I'm intrigued. Can you explain how that works? I'm not saying I don't believe you- I'll bet it is possible, but I cannot see how, and I want to!
http://zesty.ca/voting/voteline
Or, if you prefer logic:
The intuition for "something strange could happen" is that IRV only uses some of the information on the ballots. Initially it uses just the top ranking on each ballot, and then at other points in the process, it might flip to using the second ranking on some of the ballots, and so on. There are these sudden trigger thresholds that cause big discontinuities in what gets counted—and that should feel weird. We have this pile of ballots that could all count for A or B; why should that decision depend on some unrelated other ballots?
That's the mechanism by which ranking someone higher can cause them to lose. Only some of the second ranks on the ballots will get to count, and depending which ones get counted, we could get a totally different outcome. Suppose A, B, and C are all competitive in an election, with no one having an outright majority, but A has a little more support than B, who has a little more support than C. It could be the case that A is in a winning position because C supporters mostly like A as their second choice; when C gets eliminated first, those ballots count for A and A wins. But let's say B supporters all like C as their second choice, and you persuade some B supporters to switch to A. If enough of them switch away from B that B is eliminated first, then all those B supporters' ballots will count for C, and that can cause C to win. So by campaigning for A, you made A lose!
The general idea is that if B and C are close, which way it tips can have a sweeping effect on whose second choices get counted, which can affect A. It should feel intuitively scary that a small shift in one part of the election can have a magnified effect on a different part. It's not good for systems to be unstable like that.
Read "22. Does RCV satisfy the monotonicity criterion?"
Say, there's 3 candidates. You want A to win. You think A will lose to B in a 1-on-1 match, but would win C. So you vote C-A-B (or C-B-A, doesn't matter) to increase the likeliness that B got eliminated in the 1st round. Of course, you're now saying that you prefer C over A in a 1-on-1 race. But that's fine because other people will vote for A.
Concretely, let's say this is people's actual choice.
31 B-A-C; 39 A-B-C; 30 C-B-A
B wins.
Now, if 2 people preferring A (voted A-B-C) strategically change their vote to C-B-A.
31 B-A-C; 37 A-B-C; 32 C-B-A
Now, A wins.
Consider how little trust a lot of people have in polling. You would need a very high amount of trust in it if you were going to dishonesty rank your choices based on it. And if strategic voting is going to be based on polling, wouldn't we expect people then start strategically lying to pollsters?
The system has too much chaos.
What strategies will they employ in RCV elections? I don't know, but given the way campaigns already operate with continuous polling, elaborate voter databases, and modern ad-tech that can target exactly the right message to exactly the right people to influence their behavior slightly, I see election systems with wonky outcomes as an attack surface against democracy.
> The system has too much chaos.
There's too much fog-of-war for individual voters to make optimal strategic voting choices, but the Cambridge Analyticas of the world can use that to their advantage. I don't like that information asymmetry, and it seems the best way to defend against it is to have voting systems that are as close to the ideal (which is that candidates win by having the most public support) as possible.
In reality most preferential voting schemes have many candidates, and there's too much chaos to coordinate this kind of scenario.
Meanwhile plurality voting has candidates winning with way less than 50% of the vote, and claiming some mandate from god.
The sooner we ditch FPTP the sooner we get to something like Condorcet, or STV with multimember districts.
Well, that's kind of the point of the example. A normal person who wants A to win would put A as their first choice. But doing that can actually cause their preferred candidate to lose, which is exactly the opposite of the voter's intent. FPTP has a different set of problems that one could argue are worse, but one of the few good things about FPTP is that you can't say about some election result "that candidate lost because I voted for him/her".
Non-monotonic results don't happen in every election, but they're something that can happen.