During the 2012 elections in North Carolina, Republicans took nine of the state’s 13 U.S. House seats although 51 percent of the two-party vote went to Democratic candidates.
The gerrymandering that led to these results isn’t unique to North Carolina or any specific party. Both Democrats and Republicans have used it for political advantage over the years. However, new technology makes it possible to draw partisan districts with increasing precision.
“North Carolina’s 12th and 4th congressional districts in particular look a little crazy,” said study co-author and Duke senior Christy Vaughn, who is originally from Taylorsville, N.C.
Vaughn and Duke math professor Jonathan Mattingly attempted to quantify how gerrymandering actually affects recent election outcomes.
They used a statistical algorithm to randomly redraw the boundaries of North Carolina’s 13 congressional districts. The model produced thousands of versions of the redrawn map. All of them were based only on the legal requirements of redistricting, ensuring the districts represented roughly equal numbers of voters and were as geographically compact as possible, without accounting for race or political affiliation.
Next the researchers re-ran the 2012 U.S. House election on a computer and calculated what the outcome would have been for each new version of the map.
”If someone voted for a particular candidate in the 2012 election and one of our redrawn maps assigned where they live to a new congressional district, we assumed that they would still vote for the same political party,” Vaughn said.
After re-running the election 100 times, with a randomly drawn nonpartisan map each time, the average simulated election result was 7 or 8 U.S. House seats for the Democrats and 5 or 6 for Republicans. The maximum number of Republican seats that emerged from any of the simulations was eight.
The actual outcome of the election -- four Democratic representatives and nine Republicans –
did not occur in any of the simulations.