Clean Missouri Shoddy Proportionality

Missouri passed the "Clean Missouri" "Amendment 1" last November, and among other things it changes how redistricting gets done there.
tl;dr: they did some weird things I think will have messy unintended consequences

In 2021 they'll appoint a "non-partisan state demographer" who will do the first pass at map drawing. But the law has a pretty strict set of rules to design to.
In order of priority:

1. Equal Population
2. No racial bias (or could be read to require VRA style gerrymandering-for-good)
3. "Partisan fairness and, secondarily, competitiveness". Fairness being: "the difference between the two parties’ total wasted votes, divided by the total votes cast for the two parties, is as close to zero as practicable." Furthermore, competitiveness: "simulate elections in which the hypothetical statewide vote shifts by one percent, two percent, three percent, four percent, and five percent in favor of each party. The vote in each individual district shall be assumed to shift by the same amount as the statewide vote. The non-partisan state demographer shall ensure that, in each of these simulated elections, the difference between the two parties’ total wasted votes, divided by the total votes cast for the two parties, is as close to zero as practicable"
4. Contiguous
5. Respect existing political boundaries
6. Compactness

Political Fairness and Competitiveness

Their "Political Fairness" is what I call proportionality. It's interesting that they come at it from wasted vote, and I think that's probably a good measure, it's just a little inside out and backwards of how I usually frame this. I'd usually say "the party with 60% of the people should get 60% of the seats in the state legislature", but they're saying "the party with 40% of the people shouldn't have proportionally more of their votes wasted in districts they don't win than the other party wastes in districts it doesn't win". This 'efficiency gap' was a big deal in a recent case in Wisconsin that got to the Supreme Court (but SCOTUS declined to accept the efficiency gap as a sufficient measure for determining a gerrymander they'd overrule). It remains to be seen if the efficiency gap is really a good measure or a statistical fad.

Their measure of "Competitiveness" also comes directly out of recent research into gerrymandering. People have done similar analysis in Wisconsin and Michigan and determined that they disadvantaged party is not only not getting proportional representation but they would have to win huge shifts in the electorate and huge landslides in order to catch up. I'm not sure this specific proscribed simulation is actually a good statistical method though. The law goes as far as describing the statistical model, saying that each district shall be simulated to shift by a specific formula, and I kinda expect we don't yet have the best model for doing those simulations.

Trying To Predict Unintended Consequences

If we're lucky, in the best case, this system will result in a Missouri State Legislature that on average reflects the citizens of that state.

That "on average" is a hedge because I think they'll get a lot of messy districts. By messy I mean non-local, tentacled monsters; the hallmark of gerrymandering. But maybe that doesn't matter? One thing I keep running into around US culture and representative government is that people identify with their place less and less. So, I hear some voices of dissent saying that it's weird to be in a district with some random people over there connected by a squiggly district, but also I hear people not caring about that and just caring about whether they can vote for and elect someone who agrees with their values. If more people can get that, being represented by someone they kinda like, maybe that's a good thing. If the State Legislature more accurately reflects the average of the people in the state, that's a good thing, messy though this way might be.

But it could also lead to 10 years of unmitigated incumbency. Before California moved to an independent redistricting commission, they had a bipartisan gerrymander where everyone had a safe seat and incumbency was rampant. This system could easily produce that. Or it might produce half safe seats and half "competitive" seats designed to be closer to 50/50 between the two major parties. If you're living in a safe seat district, you're stuck with that and your vote (except maybe at the primary) will never really matter.

I'm curious if they'll actually break contiguity or if that will sneak back up to its traditional place next to equal population.

Lastly, with compactness at the bottom of that heap, we can pretty much ignore it.

What About Future Laws?

I also think it's just kinda fascinating that this oddly specific law passed. Missouri Republicans are contemplating a try to repeal it, but if they don't it should be a fascinating experiment in democracy. I guess we'll know how it goes some time around 2022 or 2024. I wouldn't recommend following this law as a model, but hey, let's see how it goes?