How the Supreme Court could limit gerrymandering, explained with a simple diagram

The Supreme Court heard oral arguments on October 3 for a case that could create limits on gerrymandering. The case, Gill v. Whitford, specifically challenges the way the Republican state legislature in Wisconsin drew the state assembly map to favor their own party.

But what's significant about this case is that the plaintiffs are asking the Court to come up with a standard for measuring gerrymandering. The most discussed solution is called an “efficiency gap,” developed by University of Chicago law professor Nicholas Stephanopoulos and political scientist Eric McGhee.

The concept is actually quite simple, but if you're a visual person like me, it takes a bit of doodling to understand.

Let's say the dots below represent all the people in a state.

And there are four districts, drawn thusly:

Now, we can pretty easily say that this isn't fair. After all, there are an equal number of Republicans and Democrats, but Republicans control 75 percent of the districts.

We first have to understand the strategies used to gerrymander districts.

The first is called "packing."

This means putting many voters of the opposing party into a single district:

The second is called "cracking."

This means separating the voters from the opposing party into small enough portions that they aren't a majority anywhere.

There are many ways to measure the unfairness of these districts.

The “efficiency gap” is a very clever method.

They're going through and finding the "wasted" votes.

So in our diagram, we can cross out every vote that could go away and keep the results the same. In each district, the winners needed at least 50 percent of the vote, plus one additional vote.

Even though we crossed out all those votes, Republicans still control three of four districts. And as you can see, the only "wasted" votes belong to Democrats. This means Republicans used their votes efficiently.

So a good way to measure gerrymandering is to look at what percentage of overall votes were "wasted" for each party.

Here's how it plays out in a larger map:

As you can see, this map forces Democrats to waste a lot more votes. Now, there's no way to really avoid "wasted" votes — but each party should have about the same number of them.

The researchers who came up with the “efficiency gap” argue that the maximum difference between parties should be 7 percent. The plaintiffs in Gill v. Whitford don’t ask the Supreme Court to come up with a specific number. Of course, this is just one standard that could be used to determine gerrymandering — the plaintiffs simply want there to be one.

That number isn't arbitrary. As Yale Law School dean Heather Gerken writes, any greater difference would mean it's virtually impossible for the minority party to get a legislative majority before redistricting happens again in a decade.

In other words, it tries to prevent state legislatures from gerrymandering their way to an ensured majority.

So let's fix the above map to get it under that threshold:

We often use weird district shapes to illustrate gerrymandering because that’s usually how legislators draw partisan shapes around existing populations. But it’s not about the shapes in this methodology. Rather, it’s about preventing lawmakers from redrawing district borders to keep power until the next time redistricting happens.

But here's something to keep in mind: These districts are still pretty uneven, and there's still "packing" and "cracking" happening. But when we combine all of that packing and cracking, there's a much smaller difference between the wasted votes for each party.

That's what the "efficiency gap" is all about.

Correction: A previous version of this piece miscounted wasted votes in the dot graphics. If your party wins a district, a wasted vote is every vote that account for 50 percent of the votes — plus one vote to get to the majority. The previous graphics showed that a wasted vote is any vote that is one more than the opponent’s total.

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