The statistical evidence at http://rangevoting.org/TTRvIRVstats.html seems pretty good that IRV leads to two party domination in IRV elections, while (delayed) top two runoff tends to lead to a strong multiparty system.
Why do those two methods, which seem strategically quite similar, lead to such different results? The above mentioned page has links to some speculations/explanations, which I find less than convincing. The main proposed reasons are: 1. Different strategy calculations by voters under the two systems. Voters who like a "third party" candidate seem more willing to vote for their favorite in the first round of TTR, than corresponding voters under IRV are willing to rank their favorite 1st. Why, why?!? Most IRV supporters in the US have no clue that voting their favorite 1st can ever hurt them. From my limited discussions with Australians, it seems most of them have no idea either. So why aren't Australians voting for third party candidates as their first choices, enough that they might occasionally win? While at the same time, voters in TTR countries feel free to vote for whoever they want, often enough that TTR countries tend to have flourishing multiparty systems? 2. Under IRV, if a "third party" candidate makes it to the last round, it draws little attention from the media. But under TTR, when there is a runoff, both candidates get equal attention for several weeks. This lets the finalists compete on an equal basis, so the third party has a realistic chance to win, and even if hse does not win, the party and candidate will be remembered in the next election. So, is either or both of those the real explanation(s)? Is there some evidence to back up these explanations? (I suppose statements by Australians, Irish, etc. saying that third party candidates rarely get any attention from the press, and statements by Argentinians, etc. that third party candidates _do_ get a lot of attention from the press, would be helpful.) It would be nice to put together some convincing and preferably non-technical explanation about why IRV leads to two-party domination and TTR does not. The purpose is to persuade IRV supporters to leave US TTR elections alone, or convert to something better than IRV. Thanks, - Jan ---- election-methods mailing list - see http://electorama.com/em for list info