Why does IRV lead to 2-party domination?

  1. Because it just does: The three IRV countries: Ireland (mandated in their 1937 constitution), Australia and Malta (and more recently Fiji for a brief period of IRV democracy before its coup) all are 2-party dominated (in IRV seats) – despite having many other features in their governments which would seem much more multiparty-genic than the USA with IRV added will ever have. So you can be sure the USA with IRV would be 2-party dominated too.

    E.g. "The composition of IRV seatholders in Australia is very biased against third parties: examining all 564 statehouse & federal IRV seats we found only a single one occupied by a third party."
  2. Australian analysts themselves agree that IRV leads to 2-party domination – this is not just crazy old me talking. A quote from http://www.australianpolitics.com/voting/systems/preferential.shtml is IRV "promotes a two-party system to the detriment of minor parties and independents."
  3. Because here is an example election and here is another in which, if some voters honestly order the candidates N>G>B where N=Nader is the "third party" candidate, then N and G both lose, but if those voters dishonestly vote G>N>B or G>B>N, then G wins (a better result in their view). In these example elections, just like under the present plurality system, voting Nader is therefore strategically foolish even if he is your honest favorite.

    A possible reason why the Australian House remains 2-party dominated after 80+ years of IRV is that these kinds of elections happen often enough so that strategic voters feel logically justified in thus "betraying" Nader. Hence: the third parties suffer a tremendous disadvantage, hence they die off. And hence voters observing this year after year realize the third parties have no chance, which justifies their strategic vote-exaggeration/betrayal decision all the more. (Who cares if you betray N if he had no chance anyhow? It is worth it if it increases the chance G will win – or so they reason.) Result: Vicious cycle – entrenched 2-party domination – third parties die.
    We do not know that this is the actual reason, but we will say that it would be entirely logical if it were; the only way it is not the actual reason is because of the insufficient-logic of Australian voters. If we instead assume more-mentally-challenged voters who don't know or can't understand this, then they will likely just give the two-most-likely-to-win candidates top & bottom rankings – without worrying about whether that really is logical or not – to "max out their vote's impact." That is intuitive and requires no thought at all (but nevertheless is supported by at least some deeper analysis) and with IRV will prevent a third-party candidate from ever winning.

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