See Congress polarize over the past 60 years, in one beautiful chart


The growth of partisan polarization has transformed US politics in recent decades, and the effects are especially visible in Congress. Now a new paper in PLOS One (and flagged byWonkblog’s Chris Ingraham) demonstrates this transformation in a particularly cool way.

Six researchers — Clio Andris, David Lee, Marcus Hamilton, Mauro Martino, Christian Gunning, and John Armistead Selden — have created a visualization of how likely the House of Representatives’ Democrats (in blue) and Republicans (in red) are to vote with their own party, or to cross party lines. The change over the past six decades is remarkable:

Polarized Congress

Andris, Lee, Hamilton, Martino, Gunning, and Selden, “The Rise of Partisanship and Super-Cooperators in the U.S. House of Representatives.”

You can see here that in the 1960s and 1970s it was actually quite common for members of one party to vote with the other party. The blue dots and red dots are intermixed. But gradually in the 1980s and especially in the early 1990s, partisan voting behavior grew much stronger.

By the 1993-’94 Congress — just before the Republican takeover of the House — the overlap on votes between the two parties had almost completely vanished; you can see the two groups of dots self-segregate into homogeneous clusters. Since then, the gap between the parties has remained large — and very few members of Congress have frequently crossed it. Check out the researchers’ full paper here.

WATCH: ‘President Obama on today’s political polarization’


What is political polarization?

Political polarization simply measures overlap between the two parties. A high level of political polarization means that Republicans agree with Republicans and that Democrats agree with Democrats.

There was a time, not so long ago, when this wasn’t true — when many elected Republicans agreed more with the Democrats than with other Republicans, and vice versa — and leading political scientists thought it a great crisis for our democracy. In 1950, the American Political Science Association’s Committee on Political Parties released a report calling on the two parties to sharpen their disagreements so that the American people had a clearer choice when casting their ballots.

The political scientists eventually got their wish. According to the polarization measures kept by political scientists Keith Poole and Howard Rosenthal, party polarization is higher in today’s Congress than at any time since the late 1800s:


Political polarization is sometimes used as a synonym for political extremism, which it is not. It is sometimes used as a stand-in for political incivility, which it also is not. The 1960s and 1970s were a time of incredible political controversy and tumult. But political polarization was at a low ebb, because though Vietnam and the civil rights movement and the Great Society split the country, they did not cleanly split the two political parties. The Civil Rights Act of 1965 is a good example: The law was primarily pushed by politicians in the Democratic Party, but many northern Republicans supported it while southern Democrats were its fiercest opponents.

A close examination of this period also shows why consensus should not be viewed as an unalloyed good. The de-polarized political system if the 40s and 50s relied on a bipartisan consensus in favor of segregation. Extremely conservative Southern Democrats remained in the Democratic Party so long as the Democratic Party kept protecting the architecture of southern racism. As soon as that ended, conservative Southern Democrats like Strom Thurmond migrated to the Republican Party, and the system began to polarize.

The problem with party polarization is that the American political system typically requires bipartisan coalitions in order to get big things done, but during periods of intense political polarization, it is almost impossible for those coalitions to form.


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