The mathematics behind Gerrymandering

Erica Klarreich in Quanta:

Partisan gerrymandering — the practice of drawing voting districts to give one political party an unfair edge — is one of the few political issues that voters of all stripes find common cause in condemning. Voters should choose their elected officials, the thinking goes, rather than elected officials choosing their voters. The Supreme Court agrees, at least in theory: In 1986 it ruledthat partisan gerrymandering, if extreme enough, is unconstitutional.

Yet in that same ruling, the court declined to strike down two Indiana maps under consideration, even though both “used every trick in the book,” according to a paper in the University of Chicago Law Review. And in the decades since then, the court has failed to throw out a single map as an unconstitutional partisan gerrymander.

“If you’re never going to declare a partisan gerrymander, what is it that’s unconstitutional?” said Wendy K. Tam Cho, a political scientist and statistician at the University of Illinois, Urbana-Champaign.

The problem is that there is no such thing as a perfect map — every map will have some partisan effect. So how much is too much? In 2004, in a rulingthat rejected nearly every available test for partisan gerrymandering, the Supreme Court called this an “unanswerable question.” Meanwhile, as the court wrestles with this issue, maps are growing increasingly biased, many experts say.

Even so, the current moment is perhaps the most auspicious one in decades for reining in partisan gerrymandering. New quantitative approaches — measures of how biased a map is, and algorithms that can create millions of alternative maps — could help set a concrete standard for how much gerrymandering is too much.

More here.