I recently saw a joyplot from Ian McDonald (described here) comparing DW-NOMINATE scores for Democratic and Republican members of the US House of Representatives. I’ve reproduced it below, but expanded the range to include the 71st - 114th Congress.

A lot of folks pointed out the “asymetric polarization” apparent here; over the past 50 years, Republicans have been moving further right than Democrats have moved to the left.

The joyplot is great for showing how the distributions have changed shape, but to directly compare how the left and right wings of each party have evolved I took a simpler approach. The plot below shows the absolute value of the the median, top decile, and bottom decile values of the DW-NOMINATE scores for both parties for each Congress.

There are a few interesting things here:

  1. The Deomcrats and Republicans have traded places as the more “moderate” (i.e., closer to 0) party a couple times in this period.
  2. The “asymmetric polarization” is even larger for the far right/left wings of their centers.
  3. The right wing of the Democratic party is disappearing.
  4. Median and far-left Democrats are no further from the center than they were in 1995.