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The history of the House votes-seats discrepancy, in two graphs

- November 19, 2012

My recent claim that the gap between Democratic vote share and seat share this election was not caused by redistricting has generated a lot of conversation.  The most common complaint is that the old districts were gerrymandered to favor Republicans, too.  If the new ones left that gerrymander in place…well, gerrymandering might still be a factor, and reform might still make things better.

This is a fair point:  while the impact of redistricting this year was almost certainly small, my claim that reform could only have a minor impact overstepped the data I had in front of me (see here and here for evidence that I was irrationally exuberant).  Nonetheless, I thought it would be useful to provide some broader historical context for this discussion.  How long has it been true that Republicans would likely win a majority of seats even when they don’t win a majority of the vote?  Do changes in this bias coincide with redistricting?

Toward that end, below is a graph of partisan “symmetry” bias from 1952 to 2012, with dotted vertical lines to mark the point when most (though not always all) districts have been redrawn.  Symmetry is a hypothetical:  What if the two parties both received the same share of the vote?  Would they both get the same share of seats?  If not, which party would have an advantage?  I’ve had my concerns with this measure, but it seems perfectly suited to the conversation we’re having right now.

(NB:  To calculate these numbers, I used a program called JudgeIt for R that was developed by the Monkey Cage’s very own Andrew Gelman, along with Gary King and Andrew Thomas.  The program bases its numbers on a statistical model that reduces some of the noise and allows us to explore counterfactuals.)

I take away four observations from this graph:

  1. The bias in favor of Republicans has been around since the early 1990s, but it did not directly coincide with the 1991 redistricting.  Instead, it emerged after Republicans took control in 1994.
  2. Parties generally, though not always, have a bias in their favor when they have more incumbents (i.e., when they hold a majority).
  3. Bias varies a lot from one year to the next, and is not a fixed feature of a redistricting cycle.
  4. The bias in favor of Republicans this cycle is the largest since before 1952.  The party also had more incumbents running than at any time since 1948.

In other words, as we’ve been arguing, incumbency matters to discussions of partisan advantage.  This is very much in keeping with the conclusions Gary King and Andrew came to when they examined this question 20 years ago (though the magnitude of the bias I’ve measured here is larger than they found in their paper).

What happens if we statistically remove incumbents and pretend that every district is open every year?  We get this graph:

This suggests the Republican bias has been around since the 1950s, a conclusion that is also consistent with King and Gelman’s analysis.  Moreover, shifts in the partisan direction of bias almost never coincide with redistricting.  The one exception is 1992, when a brief advantage for Democrats (that itself did not coincide with redistricting) became an advantage for Republicans.  (However, see this book for an argument why redistricting accounted for the disappearance of a pro-Republican bias that existed outside the South in the 1950s.)

This doesn’t prove that gerrymandering is irrelevant.  Perhaps we’re just living with the legacy of the lines drawn in 1991.  And it’s still possible that the bias could be alleviated with aggressive enough reform.  But to me this says there is a lot more to the question than redistricting, and that the Republican advantage has been with us for some time.