What Has Made Congress More Polarized?

By Sean Trende - May 11, 2012

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DW-NOMINATE represents an attempt to rectify this by looking at every vote cast in a given Congress. Rather than starting with a pre-conceived notion of what a conservative vote would look like and picking and choosing votes to evaluate based upon that notion, DW-NOMINATE attempts, in essence, to reverse-engineer the “ideology” from the votes, without looking at the content of the votes themselves (stick with me here).

So how do we do this? DW-NOMINATE proceeds under the assumption that legislators who occupy similar positions on the ideological spectrum will vote together more frequently than they will with legislators who hold dissimilar positions on the ideological spectrum. In other words, because Bernie Sanders votes with Barbara Boxer frequently, and votes with Tom Coburn infrequently, we may conclude that Sanders and Boxer occupy similar ideological spaces, and that Sanders and Coburn occupy dissimilar ideological spaces.

By plugging in all votes from a given Congress, the DW-NOMINATE program in essence measures the similarities and dissimilarities in members’ voting patterns, and then assigns these legislators “scores” that measure where they would fall in a given Congress.

Rather than giving a primer in linear algebra, a simple example from my thesis may be more productive. The following is the raw data I used for the 1938 Supreme Court term. A vote with the majority is classified as a “1,” a vote against the majority is a “6,” and a failure to vote is classified as a “9.” By placing the votes in rows by the justice casting them, and in columns for each opinion (for Congress, the columns represent roll call votes), a matrix is created. Note that there are no unanimous votes -- DW-NOMINATE actually excludes unanimous votes, for reasons I won’t go into (a necessary shortcoming that can actually skew its perceptions of polarization).

This is quite literally all that DW-NOMINATE ever “sees.” Just to re-emphasize what these data are, however, here’s a more user-friendly depiction of the first 11 votes.

Just a simple glance at the “raw” matrix indicates that Justices Black and Douglas rarely voted with Justices McReynolds and Butler (who vote together in every instance except two). Indeed, McReynolds and Black never voted together on a split decision during the entire 1938 term. Thus, they represent the two poles. In the middle are Justices Roberts, Hughes, Reed, Frankfurter, and Stone, in descending order of likelihood of voting with McReynolds and Butler. Thus, the expected order would go something like McReynolds, Butler, Roberts, Hughes, Reed, Frankfurter, Stone, Douglas, and Black.

DW-NOMINATE-based programs perform the analysis much more precisely than our eyeballing of the data (although in the 1938 term, the result was exactly what our eyeballing suggested it should be), though with 435 legislators casting over 1,000 votes, the analysis is much more involved. Based upon these voting pairs -- how frequently members vote with each other -- DW-NOMINATE will also estimate “ideal points” for each legislator, which is essentially the “ideological score” that everyone is referring to when they discuss DW-NOMINATE. These typically fall between -1 and 1.

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Sean Trende is senior elections analyst for RealClearPolitics. He is a co-author of the 2014 Almanac of American Politics and author of The Lost Majority. He can be reached at Follow him on Twitter @SeanTrende.

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