The growth of partisan polarization has transformed US politics in recent decades, and the effects are especially visible in Congress. Earlier this year, a new paper in PLOS One (and flagged by Wonkblog's Chris Ingraham) demonstrated this transformation in a particularly cool way.

Six researchers — Clio Andris, David Lee, Marcus Hamilton, Mauro Martino, Christian Gunning, and John Armistead Selden — 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:

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.

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