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Polarization and Ideology: Partisan Sources of Low Dimensionality in Scaled Roll Call Analyses

  • John H. Aldrich (a1), Jacob M. Montgomery (a2) and David B. Sparks (a3)


In this article, we challenge the conclusion that the preferences of members of Congress are best represented as existing in a low-dimensional space. We conduct Monte Carlo simulations altering assumptions regarding the dimensionality and distribution of member preferences and scale the resulting roll call matrices. Our simulations show that party polarization generates misleading evidence in favor of low dimensionality. This suggests that the increasing levels of party polarization in recent Congresses may have produced false evidence in favor of a low-dimensional policy space. However, we show that focusing more narrowly on each party caucus in isolation can help researchers discern the true dimensionality of the policy space in the context of significant party polarization. We re-examine the historical roll call record and find evidence suggesting that the low dimensionality of the contemporary Congress may reflect party polarization rather than changes in the dimensionality of policy conflict.


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Authors' note: A previous version of this article was presented at the 2009 Annual Meeting of the Southern Political Science Association in Atlanta, GA, and the 2010 Annual Meeting of the American Political Science Association in Washington, DC. We are grateful for comments from Jeff Gill, Frances Lee, Gary Miller, Brendan Nyhan, John Patty, Jon Rogowski, and helpful audiences at Duke University and Washington University in St. Louis. Finally, we thank Keith Poole and Howard Rosenthal for making their roll call data publicly available. Supplementary materials for this article are available on the Political Analysis Web site.



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