A historical perspective on polarization helps us better understand both its causes and consequences. We agree with the excellent points Nolan McCarty and Frances Lee have already made in their posts—that Congress is as polarized now as it ever has been, and that we are in a historically unusual period of sustained partisan competition. At the same time, looking at the data another way shows that the present period of polarization is not necessarily a historical anomaly.

Let’s say we counted the number of Democrats who were more conservative than the 10th percent, 25th percent, and 50th percent most liberal Republican in the House and Senate, and the number of Republicans who were more liberal than the 10th percent, 25th percent, and 50th percent most conservative Democrat. (Here, we’ll use DW-NOMINATE scores, which political scientists commonly use to measure ideology, although almost any other measure will work.) Those conservative Democrats and liberal Republicans are the ones who lie in what we call the “overlap” region—the region where members from both parties overlap with each other. In periods of higher ideological polarization, there will be fewer—or no—members in the overlap region, and more members in that region during periods of lower polarization.

If we simply count the number of legislators in the overlap region in each year, what becomes evident is that the present period of polarization is akin to the polarization we had for much of the nineteenth and early twentieth century. The graph below, taken from a previously published paper (ungated, gated, and discussed further here), makes this point by tracking the percentage of overlapping Democrats and Republicans over time....