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framsey 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.
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.