The US Congress has a truly prodigious re-election rate: 80-90% in the House since 1964, and ~80% in the Senate with the exception of the 1970s (when it hit a "low" of 55%-65%)

While there are a lot of different factors contributing to this incumbent advantage, it is frequently stated that gerrymandering [1] contributes significantly to it.

Is there any sort of data/research available which quantifies just how much of an impact gerrymandering has on re-election rates compared to other factors?

I'm fine if the data is specific to US Congress (slightly preferred), or US in general, or some other locale.

Gallup charts of re-election rates


  • [1] Gerrymandering is defined on Wikipedia as "a practice that attempts to establish a political advantage for a particular party or group by manipulating geographic boundaries to create partisan or incumbent-protected districts".

    In plain terms, you adjust the district so that (in terms of re-elections) the incumbent has a higher percentage of his partisans in the new district than in the old one, and higher than the percentage of opponents. This makes the district safer for the incumbent and increases the chances of winning re-election.

  • The main place where I've heard complaints about gerrymamdering is Northern Ireland.
    – TRiG
    Commented Jan 27, 2013 at 0:05
  • I do not think that gerrymandering necessarily helps incumbents. Often times a district that had a "Safe" seat for the party in power in the election will be adjusted to include areas that are politically hostile to the incumbant. Case in point Bobby Schilling had his district taken from primarily a rural conservative district and adjusted to append 2 strongly democrat urban areas made continuous only by land completely occupied by Interstate 74. Commented Apr 24, 2013 at 18:33

1 Answer 1


There is this research: http://www.mit.edu/~rholden/papers/Incumbents.pdf with a reprint ~1 year later: http://www.hks.harvard.edu/fs/jfriedm/incumbents.pdf.

Meanwhile, here is a proposal which would eliminate gerrymandering and instituting a mathematical approach: http://rangevoting.org/GerryExamples.html and (ballot initiative) http://rangevoting.org/BallInit.html.

Edit: From the abstract of the research, "The probability that an incumbent in the United States House of Representatives is reelected has risen dramatically over the last half-century; it now stands at more than 98%. A number of authors and commentators claim that this rise is due to an increase in bipartisan gerrymandering in favor of incumbents. Using a regression discontinuity approach, we find evidence of the opposite effect. All else equal, redistricting has reduced the probability of incumbent reelection over time. The timing of this effect is consistent with the hypothesis that legal constraints on gerrymandering, such as the Voting Rights Act, have become tighter over time. Incumbent gerrymandering may well be a contributor to incumbent reelection rates, but it is less so than in the past."

  • 1
    Can you please summarise the findings of the research here, to prevent link rot?
    – gerrit
    Commented Oct 16, 2013 at 20:40
  • The abstract is now incorporated.
    – xuinkrbin.
    Commented Oct 16, 2013 at 21:28
  • 2
    I don't understand how redistricting has reduced the probability of incumbent reelection over time but Incumbent gerrymandering may well be a contributor to incumbent reelection rates. Aren't those statement opposite?
    – gerrit
    Commented Oct 16, 2013 at 23:06
  • 1
    Well, redistricting =/= gerrymandering. A district could be gerrymandered after one census and a more politically neutral districting algorithm used after the next.
    – xuinkrbin.
    Commented Oct 17, 2013 at 1:23
  • 1
    @gerrit For example: "It used to give +20%. Now it gives +15%." Reduced benefit, but still a (positive) contributor. Commented Jun 9, 2018 at 0:42

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