These days, everyone seems anxious about Congress. Rarely does a news cycle pass without new stories of political dysfunction in Washington, DC. New reports of stalemates, fiscal cliffs, and failed grand bargains have begun to erode the public confidence in the ability of our representative institutions to govern effectively. In May 2013, only one in six Americans approved of the way Congress has handled its job.1–‐down–‐congress–‐own–‐representative.aspx And sadly, that level of support was a major improvement over the previous summer, when wrangling over the usually routine matter of raising the debt ceiling drove congressional approval to 10 percent.

The most common diagnoses of Washington’s ailments center on the emergence of excessive partisanship and deep ideological divisions among political elites and officeholders. In short, “polarization” is to blame. Consequently, the reform-minded have taken up the mantle of reducing polarization or mitigating its effects. In recent years, proposals for electoral reform to change electoral districting, primary elections, or campaign finance have been presented as panaceas. Other reformers have focused on changing legislative procedures such as those related to the filibuster, appropriations, and the confirmation process to limit the opportunities for polarization to undermine government.

To stimulate (and anchor) discussion, I briefly provide my assessment of the state of political science knowledge about the causes and consequences of congressional polarization and the attendant anxieties.

Congressional polarization has grown since the 1970s. Both quantitatively and qualitatively oriented scholars seem to agree that partisan and ideological divides in Congress began increasing in the 1970s. Figure 1 provides the most prominent measure of polarization, the average differences between the parties on the DW-NOMINATE scale. While some scholars attribute the increase in partisan voting to rules changes in the U.S. House, such arguments have a more difficult time accounting for the very similar changes observed in the Senate and in state legislatures.

The rise in polarization is primarily accounted for by changes in the Republican Party. Figure 2 provides the mean position on the DW-NOMINATE scale of each party by region. The trend for Northern Democrats is flat. Southern Democrats have become more liberal, reflecting both the defection of conservative and moderate members to the Republican Party and the increased numbers of African-American representatives. Republicans from both North and South have moved sharply to the right.

Polarization is a combination of ideological differentiation and partisan strategies. Lee (2009) argues that measures of polarization reflect not only an ideological divergence, but also an increasing emphasis on partisanship by members of Congress. She argues that a norm of “teamsmanship” has emerged as members’ individual interests have become increasingly linked to the fate of their parties and partisan competition for control of Congress has intensified. Lee’s primary evidence is the finding that partisan divisions on nonideological issues have grown in tandem with the divisions on ideological issues. See also Groseclose and McCarty (2001).

Polarized voters did not cause legislative polarization. The emerging consensus is that most voters have been and remain overwhelmingly moderate in their policy positions (Fiorina, Abrams, and Pope 2005; Ansolabehere, Rodden, and Snyder 2006; Levendusky, Pope, and Jackman 2008; Bafumi and Herron 2010). In studies that produce estimates of voter issue positions that are comparable to legislator positions, representatives are found to take positions that are considerably more extreme than those of their constituents (Clinton 2006; Bafumi and Herron 2010).

But sorted voters might have. Layman and Carsey (2002) and Levendusky (2009) find that over time, voters increasingly hold political views that consistently align with the parties’ policy positions. Today, fewer voters hold a mix of Democratic and Republican positions than in the past. As the parties themselves become more coherent in their policy positions, voters sort themselves more accordingly. This may well account for the finding of Bartels (2000) that partisan identification is a better predictor of voting behavior. Also because “Republican” and “Democrat” now represent very distinct clusters of policy positions, citizens who identify with one party expect the other party’s identifiers to hold very different political views. McCarty, Poole, and Rosenthal (2006) and Gelman (2009) also find that voters have become better sorted into parties by income over time. The question of whether partisan voters are more sorted by geography is more controversial (see Bishop 2009; Klinkner 2004).

The partisan realignment of the South is an important but incomplete explanation. Since the 1970s, there has been a dramatic increase in the number of Republicans representing Southern districts and states. The replacement of moderate and conservative Democrats with Republicans had two effects. First, the median Southern Democrat became more liberal. By the early 2000s, most Southern Democrats represented majority-minority districts. At the same time, the new Southern Republicans have become increasingly conservative. However, the conservative path of Southern Republicans is mirrored in non-Southern districts. Consequently, McCarty, Poole, and Rosenthal (2006) show that measures of polarization among non-Southern districts show the same patterns as those of the entire Congress.

Congressional polarization shows a striking correlation with economic inequality. Economic inequality and polarization have tracked together over the last hundred years (McCarty, Poole, and Rosenthal 2006). Unlike most other hypotheses about polarization, the inequality hypothesis can explain the decline of polarization over the first half of the twentieth century, as economic inequality fell dramatically over that period (see Piketty and Saez 2003). McCarty et al. argue that inequality and polarization are linked by a dynamic relationship (or “dance”) where increased inequality, generated by rising top incomes, produces electoral support for conservative economic policies and facilitates a movement to the right by Republicans. The resulting polarization then has a dampening effect on the policy response to increased inequality, which in turn facilitates greater inequality and polarization. Although the McCarty et al. study is limited by the fact that the correlation between inequality and polarization may be spurious in the U.S. times series data, Garand (2010) finds evidence that state-level inequality exacerbates constituency polarization within states and predicts the extremity of Senate voting behavior.

Popular arguments about the role of gerrymandering and partisan primaries are probably wrong. The Senate has polarized in tandem with the House despite never being gerrymandered. Moreover, the representatives from at-large House districts have also become increasingly extreme. More systematically, McCarty, Poole, and Rosenthal (2009) show that the increase in polarization is primarily a function of the increasingly distinct ways that Democrats and Republicans represent moderate districts. The increased numbers of safe Republican and Democratic districts have only had a marginal effect. Similarly, scholars have found that there is little difference between the candidates nominated by closed partisan primaries and those nominated in open or nonpartisan primaries (Hirano et al. 2010; McGhee et al. 2013).

The link between private campaign finance and polarization is complicated. Although the levels of campaign spending have grown in tandem with partisan polarization, there is little evidence that contributions flow disproportionately to more extreme legislators (McCarty, Poole, and Rosenthal 2006). Large donors and interest groups are marginally less likely to contribute to extremists. On the other hand, contributions from small donors are increasingly important and do favor the most liberal Democrats and conservative Republicans (Barber 2013).

Polarization combined with supermajoritarian rules contributes to gridlock. Although it can be difficult to statistically isolate the effects of polarization from other time trends, the existing evidence suggests that higher levels of polarization, especially during periods of divided government, reduce legislative productivity (McCarty 2007).

Polarization has important policy effects. The most direct effect of polarization‐induced gridlock is that public policy does not adjust to changing economic and demographic circumstances. These effects are most pronounced in the arena of social policy. One of the aims of social policy is to insure citizens against the economic risks inherent in a market system; it therefore must be responsive to shifts in these economic forces. If polarization inhibits these responses, it may leave citizens open to the new risks created by economic shifts, which are, in turn, brought on by deindustrialization and globalization. Consider the political response in the United States to increasing economic inequality since the 1970s. Most economists attribute increasing inequality to a number of economic factors such as the rise in the returns to education, exposure to trade, immigration, and changes in family structure. Nevertheless, many Western European countries faced with many of the same economic forces developed policies to mitigate the consequences so that the level of inequality changed only marginally. Similarly, Hacker (2004) has recently argued that polarization was an important factor in impeding the modernization of many of the policies that were designed to ameliorate social risks. A second issue concerns the ways in which social policies in the United States are designed. Many policies, especially those aimed at the poor or near poor, are not indexed with respect to their benefits. Therefore, these programs require continuous legislative adjustment to achieve a constant level of social protection. McCarty, Poole, and Rosenthal (2006) provide evidence for effects of polarization on the minimum wage and welfare policy outcomes.

Senate polarization has had detrimental effects on the executive and judicial branches through a broken confirmation process. McCarty and Razaghian (1999) find that delays in the confirmation of executive officials and vacancies in executive agencies rose with partisan polarization. Binder and Maltzman (2009) find similar patterns for judicial appointments.

McCarty American 1
Figure from Barber and McCarty (2014).
McCarty American 2
Figure from Barber and McCarty (2014).


Ansolabehere, Stephen, Jonathan Rodden, and James M. Snyder. 2006. “Purple America.” Journal of Economic Perspectives 20 (2): 97–118.

Bafumi, Joseph, and Michael Herron. 2010. ‘‘Leapfrog Representation and Extremism: A Study of American Voters and their Members in Congress.’’ American Political Science Review 104 (3): 519–42.

Barber, Michael. 2013. “Ideological Donors, Contribution Limits, and the Polarization of State Legislatures.” Typescript, Princeton University.

Barber, Michael, and Nolan McCarty. 2014. “The Causes and Consequences of Polarization in Congress.” In Negotiating Agreement in Politics: Report of the American Political Science Association Presidential Task Force 2012-13, eds. Jane Mansbridge and Cathie Jo Martin: 19–54.

Bartels, Larry. 2000. “Partisanship and Voting Behavior 1952–1996.” American Journal of Political Science 44 (1): 35–50.

Binder, Sarah A., and Forrest Maltzman. 2009. Advice and Dissent: The Struggle to Shape the Federal Judiciary. Brookings Institution Press.

Bishop, Bill. 2009. The Big Sort: Why the Clustering of Like-Minded Americans Is Tearing Us Apart. New York: Mariner Books.

Clinton, Joshua D. 2006. ‘‘Representation in Congress: Constituents and Roll Calls in the 106th House.’’ Journal of Politics 68 (2): 397–409.

Fiorina, M. P., Samuel J. Abrams, and Jeremy C. Pope. 2005. Culture Wars? The Myth of Polarized America. New York: Longman.

Garand, James C. 2010. “Income Inequality, Party Polarization, and Roll-Call Voting in the US Senate.” Journal of Politics 72 (04): 1109–1128.

Gelman, Andrew. 2009. Red State, Blue State, Rich State, Poor State: Why Americans Vote the Way They Do. Princeton, NJ: Princeton University Press.

Groseclose, Timothy and Nolan McCarty. 2001. “The Politics of Blame: Bargaining before an Audience,” American Journal of Political Science. 45(1):100-119.

Hacker, Jacob S. 2004. “Privatizing Risk without Privatizing the Welfare State: The Hidden Politics of Social Policy Retrenchment in the United States.” American Political Science Review 98 (2): 243–260.

Hirano, Shigeo, James M. Snyder Jr., Stephen Daniel Ansolabehere, and John Mark Hansen. 2010. “Primary Elections and Partisan Polarization in the U.S. Congress.” Quarterly Journal of Political Science 5 (2): 169–191.

Klinkner, Philip A. 2004. “Red and Blue Scare: The Continuing Diversity of the American Electoral Landscape.” The Forum (2)2.

Lee, Frances. 2009. Beyond Ideology: Politics, Principles, and Partisanship in the U.S. Senate. University of Chicago Press.

Layman, Geoffrey, and Thomas Carsey. 2002. “Party Polarization and ‘Conflict Extension’ in the American Electorate.” American Journal of Political Science 46 (4): 786–802.

Levendusky, Matthew. 2009. The Partisan Sort: How Liberals Became Democrats and Conservatives Became Republicans. University of Chicago Press.

Levendusky, Matthew S., Jeremy C. Pope, and Simon Jackman. 2008. ‘‘Measuring District Level Preferences with Implications for the Analysis of U.S. Elections.’’ Journal of Politics 70 (3): 736–53.

McCarty, Nolan. 2007. “The Policy Consequences of Political Polarization.” In The Transformation of the American Polity, eds. Paul Pierson and Theda Skocpol. Princeton, NJ: Princeton University.

McCarty, Nolan, Keith T. Poole, and Howard Rosenthal. 2009. “Does Gerrymandering Cause Polarization?” American Journal of Political Science 53 (3): 666–680.

—–. 2006. Polarized America: The Dance of Ideology and Unequal Riches. Cambridge, MA: MIT Press.

McCarty, Nolan, and Rose Razaghian. 1999. “Advice and Consent: Senate Response to Executive Branch Nominations 1885–1996.” American Journal of Political Science 43 (3): 1122–43.

McGhee, Eric, Seth Masket, Boris Shor, Steve Rogers, and Nolan McCarty. 2013. “A Primary Cause of Partisanship? Nomination Systems and Legislator Ideology.” American Journal of Political Science. doi: 10.1111/ajps.12070

Piketty, Thomas, and Emmanuel Saez. 2003. “Income Inequality in the United States 1913–1998.” Quarterly Journal of Economics 118 (1): 1–39.