Quantifying the Effects of Gerrymandering

Quantifying the Effects of Gerrymandering December 2, 2014

All over the country we’ve seen the effects of partisan gerrymandering by Republicans after they took control of so many state legislatures in 2010. A new study out of Duke University quantifies the effects of that gerrymandering and the results are exactly what one would expect.

In 2012, Democratic U.S. House candidates in North Carolina received 81,190 more votes that Republicans. Republicans received just under half of the votes earned by the two parties. And yet, the GOP walked away with 9 of the state’s 13 congressional districts. So, despite the fact that they earned just over 49 percent of the two-party vote, Republicans won nearly 70 percent of the state’s congressional seats.

Common sense dictates that the legislative maps that could produce such a result must be deeply flawed — and that they must be biased towards Republicans, the same party that controlled both houses of the state legislature when these maps were drawn (although the state had a Democratic governor at the time of the redistricting, the governor has no veto power over congressional maps). A new study by Duke Mathematics Professor Jonathan Mattingly and undergraduate Christy Vaughn seems to confirm this insight. Their study confirms that it is highly unlikely that a fair redistricting process would have produced a map as skewed towards one political party as North Carolina’s congressional map is.

Mattingly and Vaughn’s study redrew numerous random congressional maps, all of which complied with three rules: the districts must be “connected,” they must “come as close as possible to having [an] equal number of people,” and “they should be as compact as possible.” They then ran eight different simulations, some of which gave greater preferences to compact districts over equal population, while others placed greater emphasis on maintaining exact population. Seven of the eight simulations did not produce a single map where Democrats won less than five congressional seats, assuming that every voter who cast a vote for a Democrat or a Republican in 2012 would have cast the same vote under the simulated maps. The one simulation that did produce a handful of outlier maps where Democrats won only four seats did so “in less than 5% of the samples.”

Thus, the actual result of the 2012 elections — four Democratic congressional seats in North Carolina — did not even show up in all but one of Mattingly and Vaughn’s simulations. In the simulation where it did arise, it did so only in a few unusual cases. It is exceedingly unlikely that North Carolina’s GOP-friendly maps could have arisen organically. Rather, as Mattingly and Vaughn demonstrate, they are almost certainly the product of a legislature that carefully designed the maps to produce a desired result. The study’s authors argue that this result cries out for an independent check on redistricting — “The fact that the election outcomes are so dependent on the choice of redistrictings demonstrates the need for checks and balances to ensure that democracy is served when redistrictings are drawn and the election outcome is representative of the votes casted.”

The same thing happened here in Michigan, where Democratic candidates for the U.S. House got 515,716 votes (49.15%)

while Republicans got 1,463,854 votes (47.47%), but Republicans nonetheless control 9 of the state’s 14 seats in Congress. In the statewide election for Senator, where gerrymandering doesn’t matter because it’s only the total state vote that counts, the Democrat won easily. This is why the redrawing of districts should not be done by elected officials at all. It should only be done by a non-partisan board of demographics experts.

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