> > I believe there have to be only 3 candidates and it has to be a close > 3-way election for the 20% to be valid. > As long as the odds are low enuf, it doesn't matter that much. It just > says that in some cases, some folks will have sour grapes. >
As Jameson says, it depends how you simulate the voters. But there are ways in which I think IRV encourages spoiler scenarios. Consider a city that has a slight Democratic bias. That is, Democrats win by 8-10% most of the time. Step 1) A "Progressive" party starts up that thinks the Democrats have gotten complacent and are not ambitious enough. This resonates with people but they're scared to vote for third parties. Step 2) IRV supporters tell everyone it's safe to rank your favorite first. Some people rank the Progressives first, and nothing bad happens! People gain confidence in IRV and ranking your favorite first. More people start ranking the Progressive first. Step 3) Eventually the Progressive party overtakes one of the top two parties to make it into the final round. The party overtaken is almost certainly the Democrats (because the Progressive party wasn't taking any votes from the Republicans). So who did the Democrats put for their second choice? If even a small fraction of the Democrats put the Republican second, then the Republican will win, and we have had a successful spoiler scenario. (Yes the Progressive was the spoiler even though they made it to the final round.) Voting for the Progressive caused the Republican to win. Progressives should've put the Democrat first. Step 4) Progressives and Democrats get very angry at each other and everyone is angry at the Republicans. A majority is angry at IRV. Yes, it's sour grapes, but it's not hard to see that it's a really bad outcome. A city that starts 45% Republican and 55% Democrat and then the only change is that some of the Democrats put the Progressive first and the Democrat second should not cause the Republican to win. (Even if a few Democrats put the Republican second.) This was described from the perspective of the left, but it could have been done just as easily in terms of Republicans and a "Libertarian" or "Tea" party. All that's required is that the growing third party be a little more extreme than the two major parties (or, as Jameson put it, that the three parties basically fall on a one-dimensional spectrum). Then these steps become natural, even probable. The worst part is that IRV, and IRV supporters by telling people that it's safe to rank your favorite first, encourage the progression down this path-to-spoilers. ~ Andy
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