| Summary: | Social identity voting (SIV) is a new model of voting behavior based on the principles of social identity theory. We introduce and use this model to analyze roll call votes for the 35 th through 112 th US Congresses. Comparing out-of-sample accuracy of SIV and Poole and Rosenthal’s Weighted NOMINAL Three-step Estimation (W-NOMINATE), we find that SIV performs better than the one- or two-dimensional W-NOMINATE model and that generally, W-NOMINATE needs up to 10 dimensions to produce accuracy comparable to that of SIV. The differences between SIV and W-NOMINATE are further clarified in three case studies: first, a longitudinal examination of all Congresses; second, an analysis of the 112 th House of Representatives; and third, a study of the Tea Party caucus in the 112 th House of Representatives. Each study sheds new light on the potential motivations driving voting behavior, supporting our assertion that the SIV and W-NOMINATE models provide two distinct approaches to understanding voting records. SIV, with its emphasis on political identity derived from group membership, expresses combinations of individual and group preferences which contribute to legislators’ ideological classifications.
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