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Gerrymandering is often given as one explanation of why the Democrats lost congressional seats to the Republicans. Is there any good analysis about how many seats were lost because of gerrymandering that happened after 2008? To what extent have changes in district lines resulted in the Republicans gaining more seats?

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    One of the issues is that it is very difficult to say what exactly is and isn't gerrymandered in any empirical way. There are legitimate reasons to draw "whacky" district lines, which is why they're drawn in the first place, instead of just using a simple grid system. – Martin Tournoij Dec 21 '17 at 0:48
  • @Carpetsmoker : I edited my question to make it more clear that I'm talking about the effected of changes in district lines since 2008. – Christian Dec 21 '17 at 0:52
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    Is it possible to attribute the loss of a particular seat to gerrymandering? – Andrew Grimm Dec 21 '17 at 2:07
  • What is the definition of gerrymandering as used in this question? – Drunk Cynic Dec 21 '17 at 3:37
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    According to 538 (including last week's Gerrymandering podcast as well as earlier posts), negligible effect. Most of the effect is from self-sorting of population, and gerrymandering has at best marginal effect, further reduced by the fact that gerrymanderings in different locales cancel each other out (both partisan ones and other ones) – user4012 Dec 21 '17 at 15:13
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Based on the Cook Partisan Voting Index, the effect seems to be negligible.

Looking at the PVI for the 2010 election, which was the last one to be used under the old district boundaries, the Republican candidate outperformed their national result in 235 districts, whereas the Democratic candidate outperformed in 192 districts. 8 districts were even with the national result. Source

These numbers are not significantly different ahead of next year's midterms. The Republican candidate outperformed their national result in 238 districts, compared to 189 for Democrats. Again, 8 were even with the national result. Source

The swing here is only three seats, which in my opinion is not statistically significant.

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According to analysis by the Associated Press, the GOP benefited a bit from gerrymandering:

The AP analysis also found that Republicans won as many as 22 additional U.S. House seats [in 2016] over what would have been expected based on the average vote share in congressional districts across the country.

The NYT shows that the effect was a bit less in 2012:

Most of the political-science-based estimates suggested that gerrymandering cost the Democrats a net of 7 to 12 seats in 2012 (with a few estimates coming in higher and lower).

Analysis by the Brennan Center concludes:

[A] new report from the Brennan Center for Justice at NYU School of Law finds that extreme partisan bias in congressional maps account for at least 16-17 Republican seats in the current Congress

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