Polarization is Real (and Asymmetric)

This post is co-authored with Keith Poole, Howard Rosenthal, and Chris Hare.  It is cross-posted at voteview blog.

The recent outburst of scholarly and popular interest in political polarization has attracted attention to the methods we use to measure this phenomenon. One frequently voiced concern (see a recent column by Sean Trende) is that Congress may not have polarized as we have claimed in publications and blogs stretching as far back as 1984. The concern is that the meaning of ideological (NOMINATE) scores are tied to the legislative and historical context of the roll call votes that are used to estimate them. For example, the content of roll calls votes cast by members of 90th Senate that dealt with the Vietnam War, civil rights, and funding for LBJ’s “Great Society” programs are quite different than those votes cast in the current Senate. Thus, being the most conservative Senator (with a score of 1.0) in 1968 would mean something different than having an identical 1.0 score in 2012.

Indeed, temporal comparisons should not be made for ideal points generated from static scaling methods. Static methods (like W-NOMINATE) treat each legislative session separately and there is no valid way to compare the scores of legislators from different years. However, we developed a dynamic methodology, DW-NOMINATE (McCarty, Poole and Rosenthal 1997), to allow for over-time comparisons of legislator ideological positions. The key innovation is the use of “bridge” legislators—members of Congress (MCs) who have served in multiple sessions—to compare the positions of legislators who have never served together.

A sports analogy to the overlapping cohorts method is the “common opponents” statistic. If we want to compare two teams who have not played each other, we compare their performances against a common opponent(s). Likewise, MCs who have not served together can be compared with the use of a “bridge” legislator who has served with both. For example, if we know that Sen. George McGovern (D-SD) is more liberal than Sen. Patrick Leahy (D-VT), and that Sen. Leahy is more liberal than Sen. Max Baucus (D-MT), then we can say that Sen. McGovern is more liberal than Sen. Baucus. Though intransitivities may arise cases involving 3 or more sports teams, Poole shows in his 2007 Public Choice article “Changing Minds? Not in Congress!” that MCs remain remarkably static in their ideological positions over the course of their careers. Thus, we are on much firmer ground in making over-time comparisons between MCs with the caveat that we cannot compare members outside of one of the stable, two-party periods of American history. For that reason, when we discuss current polarization we focus on the period from the end of Reconstruction in 1877 to the current period.

With the use of overlapping cohorts, we can make the over-time comparisons needed to analyze polarization. A good example is Senator Richard Lugar (R-IN), who, after his primary defeat last week, will have served in the Senate between 1977 and 2013. As David Karol points out, Lugar himself did not change very much over time: he was a reliable conservative who moved only somewhat towards the center during a 30-plus year career (from a DW-NOMINATE first dimension score of 0.348 to 0.241). DW-NOMINATE scores range (with slight simplification) from  minus 1 to 1 or a band of two units. So in 30 years, Senator Lugar moved just five percent on the liberal-conservative dimension.1

For Lugar, what is more dramatic is the change in his ideological position relative to the Senate Republican Caucus. In his first term in Congress, Senator Lugar was the 23rd most moderate Republican in the Senate; in the most recent term (through 2011), he was the fifth most moderate. Even if he had maintained his freshman score of 0.341, he would still have been the 12th most moderate Republican in the 112th Congress. This repositioning occurred because almost every new cohort of Republican Senators has been more conservative than Senator Lugar. That fact is the basis for our claim that the Republican party has moved to the right.

To be sure, political polarization is not entirely asymmetric. Congressional Democrats have moved slightly to the left during this period, but most of this is a product of the disappearance of conservative Southern “Blue Dog” Democrats. But the northern Democrats of the 1970s are ideologically indistinguishable from their present-day counterparts, with average scores around -0.4.

Though Democrats have not moved nearly as much to the left as the Republicans have to the right, they have also contributed to polarization, in our opinion, by embracing identity politics as a strategic tool. In Roosevelt’s New Deal, the Democrats advocated redistribution and regulation of business. These issues remain active to some extent, but with time emphasis has shifted to issues centered on race, gender, ethnicity, or sexual preference (Gerring 1998). This distinction, however, is not necessarily picked up in roll call voting. But it does represent an important rhetorical shift from the Roosevelt and Truman-era Democratic Party that likely worsens political and social divides.

Nonetheless, we should be careful not to equate the two parties’ roles in contemporary political polarization: the data are clear that this is a Republican-led phenomenon where very conservative Republicans have replaced moderate Republicans and Southern Democrats. Thomas Mann and Norm Ornstein do an excellent job of navigating these trends in their new book: It’s Even Worse Than It Looks: How the American Constitutional System Collided With the New Politics of Extremism.

Moreover, the rise of the “Tea Party” will likely only move Congressional Republicans further away from the political center. For example, the five Tea Party-backed Senators elected in 2010 (Senators Rubio, Paul, Toomey, Lee, and Johnson) have an average first dimension DW-NOMINATE score of 0.795. Moderate MCs (especially Republicans) are increasingly likely to be “primaried” out (e.g., Sens. Bob Bennett (R-UT), Joe Lieberman (D-CT), and Lisa Murkowski (R-AK), as detailed in a recent post on voteview.com).

The public policy consequences of polarization are immense. Bipartisan agreements to address looming issues like the budget deficits, spending on entitlement programs, and immigration are now almost impossible to reach. In contrast, during Ronald Reagan’s administration, about 40% of the members of Congress could be described as moderates. Reagan was thus able to forge major bipartisan agreements to cut taxes in 1981, raise taxes in 1982, fix Social Security (the Greenspan Commission) in 1983, and pass immigration reform (which included amnesty) and major tax simplification in 1986.

As shown in the second pair of figures below, only about 6% of Representatives and 13% of Senators in the 112th Congress can be described as moderates (defined as having a first dimension DW-NOMINATE score between  minus 0.25 and  plus 0.25). This absence forces major legislation, such as President Obama’s health care package, to be passed by one party. But unlike major bipartisan efforts (e.g., the Social Security Act of 1935, the Civil Rights Act of 1964, or welfare reform in 1995), legislation passed by one party is less likely to earn popular acceptance (as evidenced by the partisan breakdown in opinion on “Obamacare”).

Polarization is real. Arlen Specter was reelected to the Senate as a moderate Republican in 2004. In the 2010 election, he was replaced by Pat Toomey. Do academics and pundits really want to argue that Republicans have not moved to the right and that Pat Toomey might be more moderate than Arlen Specter because the congressional agenda has changed? Let’s not get picky about polarization. It’s for real, and it is making the United States dysfunctional.

1 Legislators’ DW-NOMINATE scores are allowed to move as a linear function of time, while a single coordinate is estimated for each legislator with the Common Space procedure; methodological issues aside, the linear and constant methods produce yield the same pattern of contemporary political polarization.


Gerring, John. 1998. Party Ideologies in America, 1828-1996. Cambridge: Cambridge University Press.

McCarty, Nolan M., Keith T. Poole, and Howard Rosenthal. 1997. Income Redistribution and the Realignment of American Politics. AEI Studies on Understanding Economic Inequality. Washington, DC: AEI Press.

14 Responses to Polarization is Real (and Asymmetric)

  1. DonM May 15, 2012 at 6:58 pm #

    I would say that a congress that sees a trillion dollars more spending each year than they accepted in income was disfunctional. Trillion dollar deficits should not be polarizing, rather they should be universally condemned. That they are not shows not polarization, but rather fantasy.

    Why didn’t the Tea Party shut down the government to stop spending? Because Obama was seen as too radical to do it responsibly. If Romney gets elected, the government can be shut down by someone who has a record of cutting fat and keeping productive elements on line. Unconstitutional departments like Education, Energy, Agriculture, Health and Human Services, Housing and Urban Development, and Labor can be shut down and not restarted. The small constitutional parts (like the Dept of Energy units responsible for nuclear weapons) can be shifted to constitutional departments.

    And then all will be well.

    • Daniel Francis May 17, 2012 at 12:14 am #

      Trust me, if even one of your proposed items got pushed there would be nothing short of Political Chaos. Amateurish. Really.

    • iLarynx May 17, 2012 at 7:48 am #

      “And then all will be well.”

      Talk about fantasy! (Not to mention ignorance of the Constitution.)

  2. Andrew Gelman May 15, 2012 at 7:41 pm #

    Hey Nolan. Whassup with this carefully-argued, well-documented blog post? You’re setting the bar pretty high for the rest of us!

  3. Number Three May 15, 2012 at 7:42 pm #

    Isn’t the most telling criticism not that “times change” but that DW-NOMINATE can’t actually distinguish partisan polarization from ideological polarization, and that lazy reliance on easy-to-obtain measures conflates “partisan” with “ideological”? So a vote against Bush’s expansion of Medicare to prescription drugs is a “liberal” vote (because Dems voted against, and the GOP voted for) . . . but a vote for “Obamacare,” a GOP program based on the policies of the current (presumptive) GOP presidential nominee is “liberal,” because Dems voted for Romneycare?

    • Jack C. May 15, 2012 at 10:17 pm #

      When you aggregate thousands of roll calls over several decades, these types of things cancel out and you actually can back out a latent dimension of ideology. We might conflate ideology and partisanship in analyses of individual roll call votes, but when you aggregate, this becomes a non-issue.

      Also, partisanship is quite likely a reflection of one’s own ideology. Liberals self-select into the Democratic Party, so obtaining a valid and accurate measure of partisanship–independent of the underlying ideology–is somewhat of a Holy Grail for Congress researchers.

      • Number Three May 16, 2012 at 8:10 am #

        Seriously? “[P]artisanship is quite likely a reflection of one’s own ideology?” This was NOT the case for a very long stretch of U.S. history, when the most conservative members of Congress and the most liberal were in the same party.

        • Manju May 16, 2012 at 9:52 pm #

          This is the popular narrative, because “conservative” is conflated with “segregationist” . But outside of race issues, Southern Dems were well to the left of Republicans. Thats what the chart above shows (civil rights is carved out onto another dimension, not shown here).

          I you just pull up some anecdotal roll calls, like say social security or medicare, you’ll see that Southern Dems generally supported them (albeit lukewarmly) more than even so-called Liberal Republicans (whose liberalism was largely racial).

          • Number Three May 17, 2012 at 7:16 am #

            But doesn’t that “albeit lukewarmly” support the premise that these Southern Dems were voting for their co-partisan president’s proposals, for partisan reasons, as opposed to their “sincere” preferences? And liberal Republicans voting against the other party and not in line with their “sincere” preferences?

            There were a few Southern liberals. But that label does not describe most Southern senators of this period: George, Russell, Stennis, Sparkman, Eastman, Harry Byrd (wasn’t a liberal!) . . . .

            • Manju May 17, 2012 at 11:52 am #

              There were a few Southern liberals. But that label does not describe most Southern senators of this period: George, Russell, Stennis, Sparkman, Eastman, Harry Byrd (wasn’t a liberal!) . .

              . .

              You’re cherrypicking. Here are some unpicked cherrys: Bill Fulbright, Clinton’s mentor, was a Liberal icon. Clinton called Gore Sr a Progressive, because he was one.

              Claude Pepper was so left of center that Truman thought him a communist. He had Gorgeous George unseat him. Smathers was a segregationist in the vein of Fulbright, and was considered for the #2 spot by Kennedy. That brings us to LBJ, a well-known liberal and (pre-64, or 57 depending on you point of view) segregationist…indeed a lot more vicious a segregationist than had been known until Robert Caro went digging.

              Sam Earvin of Watergate fame. Huey Long criticized the New Deal from the Left. Russel Long , Orville Faubus, George Wallace: all strong New Dealers. Robert Byrd.

              Just eyeballing this sorry list of racists, I’d say they would average out, on issues outside of race, as bluedogs who lean left. That’s where DW-nom puts them.

    • Manju May 17, 2012 at 12:56 am #

      So a vote against Bush’s expansion of Medicare to prescription drugs is a “liberal” vote (because Dems voted against, and the GOP voted for)

      I think DW-Nom is actually revealing an important ideological divide here.

      IIRC, Dems voted against the bill, not b/c they opposed expansion, but because Bush was going to pay for it with tax cuts, ie he was not going to pay for it at all. He was going to borrow more money.

      What this tell me about the ideology of Repubs is that they believe the Laffer curve bends much earlier than it actually does. They believe tax cuts pay for themselves. Their ideology is voodoo economics.

      In contrast, Dems range from neo-liberal to Keynesian. Boilerplate Keynesianism tells us to save borrowing & spending for downturns. Ergo, during normal times, additional welfare spending must be paid for by increasing taxes or a cutting spending elsewhere.

      Medicare Part D is arguably the most fiscally irresponsible thing Bush ever did. To pin that on Liberalism, on Keynesianism, or even Clinton Neo-liberalism, is perverse. Nominate stuck this stinker with the right ideology.

      • Number Three May 17, 2012 at 7:17 am #

        Post hoc rationalization. No one would ever code expanding Medicare as “liberal” ex ante.

  4. Kendall Kaut May 15, 2012 at 10:25 pm #

    This survey is useful and surely correct that the Republican Party has moved to the right. However, it seems moving to the right only matters in a few instances. If republicans symbolically vote against minor pieces of legislation or stall certain regulations from taking effect the impact on the county is less pronounced and less important. The more important issues seems to be breaking down how the two parties act on the major areas you discuss: entitlements, immigration, taxation and defense. It is true that Pat Toomey and Marco Rubio are more conservative members, however they may actually help solve problems. Rubio has offered his own version of the DREAM Act. Toomey offered billions of dollars in revenue during the negotiations to avoid the sequestration from kicking in. Examining major bills is also more useful because it seems these discussions require more than quantitative analysis of where an individual sits on a partisan spectrum. Did the republican party have legitimate reasons for opposing the healthcare bill or were they simply moving to the right to deny Obama a victory? It also begs the question of some of this legislation for determining what is Conservative? Is it Conservative to maintain a private healthcare system and adopt a model similar to what was used in Massachussets or at least fairly moderate, or is a bill that requires community rating provisions and expands Medicare Part D quite liberal? While it is a headache to deal with obstructionism on minor bills, the major bills are really what matters and what is bracketed off and discussed as why any of this matters in most studies. It is essential to narrow the focus for anyone proclaiming today’s republican party to be unimaginably conservative and obstructionist.

  5. Tristan May 16, 2012 at 5:33 pm #

    Any speculation/research that points to what happened in 1976?

    I’d like to blame Baby Boomers beginning to become politcally active, but I would say I have biases towards that conclusion.