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McCain For President?

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Does Gerrymandering Cause Polarization?

Political Polarization in the 110th Congress is the Highest in a 120 Years

Joe Lieberman in Polarized America

Corrections to the Hard Cover Edition of Polarized America

Polarized America: The Dance of Ideology and Unequal Riches

(June 2006, MIT Press)

Nolan McCarty, Keith T. Poole, and Howard Rosenthal


Political polarization, income inequality, and immigration have all increased dramatically in the United States over the past three decades. The increases have followed an equally dramatic decline in these three social indicators over the first seven decades of the twentieth century. The pattern in the social indicators has been matched by a pattern in public policies with regard to taxation of high incomes and estates and with regard to minimum wage policy. We seek to identify the forces that have led to this observation of a social turn about in American society, with a primary focus on political polarization.

Our primary evidence of political polarization comes from analysis of the voting patterns of members of the U.S. House of Representatives and Senate. Based on estimates of legislator ideal points (Poole and Rosenthal 1997 and McCarty, Poole, and Rosenthal 1997), we find that the average positions of Democratic and Republican legislators have diverged markedly since the mid-1970s. This increased polarization took place following a fifty-year blurring of partisan divisions. This turning point occurs almost exactly the same time that income inequality begins to grow after a long decline and the full effects of immigration policy liberalization are beginning to be felt.

Some direct causes of polarization can be ruled out rather quickly. The consequences of “one person, one vote” decisions and redistricting can be ruled out since the Senate, as well as the House of Representatives, has polarized. The shift to a Republican South can be ruled out since the North has also polarized. Primary elections can be ruled out since polarization actually decreased once primaries became widespread.

It is more difficult to find the causes of polarization than to reject them because social, economic, and political phenomena are mutually causal. For example, immigration might lead to policies that increase economic inequality if immigrants are at the bottom of the income distribution and do not have the right to vote. We document an upward shift in the income distribution of voting citizens. In turn, dispersal in income might lead to polarization. It also might lead to laxity toward immigration if inexpensive immigrant labor in the form of domestic and service workers is a complement to the human capital of the wealthy.

In additional to our focus on the polarization of elected office-holders, we look at patterns of polarization among economic elites. By examining campaign contributions, we find very high levels of polarized giving. While some billionaires clearly spread their contributions to both parties to buy access, increasing numbers concentrate their largess on the ideological extremes. This polarized campaign giving, coupled with the emergence of the soft money loophole has arguably contributed to the ideological extremism of political parties and elected officials.

Finally, we also examine polarization among the electorate. While it is fairly clear that the views of most citizens have not become more extreme, those with strong partisan identifications have (DiMaggio, et al., Fiorina). Consistent with other findings (King, Jacobson), we find that partisans are more likely to apply ideological labels to themselves and a declining number of them call themselves moderate. Strong party identifiers are the most likely to define politics and ideological terms while the differences in the ideological self-placements of Republicans and Democrats have grown dramatically since the 1980s. Given Bartels’ findings that partisanship has become a better predict of vote choice, this polarization of partisans has contributed to much more ideological voting behavior.

We also find that the polarization of the electorate has increasingly taken place along economic or class lines. Unlike the patterns of the 1950s and 1960s, upper income citizens are more likely to identify with and vote for Republicans than are lower income voters. However, we find that class polarization is most likely a result of the ideological shift of the Republican Party towards a more economic libertarian position. This shift to the right was aided by a number of social, political, and economic factors. First, as American society has become wealthier on average, a larger segment of society prefers to self-insure rather than depend on government social programs. Such voters have become more attracted to the Republicans and their agenda for an “Ownership Society.” Second, due to patterns of immigration and incarceration, members of lower income groups are less likely to be part of the electorate. This has the effect of moving the median income voter closer to the mean income citizen, reducing the demand for redistribution (Romer, Meltzer and Richard). Third, middle-income voters in the so-called “Red states” increasingly sympathize with Republican positions on social, cultural, and religious issues (e.g. Franks). The Republican advantage on these issues has mitigated any loss of votes that might have been associated with their shift on economic issues. Finally, the emergence of a class-based, two-party system in the United States has benefited the Republicans and mirrored the patterns of economic polarization found in other regions.

Finally, we examine the policy consequences of the fall and rise of political polarization. The separation of powers makes it difficult to generate coalitions large enough to produce policy change even when opinion shifts. We exploit this observation to get some leverage in disentangling the effects of political, economic, and social policies. For much of the period when polarization fell, immigration policy was restrictive and unchanged while income and estate taxes, defined in nominal terms, became more onerous. For the period since the onset of renewed polarization, we find strong evidence that “gridlock” has resulted in a less activist federal government. The passage of new laws has been curtailed due to the increasing difficulty of generating the requisite bipartisan coalitions. The effects on social and tax policy have been especially dramatic as real minimum wages have fallen, welfare devolved to the states, and tax rates have diminished. We also show how polarized politics has affected administrative and judicial politics.

(Of related interest! Note the Patterns. Added, July 2011)

For further information about McCarty, Poole, and Rosenthal's work on Polarization see:

The Next Big Issue: Inequality in America, by Godfrey Hodgson

Interview of Nolan McCarty by The American Prospect

The Decline and Rise of Party Polarization in Congress During the 20th Century

Growing Apart: The Mathematical Evidence for Congress' Growing Polarization, by Jordan Ellenberg
(Note that the links to voteview.uh.edu should be pooleandrosenthal.com. The pages are still on the website.)

Party Polarization: 1879 - 2010

Updated 11 January 2011

Below is a graph of the difference between the Republican and Democratic Party means on the first DW-NOMINATE dimension from the end of Reconstruction through the 110th Congress. This difference in first dimension means is a good measure of the level of political polarization. By this measure polarization is now at a post-Reconstruction high in the House and Senate. [Above are three more graphs that show the relationships between political polarization, income inequality, and immigration. These three graphs are from the first chapter of our (Nolan McCarty, Keith T. Poole, and Howard Rosenthal) forthcoming book Polarized America: The Dance of Ideology and Unequal Riches (MIT Press, spring 2006). An abstract of our book is shown above.]

With few exceptions, roll call voting throughout American history has been simply structured. Only two dimensions are required to account for the great bulk of roll call voting. The primary dimension is the basic issue of the role of the government in the economy, in modern terms liberal-moderate-conservative. The second dimension picked up regional differences with the United States -- first slavery, then bimetalism, and after 1937, Civil Rights for African-Americans. With the passage of the 1964 Civil Rights Act, the 1965 Voting Rights Act, and the 1968 Open Housing Act, this second dimension slowly declined in importance and is now almost totally absent. Race related issues - affirmative action, welfare, Medicaid, subsidized housing, etc. - are now questions of redistribution. Voting on race related issues now largely takes place along the liberal-conservative dimension and the old split in the Democratic Party between North and South has largely disappeared. Voting in Congress is now almost purely one-dimensional - a single dimension accounts for about 93 percent of roll call voting choices in the 110th House and Senate - and the two parties are increasingly polarized.

Polarization declined in both chambers from roughly the beginning of the 20th Century until World War II. It was then fairly stable until the late 1970s and has been increasing steadily over the past 20 years. Our (Poole and Rosenthal, 1997) original D-NOMINATE estimation ended with the 99th Congress. Interestingly, Congresses 100- 110, if anything, mark an acceleration of the trend (especially in the House). Note, however, that the acceleration is smooth and does not show a particular jump in polarization induced by the large Republican freshman class elected in 1994. Polarization in the House and Senate is now at the highest level since the end of Reconstruction.

In addition, the percentage of moderate Representatives and Senators continues to plummet. In the House the percentage of moderates (-.25 to +.25 on the first DW-NOMINATE dimension) has declined to about 10 Percent in both Chambers.

The files below were used to construct the figure. Each file has 12 variables. In order these are: Congress #; first year of Congress; Chamber Mean 1st dimension; Chamber Mean 2nd dimension; Democratic Party Mean 1st dimension; Democratic Party Mean 2nd dimension; Republican Party Mean 1st dimension; Republican Party Mean 2nd dimension; Northern Democrat Mean 1st dimension; Northern Democrat Mean 2nd dimension; Southern Democrat Mean 1st dimension; Southern Democrat Mean 2nd dimension.

Note that all 2nd dimension coordinates have been weighted. See the DW-NOMINATE page for an explanation.

Political Party Means 46th to 111th Houses
Political Party Means 46th to 111th Senates

Below are graphs of party unity scores for the House and Senate. A Party Unity vote is one that pits a majority of one party against a majority of the opposite party. The Proportions shown in the plots are the proportions of the members voting with their party on Party Unity votes. These figures tend to track the polarization figures.

In the file below the first column is the congress number, the second is the first year of the Congress, the third is the number of roll call votes in the House, the fourth is the number of party unity roll calls (at least 50% of one party against 50% of the opposite party), the fifth colum is the percentage of party unity votes, the sixth is the proportion of Republicans voting with the majority of their party, the seventh is the proportion of Democrats voting with the majority of their party, the eighth is the number of roll call votes in the Senate, the ninth is the number of party unity roll calls in the Senate, the tenth is the percentage of party unity votes, the eleventh is the proportion of Republicans voting with the majority of their party, and the twelve is the proportion of Democrats voting with the majority of their party.

Political Party Unity Scores 46th to 111th Houses and Senates
Political Party Unity Scores for individual members of the House and Senate for the 46th to 111th Congresses

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