The U.S. Supreme Court has agreed to hear Gill v. Whitford, a case where the issue is whether the Republican-dominated Wisconsin legislature drew the state's voting district boundaries in such a way as to give their candidates an overwhelming advantage. Republican candidates garnered just 48 percent of the vote statewide in 2012, but took 60 of 99 seats in the state legislature. Earlier this year, a federal appeals court ruled that the Wisconsin's legislature's latest redistricting plan "constituted an unconstitutional partisan gerrymander." The court ordered the legislature to devise and submit a fairer redistricting plan by November 1, 2017.
The practice of drawing district boundaries to establish an advantage for a particular party is called gerrymandering. The name comes from Massachusetts Gov. Elbridge Gerry, who in 1812 signed an egregious redistricting bill. One of the voting districts it created resembled the shape of a salamander; thus, "gerrymander."
Gerrymandering is generally achieved by either "packing" or "cracking." Packing concentrates the opposing party's voters in one district to reduce their voting power elsewhere. Cracking dilutes the voting power of the opposing party's supporters by spreading them across many districts.
With the exception of scrutinizing districts clearly designed dilute the power of black voters, federal courts have been reluctant to involve themselves in redistricting fights. This reluctance stems from courts' difficulty identifying any simple and objective way to determine the extent of gerrymandering. But mathematicians and statisticians have recently turned their attention to the issue, and they may be able to provide some guidance to the courts.
In Gill V. Whitford, the federal appeals court that ruled against the state cited a measure called the efficiency gap. Devised by Nicholas Stephanopoulos, a law professor at the University of Chicago, and Eric McGhee, a political scientist at the Public Policy Institute of California, the efficiency gap scheme measures a state's "wasted" votes. (Basically, votes are "wasted" if they are cast for a defeated candidate or cast in excess of those needed to elect a winning candidate.) In Stephanoupoulos' calculation, the efficiency gap is "the difference between the parties' respective wasted votes in an election, divided by the total number of votes cast." If a party is simultaneously getting an unusually high number of landslide victories and an unusually high number of crushing losses, that would be a sign of gerrymandering.
"Based on their calculations of the efficiency gaps in all redistricting plans over the past 40 years, Stephanopoulos and McGhee suggest setting thresholds above which redistricting plans would be presumptively unconstitutional; if the efficiency gap is 8 percent or more, or if it is enough to change at least two congressional seats, that would be enough to justify a constitutional challenge. In North Carolina's 2012 congressional election, for example, the efficiency gap was 21 percent,, which resulted in the Democratic candidates winning only 4 out of 13 seats. "
Meanwhile, the Duke mathematicians David Mattingly and Christy Graves have devised a program that draws voting district boundaries based on contiguity, geographical compactness, and a difference in population of no more than 0.1 percent. Although Democrats won 50.3 percent of the vote in 2012 in North Carolina, they captured only four of the state's 13 seats in the House of Representatives. In three of the districts drawn by the Republican-dominated legislature, voters were more than three-quarters Democrat. This is a classic example of packing.
The program devised by Mattingly and Graves creates thousands of randomly drawn district maps. Of those maps, they find that on average 7.6 seats would go to Democrats, compared with the four they actually won.
Other researchers are trying to devise fair and objective ways to set voting district boundaries. For example, Nature reports: "At the University of Illinois Urbana–Champaign, political statistician Wendy Tam Cho has designed algorithms to draw district maps that use the criteria mandated by state law, but do not include partisan information such as an area's voting history….Cho measures how closely a state's existing legislative districts line up with billions of non-partisan maps drawn by her supercomputing cluster. If they diverge significantly, then the people who drew the districts probably had partisan motives for placing the lines where they did, Cho says."
District boundaries are often drawn with the goal of protecting incumbent politicians from competition. And indeed, the incumbency rate for the House of Representatives in 2016 was 97 percent. Gerrymandering also discourages contested elections. Consider that the percent of voters whose state senators and legislators ran unopposed increased respectively from 11.3 and 22.2 percent in 1973 to 32.8 and 40.4 percent in 2014.
Researchers have clearly devised some pretty good methods for identifying and avoiding gerrymandering. It is now up to the Supreme Court to decide if the practice is unconstitutional. Stay tuned.