Back in the 1960s, U.S. Supreme Court Justice Potter Stewart offered a simple definition of pornography that quickly became famous: “I know it when I see it.”

The reverse is true of redistricting. Everyone understands when a map is drawn to heavily favor one party. But few people, especially judges, seem to agree on what exactly that looks like.

On Monday, the U.S. Supreme Court ruled that North Carolina's congressional map is an unconstitutional racial gerrymander. The ruling also hinted that the court could be open to finding maps unconstitutional on the basis that they are too partisan, but the justices still haven't made it clear exactly what they think that would look like.

But social scientists have come up with a way of measuring unconstitutional partisan gerrymandering.

Known as the “efficiency gap,” the measurement was at the heart of a successful challenge brought against Wisconsin and is also key to pending cases in Maryland and a separate North Carolina case. It works by looking at how many votes for one party are wasted. In other words, say a Democrat carries a district by 30 percentage points; 29.9 percent of those votes are wasted, or unnecessary for a simple majority win. If there are too many lopsided districts, that shows that GOP mapmakers intentionally packed Democratic voters into a small number of districts, guaranteeing the party a few surefire wins and lots of losses elsewhere.

Democrats were able to convince a federal district court last November that Wisconsin Republicans had pursued exactly that strategy. The efficiency gap in Wisconsin, as measured by vote totals from the 2012 elections, was about three times as high as the average legislature’s and significantly favored Republican candidates. In January, the court ordered Wisconsin to draw new maps in time for next year’s legislative elections. The state is appealing.

“It’s clear that partisan gerrymandering is going back to the Supreme Court, most likely next term,” says Michael Li, a redistricting expert at the Brennan Center for Justice at New York University.

But even if courts accept the efficiency gap as evidence of gerrymandering, they will never rule that it must be eliminated altogether. Efficiency gaps might be the result of neutral factors, such as efforts to comply with Voting Rights Act requirements or geography, with some areas packed more densely with one side’s partisans. What’s more, motivation is also key in determining if a legislature gerrymandered a map. Plaintiffs will have to make the case that inefficient redistricting was performed with the intent to create an unfair advantage.

It doesn’t seem like that should be difficult. Every exercise in redistricting is an attempt to increase the chances that one side will win or keep control of the maximum number of seats possible. And courts have not shied from calling out legislators for bad intent when it comes to racial gerrymandering. Texas’ congressional map was thrown out in March by a federal court that found legislators “intentionally diluted minority voting strength in order to gain partisan advantage.”

But coming up with a way to statistically prove intent when it comes to partisan gerrymanders has been the missing piece for complainants.

“We needed a test to determine how much partisanship is too much,” says Ruth Greenwood of the Campaign Legal Center, which helped challenge the Wisconsin map.

They may have it now.

*This story has been updated from the print version to reflect a U.S. Supreme Court ruling.