What Has Made Congress More Polarized?

What Has Made Congress More Polarized?

By Sean Trende - May 11, 2012

Thomas Mann and Norm Ornstein ignited a bit of a firestorm with their column describing Republicans as main drivers behind Washington's problems. The Washington Post's Chris Cillizza has made some excellent points with regard to Mann and Ornstein's qualitative arguments, and other thorough responses abound. Rather than revisiting these points, I would like to focus my attention on the quantitative arguments made at the end of the article. Mann and Ornstein note that:

“[P]olitical scientists Keith Poole and Howard Rosenthal, who have long tracked historical trends in political polarization, said their studies of congressional votes found that Republicans are now more conservative than they have been in more than a century. Their data show a dramatic uptick in polarization, mostly caused by the sharp rightward move of the GOP.”

Mann and Ornstein are referring to data generated by Poole and Rosenthal’s famous DW-NOMINATE program -- one of the great achievements of modern political science. Poole and Rosenthal’s results regarding polarization have received a fair amount of media attention lately, popping up on “The Rachel Maddow Show,” NPR, and in the work of the American Prospect’s outstanding analyst Jamelle Bouie. The claim that Republicans are mostly to blame for the increase in polarization is usually accompanied by the claim -- also based upon DW-NOMINATE data -- that Republicans are now more conservative than they have been at any point since the 1880s.

This is a misapplication of DW-NOMINATE. As we’ll see below, DW-NOMINATE scores don’t really tell you how conservative or liberal a member of Congress is, at least not in the sense that most pundits use the term. It tells you how conservative or liberal a member of Congress is relative to other members of Congress. The latter is useful information, to be sure, but it really is an important distinction to be aware of.

Before explaining why I think this is a misapplication of DW-NOMINATE -- at least as pundits are using the data -- I should give a basic account of where I’m coming from in the big picture. On the one hand, I’m happy to see more rigorous analytical techniques make their way into political journalism. Having spent 18 months of my life applying a first cousin of DW-NOMINATE (known as OC, or Optimal Classification) to measure Supreme Court voting patterns for my master’s thesis (portions of which are paraphrased for this piece), my initial reaction was that the increased use of it was a good thing.

On the other hand, I’m concerned about relatively sophisticated statistical techniques like regression analysis and DW-NOMINATE being increasingly used by a journalist class that doesn’t really understand the assumptions or limitations behind them. With some of these techniques, journalists and their audience simply aren’t equipped to tell whether a technique is being misused or not. The result can be embarrassing misapplications, which the audience is then ill-equipped to analyze critically.

Remember, while there are a few political scientists who behave like partisans once they enter the public arena, the vast majority don’t. What occurs instead is what we’ve seen with regression modeling. These techniques all have certain assumptions and limitations built into them -- with which political scientists are familiar. When they speak to each other, there’s no need to list all of the caveats. The problem is that this manner of speaking is habit forming, and when addressing the general public, these assumptions and limitations really do need to be explained. All too often, however, they’re glossed over.

So it’s important to set forth at least a rudimentary explanation of what DW-NOMINATE is and how it works before analyzing this particular application. DW-NOMINATE grew out of a disenchantment with interest group ratings (think Americans for Democratic Action or American Conservative Union scores) as a measure of ideology. Interest groups typically cherry-pick 20 or so “key” roll call votes out of hundreds cast in Congress, and rate legislator ideology on this basis. Political scientists had noted that these scores often failed to grasp complexities in a legislator’s voting patterns, and could artificially place a lawmaker on the extremes of Congress.

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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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