I agree with what /u/zestyping says in the other comment, so I won't repeat it. But here's a video explaining the strategic voting issue: https://www.youtube.com/watch?v=JtKAScORevQ
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.
But expressing your preferences doesn't actually accomplish anything useful.
You can do it, but what does it get you?
Instant runoff is not monotonic: there are times when the best strategy is to vote your favored candidate second and your second choice first so that the second choice can push out your least favored candidate.
Actually the same happened in my country. I was thinking if this is a property of voting system. I like this one https://en.wikipedia.org/wiki/Instant-runoff_voting which theoretically helps to choose compromise candidate for most of the voters. But it is just my opinion I don't know how this works in reality.
Yeah, I'm not sure where the qualms about monotonicity are coming from. In an IRV the lower-ranked preferences do not count until the higher-ranked preferences have already been eliminated for not reaching majority
Monotonicity intuitively seems like it would be desirable, but I'm struggling to see how the effect described in the video actually matters in practice. It seems vanishingly unlikely that candidates 2&3 would be eliminated in a tie (and you can just change the rules to force a re-vote/runoff in that case if you really think it's a bad outcome, similar to how you'd handle a tie in FPTP). And even then, the outcome of picking the first candidate when 2&3 are tied doesn't seem explicitly unfair. I'm sure there are other cases where this can show up too though, and maybe some of those do seem unfair.
I can see how ex post, one could say "if I had ranked my choice lower, they would have done better", but it's not clear to me that this property actually informs any real-world voting strategy ex ante.
Basically, this seems like a somewhat academic property that most voters probably wouldn't care about, so it's not clear that a lack of monotonicity would actually impede IRV from popular acceptance.
Instant-runoff voting can punish voters for prioritizing a 3rd party candidate. IRV does not satisfy the monotonicity criterion. See this article [1] for more information.
In general, IRV is a poor system that is almost as bad as plurality voting in terms of how the outcomes reflect the voters' preferences (plurality voting is what the United States uses for most elections).
There are many systems that are better than IRV or plurality. I like approval voting [2], which is a system where you can vote for any number of candidates (instead of one candidate like plurality). Unlike IRV, you don't rank your choices. Approval voting is very simple (the simplest option other than plurality) and produces good outcomes.
I still find the nonmonotonicity of instant runoffs problematic. Something seems wrong about a system where you can lose by gaining votes. I'd expect instability to be higher than a similar system that is monotonic.
> You don’t have to game theory your choice of the lesser of two evils, you rank who you prefer
You "have" [1] to game-theory your choice no matter what the voting system is. Ranked-choice is apparently believed [2] to be less susceptible to tactical voting than some other systems, though. (Not sure if this is quantitative or qualitative.)
> IRV may incentivize forms of tactical voting (such as compromising) when voters have sufficient information about other voters' preferences, such as from accurate pre-election polling. FairVote mentions that monotonicity failure can lead to situations where "having more voters rank [a] candidate first, can cause [the candidate] to switch from being a winner to being a loser."
> "increasing your support for your genuine favorite can actually hurt their odds of winning"
This is referring to the monotonicity criterion[0]. The page has both toy examples and real world ones. It includes some hints about possible violations in Australia, but your voting system isn't very transparent so it is difficult to audit these situations.
As for vote splitting, I think it would be better to look at Favorite Betrayer[1][2]. Sometimes also called the strong spoiler effect. This is actually a big problem. It is why candidates like Bernie Sanders pledge to not run as independent after losing the primary. If you don't violate this condition such pledges are entirely unnecessary.
The problem with IRV is that it asks for a full list of preferences but doesn’t actually respect the voters full preference, and specifically it violates monotonicity.
This short video gives an example that’s easy to understand:
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