Why Ranked Choice Voting Isn't Great

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Several friends are collecting signatures to put Instant-runoff Voting, branded as Ranked Choice Voting, on the ballot in Massachusetts ( Ballotpedia, full text). I'm glad that an attempt to try a different voting method is getting traction, but I'm frustrated that they've chosen IRV. While every voting method has downsides, IRV is substantially worse than some other decent options.

Imagine that somehow the 2016 presidential election had been between Trump, Clinton, and Kasich, and preferences had looked like:

• 35% of people: Trump, Kasich, Clinton
• 14% of people: Kasich, Trump, Clinton
• 17% of people: Kasich, Clinton, Trump
• 34% of people: Clinton, Kasich, Trump

In a traditional election we'd have 35% for Trump, 34% for Clinton, 31% for Kasich, and declare Trump the winner. Under IRV we would eliminate the last-place candidate, Kasich, and divide their votes based on who was put down in second place:

• 49% of people: Trump, Clinton
• 51% of people: Clinton, Trump

But let's take a look at how people would have voted in one-on-one elections:

• Trump vs Kasich: Kasich wins 65% to 35%
• Clinton vs Kasich: Kasich wins 66% to 34%
• Trump vs Clinton: Clinton wins 51% to 49%

Additionally, some proponents of IRV will say that voting your true preferences will never hurt you: that under IRV you never need to hold your nose and put someone first when really you prefer someone else. But there are cases where you can get an outcome you like more by putting a higher ranking on a candidate you like less. Consider an election like:

• 32% of people: Clinton, Sanders, Trump
• 33% of people: Sanders, Trump, Clinton
• 35% of people: Trump, Clinton, Sanders

Now, this is a terrible election and there's no clear winner because people's preferences point around in a circle. Under traditional voting Trump wins, while under IRV Clinton is eliminated and Sanders wins. But if a few people preferring Trump > Clinton > Sanders had instead voted just Clinton > Trump > Sanders we could have had:

• 32% of people: Clinton, Sanders, Trump
• 33% of people: Sanders, Trump, Clinton
• 32% of people: Trump, Clinton, Sanders
• 3% of people: Clinton, Trump, Sanders

Then Trump would have been eliminated first, with only 32% of the first place votes, and his second place votes would have gone to Clinton, making her win. So by voting for a candidate they liked less, these voters got an outcome they liked more. Staying home and not voting would similarly have helped them.

I do want to be clear: no voting system handles situations like this well, and all reasonable voting systems have situations where you can come out ahead by voting in a way that doesn't reflect your true preferences. But "IRV means you can always vote your heart" isn't true.

So what do I think we should do? Since every voting system has downsides it's not enough to say that a voting system is flawed and so we shouldn't use it. But at the same time some voting systems are still better than others.

For example, a clear improvement over IRV would be to say that if at any point there's a candidate who would win a one-on-one election against every other candidate (a Condorcet winner) then that candidate should win. While it does make IRV a little bit more complicated, it means IRV will no longer eliminate candidates who are the closest there is to a consensus choice. This is called Condorcet-IRV or WoodSIRV, and turns IRV into a voting method that I would be very strongly in favor of.

(I think approval voting is probably a better choice, but it would be good to see more examples of it in practice in competitive elections first.)

[1] A bit of historical irony: in 2008 Greg Dennis, an IRV supporter, wrote in Why I Prefer IRV to Condorcet:

In this scenario, the presence of a candidate with strong core support causes a Condorcet winner with little core support to lose. Fortunately, despite the theoretical possibility of this scenario, the empirical evidence suggests that it is vanishingly rare in practice. In fact, if you look at the ballot data that is publicly available for the IRV elections held in the US (San Francisco, CA, Burlington, VT, and Pierce County, WA), for instance, you'll see that IRV and Condorcet agree on the winner every single time.

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for each other candidate on offer:
    about 2/3 of people prefer Kasich to that candidate

there exists a group of about 2/3 of people that:
    for each other candidate on offer:
         everyone in the group prefers Kasich to that candidate