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bryjnar comments on Imperfect Voting Systems - Less Wrong
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I think people often dismiss systems like STV/IRV by essentially saying "Arrow's theorem implies you can still vote tactically, so it's just as bad". But there's a big difference: in STV it's much harder to figure out how to vote tactically.
In First Past The Post systems, tactical voting is blindingly obvious: if there are two candidates you like, but you don't think that your favourite has enough popularity to win outright, then you should vote for the other one, to avoid splitting the vote. This is easy to understand, and it's also easy to detect circumstances where it would be beneficial for you to vote other than your preferences.
OTOH, even though there are times where you can vote tactically in STV, they're harder to understand, and crucially, it's much harder to recognise such opportunities: you need a lot more information.
This means that, in general, STV would cut down on tactical voting a great deal, simply because it makes it harder.
This. In FPTP, tactical voting will have a major influence on the results, since it's so obvious and thus lots of people will end up doing it. In STV, most people won't even realize that the opportunity for tactical voting exists, so the amount of people doing it will be much smaller.
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Because in electoral politics, you are not a unique snowflake. Failure to take into account collective action of people thinking along the lines you're thinking drastically understates the influence of those thoughts.
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It's not that your decision affects the others. That's messed up causality. It's that your decision shares a lot of its causes with other decisions in other people. If you take yours as typical of a particular subset, and decide not to vote, then that suggests that others in that subset might as well. Gathering a large bloc of equivalent voters raises the voting power from approximately 0 to... well, some nontrivial number. If it's really large, it's exactly 1 (voting power is not probability, so 1 is a legit answer).
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It's not 'will be the same regardless of which side I come down on', like we're in a state
A |Luke votes & Joe votes> + B|Luke doesn't vote & Joe doesn't vote>
It's 'Is this a good plan of action? Let's look at its consequences. First order (if I do it): OK. Second order (if everyone does it): not OK.'
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Possible counterexample:
In the USA, tactical voting for President is usually completely impossible, due to the electoral college system. Your vote only affects the election outcome if the election as a whole is a toss-up and if your home state is in play. But for a Texan like me, that will never occur, not even relative to the one-in-a-million odds you'd normally expect from a national election. If the Democratic candidate stands any chance of winning here, they'll be winning the rest of the country in a landslide. Likewise for a Californian: any situation in which the Republican might win California is a situation where they've surely already won nearly everywhere else. Likewise for most of the country; only a handful of states, known well in advance, are "swing states" in a given election.
Yet, other than a few weirdos like me, does anyone use this freedom-means-nothing-left-to-lose situation to vote for a third party and thereby at least influence news results? Granted that most people usually prefer the two leading parties' candidates, but I haven't seen any statistics which would suggest that the minority of voters who disagree are likely to vote their consciences except in the swing states.
It may be that, even if many Democratic/Republican voters would legitimately prefer other alternatives, they appear to vote tactically due to some other bias: the human desire to "vote for a winner" tempered by the desire to avoid voting for a completely unpalatable candidate, or the desire for group affiliation expressed in this case via political party identification, perhaps.
What baffles me is that even the news media here does first-past-the-post polling! Asking people to give ranked preferences to generate official election results might be a serious legal undertaking, but why in the world doesn't a single major news company or polling organization ask for ordered preferences either, even for primary elections when there are often a half dozen serious contenders in play?
I imagine that if polls showed that we were in a situation where strategic voting might be useful for people with certain preferences, the news media would report on it and people would learn about it.
I can see the headline now: "Mathematician says that if your preferences are 'A > B > C', you should vote 'B > A > C' in November!"
Such situations could be recognized by poll questions like "What is your preference ordering over these 3 candidates?" Candidate B's campaign would have a large incentive to publicize this information.
I think the more important point is that you simply require too much information about too many people's preferences to be able to do that, except in rare cases. The amount of data you'd need would be tantamount to just knowing how everyone would vote. If you know that, then this sort of thing might be feasible, but that's a big ask!
In IRV, cases of clear opportunity for strategic voting are not at all rare, nor are they hard to detect. All you need is to see that a compromise candidate is in third place by lead preference, and that your wing candidate would probably lose in the runoff. This is hardly inaccessible information, requiring only one horse-race poll and the most obvious head-to-head poll.
What you face is a flat cost in terms of settling for a non-first-choice, in return for bolstering your chances of avoiding a strongly non-preferred outcome. It's a chicken strategy, rather than the berserker strategy that's your only opportunity with Condorcet.
In effect it pulls IRV back towards FPTP in terms of voting behavior. Not all the way, to be sure - you're perfectly safe putting the extremely silly party on the top of your ticket, and if there are more than three major parties, each with a credible chance at winning, it gets to be sufficiently difficult to project consequences that the benefits of defensive strategy are no longer clear.
OTOOH, the people who do figure it out effectively get more power over choosing the result than people who don't. In most democracies, this would be considered a negative. Not that real-life elections are totally fair either, of course.