How Computers Turned Gerrymandering Into a Science

How Computers Turned Gerrymandering Into a Science, by Jordan Ellenberg.

About as many Democrats live in Wisconsin as Republicans do. But you wouldn’t know it from the Wisconsin State Assembly, where Republicans hold 65 percent of the seats, a bigger majority than Republican legislators enjoy in conservative states like Texas and Kentucky.

The United States Supreme Court is trying to understand how that happened. …

Gerrymandering used to be an art, but advanced computation has made it a science. Wisconsin’s Republican legislators, after their victory in the census year of 2010, tried out map after map, tweak after tweak. They ran each potential map through computer algorithms that tested its performance in a wide range of political climates. The map they adopted is precisely engineered to assure Republican control in all but the most extreme circumstances.

In a gerrymandered map, you concentrate opposing voters in a few districts where you lose big, and win the rest by modest margins. But it’s risky to count on a lot of close wins, which can easily flip to close losses. …

To gain control of the State Assembly, the authors estimate, Wisconsin Democrats would have to beat Republicans by 8 to 10 points, a margin rarely achieved in statewide elections by either party in this evenly split state. As a mathematician, I’m impressed. As a Wisconsin voter, I feel a little ill. …

Republican legislators argue that any Wisconsin map will look biased, because Democratic voters tend to congregate in big cities like Milwaukee and Madison. That packs the Democratic half of the state into a small cluster of districts. “Why are you gerrymandering yourselves?” they ask.

They’re partly right. … [But] the Wisconsin district map … is an “outlier” — so far outside the ordinary run of things that it can’t be mistaken for a map without partisan purpose.

Research by the political scientist Jowei Chen suggests that the Wisconsin district map does much worse on traditional districting criteria than neutral maps do, despite the Wisconsin Constitution’s requirement that districts be “in as compact form as practicable.”

Outlier detection is a critical part of data analysis, and mathematicians have gotten really good at it by now. That’s the good news about advanced computation: You can use it to make electoral mischief, but you can also use it to detect and measure that mischief. It’s not math versus democracy; it’s math versus math, with democracy at stake. …

Now that gerrymandering is organized and detectable by rocket scientists, will it be outlawed? Will it spread throughout the democracies?

This is a momentous case with major implications for American democracy. …

There will be many cases, maybe most of them, where it’s impossible, no matter how much math you do, to tell the difference between innocuous decision making and a scheme — like Wisconsin’s — designed to protect one party from voters who might prefer the other.

Writes a UK reader:

We have had the same thing in the UK with Parliamentary constituencies, although I doubt there is anything like the same degree of sophistication behind it. The Labour party was very upset by the last set of boundary map changes which made an effort to give constituencies about the same number of voters and are set by an independent commission. Previously, some northern constituencies, usually Labour strongholds, had as little as half the voter numbers of mainly southern constituencies. It is one obstacle to a Corbyn victory and on the face of it seems to be a reasonable one.

hat-tip Bob