non-U.S. legislatures, and supranational organizations. Yet one potentially fruitful laboratory for testing spatial theories, the individual U.S. states, has remained relatively unexploited, for two reasons. First, state legislative roll call data have not yet been systematically collected for all states over time. Second, because ideal point models are based on latent scales, comparisons of ideal points across states or even between chambers within a state are difficult. This article reports substantial progress on both fronts. First, we have obtained the roll call voting data for all state legislatures from the mid-1990s onward. Second, we exploit a recurring survey of state legislative candidates to allow comparisons across time, chambers, and states as well as with the U.S. Congress. The resulting mapping of America's state legislatures has great potential to address numerous questions not only about state politics and policymaking, but also about legislative politics in general.


This article emerges from earlier work with Chris Berry (Shor, Berry, and McCarty 2010). We thank Project Vote Smart for access to NPAT survey data. The roll call data collection has been supported financially by the Russell Sage Foundation and the Woodrow Wilson School. Special thanks are due to Michelle Anderson for administering this vast data collection effort. We also thank the following for exemplary research assistance: Johanna Chan, Sarah McLaughlin Emmans, Stuart Jordan, Chad Levinson, Jon Rogowski, Aaron Strauss, Mateusz Tomkowiak, and Lindsey Wilhelm. We thank Michael Bailey, William Berry, Andrew Gelman, Will Howell, David Park, Gerald Wright, Chris Berry, the Harris School Program on Political Institutions, and seminar participants at Stanford, Chicago, Essex, and Texas A&M universities. Any remaining errors are our own.

The authors also provide a supplemental online Appendix (available at