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

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.

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

  1. Nick Goedert November 19, 2012 at 2:32 am #

    The first graph indeed pretty much conforms to my general impressions for what bias has looked like in House elections over the last several decades…although I’ve noticed that the Democratic bias pre-1994 seems almost entirely confined to the South. But I was surprised by how much eliminating incumbency changed these trends as reflected in the second graph!

    My only concern with the second graph is that incumbency is not compeletely causally prior to the election outcome….incumbents often retire instead of running again when they find themselves at the greatest risk of losing. And in particular, retirements are commonly induced by redistricting. So might you be controlling away some of the real effects of redistricting by assuming that every seat is open?

  2. Mikey November 19, 2012 at 8:40 am #

    This line stuck out at me: “Parties generally, though not always, have a bias in their favor when they have more incumbents (i.e., when they hold a majority).” Does this make lie of the whole idea of a bias to begin with?

    We can have a back-and-forth on the second graph. So many incumbents are re-elected as a matter of course that I think it’s tough to adjust them out. Or perhaps the real “bias” is just incumbency: once they’re in, almost everyone stays in, and it doesn’t matter how the legislature draws the lines.

    I also think it’s tough to swallow that “the Republican bias has been around since the 1950s.” How soon we forget that, right up until 1994, the Democrats were perceived as having such a permanent advantage that most people thought Republicans would *never* take over the House. Finding that “GOP bias has been around since the 50’s” would be the type of contrary-to-common-sense conclusion that would send me back to the drawing board.

  3. Mikey November 19, 2012 at 8:41 am #

    Good work on all this btw.

  4. Eric L. November 19, 2012 at 10:24 am #

    For an excellent discussion of the Republican gerrymander of the 1950s and its decline in the 1960s, see Bob Erikson’s discussion here:

    Malapportionment, Gerrymandering, and Party Fortunes in Congressional Elections, American Political Science Review , Vol. 66, No. 4 (Dec., 1972), pp. 1234-1245

    From the conclusion:

    “It is particularly striking that neither malaportionment nor deliberate partisan manipulation of district lines played the major role in the creation of the one-time Republican advantage in congressional districting. Instead, the explanation seems to be that the Republicans are given the edge in the way Democratic and Republican voters are geographically distributed. Possibly the Republican gerrymander is now only in a temporary state of remission, to return once again, following the eventual retirement of the Democratic Congressmen who now survive in normally Republican districts with the help of their inflated incumbency advantage.”

    • Mark November 19, 2012 at 3:44 pm #

      That conclusion is about only one region of the country. From the second paragraph: “Nationwide, the Democrats are the party most likely to win a majority of the seats with only a minority of the votes…”

      Bottom line is Democrats controlled the House with only two breaks between ’32 and ’94. To talk about a Republican gerrymander during this era is like saying nobody drowned in six inches of water when they should have drowned in six feet; it’s nonsense.

      • Mark November 19, 2012 at 3:45 pm #

        *somebody drowned

  5. Matt November 19, 2012 at 10:42 am #

    Pardon me for asking this, but how exactly is it possible to remove the effects of incumbency for the second graph? Doesn’t that simply assume that voters in incumbent districts would vote in patterns similar to open ones, an assumption which would essentially beg the question of what is the effect of incumbency? I feel like I must be missing something here. If you could provide a little more information on the logic of the process of “statistically removing incumbents” that would be great.

  6. Nick November 20, 2012 at 8:33 pm #

    I am wondering whether the Democratic incumbency bias is not a byproduct of incumbents holding the advantages we commonly associate with incumbency, but rather the fact that Southern Democrats were ideologically different than their brethren from the rest of the country. This means that while electoral coalitions changed at the presidential level during the 60s, Southern Democrats still stood for same things the old democratic party stood for and they didn’t give their constituents the incentive to switch parties like they did at the presidential level. In other words, this is less an issue of incumbency and more an issue of abnormal partisan labeling.

    This theory would explain why the democratic bias went away in the early nineties -right at the point the last Southern Democrats were leaving Congress and why once we take away the incumbency bias, we find a consistent Republican bias in gerrymandering throughout the past 50 yeas. I mean, you could even argue that the sole spike in Democratic gerrymandering was due to Southern Democrats who, after understanding which way the wind blew, did a last-ditch effort to protect themselves.