It can be difficult to prove, legally. There’s no one standard to determine whether a map has been gerrymandered, and mapmakers rarely admit to doing it.
But it has been possible to demonstrate in a handful of states, including Pennsylvania, where the state Supreme Court in 2018 found Republicans in charge maximized the number of congressional seats for their party while disadvantaging Democrats. Members of that party won just five of the 18 seats with the Republican-drawn map, despite winning slightly more than half of the statewide vote.
“We showed that the features of the map could not be explained by anything other than partisan manipulation,” said Ben Geffen, a lawyer with the Public Interest Law Center who worked on the case.
Pennsylvania’s state House and Senate maps, which were drawn by Republicans and approved by the state Supreme Court in 2012, haven’t faced the same partisan scrutiny. Still, experts say that some of the same computer-based techniques used in the 2018 legal case show the maps skew toward Republicans, and that it is unlikely this happened organically.
When lawmakers go back to the drawing board later this year to make Pennsylvania’s new maps, experts and analysts will apply these same tests to evaluate whether there are any partisan asymmetries that can’t be explained by chance.
Efficiency gap
If the goal of partisan gerrymandering is for one party to do everything possible to secure as many seats as it can, the way to do that is by packing and cracking districts. Packing crams voters of the rival party into a few districts to give that party overwhelming wins. Cracking spreads members of the rival party across a large number of districts so their votes don’t matter as much.
A concept called the efficiency gap, developed by Harvard Law professor Nick Stephanopoulos and political scientist Eric McGhee, measures the extent that districts have been cracked or packed by looking at the number of collective “wasted” votes in each district.
Any vote that is above what a candidate needs to win is an extra, or wasted, vote. Likewise, any vote for the candidate who ends up losing is also considered wasted.
The efficiency gap takes the difference between the parties’ respective wasted votes and divides that by the total number of votes cast.
“It captures, in a single tidy number, all of the packing and cracking decisions that go into a district plan,” Stephanopoulos and McGhee wrote in a 2014 working paper.
In all Pennsylvania House and Senate districts that were drawn in 2012, Democrats had 1.1 million to 1.2 million more wasted votes than Republicans. The more wasted votes a party has, the more likely that party is to have been disadvantaged.
U.S. Chief Justice John Roberts has called the efficiency gap “sociological gobbledygook,” a phrase embraced by the Republican State Leadership Committee. The national organization, which works to elect GOP candidates at the state level, claims Republicans win more seats than the total vote share because they have stronger candidates and policies that appeal to voters.
While the efficiency gap has been used in legal arguments where federal courts, including in North Carolina and Wisconsin, have struck down maps, the U.S. Supreme Court has sidestepped questions about what standards, if any, should be used to determine partisan gerrymandering.