You've probably seen a couple of the snappy, well-enunciated videos produced by C. G. P. Grey; he's done pieces on the evil of pennies, the problems with copyright, and various aspects of British history, as well as a number of videos on political theory. I want to talk about his video The Alternative Vote Explained. The Alternative Vote is the British name for what Americans may better know as instant runoff voting, or perhaps as ranked choice voting, which are all mechanically identical; I will refer to it as IRV here. The first three minutes of the video are an excellent explanation of the method, including cute fuzzy animals in an easy-to-follow example. But right at the 3:00 mark, Grey makes an egregious mistake.
Grey claims that IRV's crowning achievement is that, unlike plurality voting, IRV doesn't have spoilers, but that isn't true, and it's very easy for me to show it. Consider first his "spoiler-proof" example. Gorilla and Leopard are running for office; Gorilla is about to receive 45% of the vote, and Leopard 55%; Leopard is going to win. But at this point, Tiger enters the race, and 15% of voters move from Leopard to Tiger, making the vote totals Gorilla 45%, Leopard 40%, and Tiger 15%:
Under plurality, that makes the winner change to Gorilla, and that's bad. Grey's claim, and this much is true, is that under IRV, those 15% of Tiger-favoring voters can have their votes reassigned to their second choice, Leopard, putting everything right with the world by making Leopard the winner again.
But what happens if Tiger is a bit more convincing? Consider the case where Tiger convinces not just 15% of Leopard voters to rank him first, but 30%. That makes the standings Gorilla still at 45%, Leopard 25%, and Tiger 30%:
This leads us into some uncharted territory, because now it's Leopard who is eliminated, and we don't know where Leopard's voters would now go for their second choices. But we can make some educated guesses. Taking a look at the video's earlier example (at the 1:47 mark):
We can see that there is a large contingent—amounting to 25%—of Owl-loving voters, who find themselves torn between Gorilla and Leopard. (Pedantic aside: You can imagine that Leopard gets 15% and Tiger 30% if you want to align this example with our alternate "persuasive Tiger" scenario.) If Owl were running in this race, those 25% of voters would have listed him first. But since he is not, they instead voted for other candidates. It seems that 3 out of 5 owls chose to vote for Gorilla (Turtle's 5% plus Gorilla's 25%, plus 15% from Owl, equals the 45% Gorilla is currently enjoying) while the other 10% are supporting Leopard. There is clearly a split in the Owl community about who the best non-Owl choice is. Suppose that the remaining 10% of Owl supporters, after the elimination of Leopard, re-join their brethren by supporting Gorilla as their second choice in this election, while the other Leopard voters all throw in with Tiger:
That would be a logical assumption. And it means that the final standings are now Gorilla 55%, Tiger 45%:
Let's take a look at that again: If just Gorilla and Leopard run, then Leopard wins. But if Tiger chooses to run, and is able to convince a few more Leopard voters to vote for him, then not only does Tiger still fail to win, he makes it so that Leopard doesn't win either. He actually makes it so that Gorilla wins, which is the exact same problem that we saw with plurality. In other words, we have a candidate who doesn't win, but by entering the race, can change the winner. This is the very definition of a spoiler and it (yes, once again) shows that IRV does, in fact, have spoilers.
Calling C.G.P. Grey
Let me thank Grey for his enlightening videos and his clearly-superior-to-my-own abilities with graphic arts (images not used with permission, but I assert that my usage would qualify as Fair Use.) But this portion of his video is promoting a demonstrably wrong version of the facts, but one which has nonetheless gained a large amount of mindshare and garnered a frightening number of repetitions, both before Mr. Grey's video and more so since. I call on Mr. Grey to address this error, and if he is interested in creating videos to promote actually spoiler-free election method reforms, I would like to point him to approval voting.