Agreement Scores, Ideal Points, and Legislative Polarization

A number of scholars have described a steady increase in ideological polarization between Democrats and Republicans in the U.S. Congress. This research often relies upon ideal point estimation methods that assume legislators are atomistic individuals registering a sincere, independent preference on each roll call vote. Yet scholars who employ social network techniques, such as estimating each legislator’s centrality score, typically assume that legislators’ voting decisions are a consequence of social influences. This paper examines the distinctions between these two approaches. We introduce the agreement score, which measures the extent to which pairs of legislators vote the same way. We demonstrate the relationship between both agreement scores and centrality scores, a common measurement of influence in social network analysis, as well as between centrality scores and ideal point estimates. We then focus the ways in which agreement scores can be used to illuminate theories of legislative polarization for members of the U.S. House from 1967 to 2008. Agreement scores are not only more conducive to the assumptions typically made in social network analysis but also the empirical patterns of these scores are consistent with major explanations of legislative partisanship.