The web page listed some interesting factors that may have effect but
I think it didn't provide a complete plausible explanation to the two
party domination questions and the role of IRV and top-two runoff in
this.
> Why do those two methods, which seem strategically quite similar,
lead to such different results?
I think the methods are behaviourally closer to each others than what
the web page said. There are also other reasons behind two party
domination. I'll try to address some of them below.
> 1. Different strategy calculations by voters under the two systems.
I didn't find convincing evidence here. There are many strategies the
voters could try, sensible and less sensible. Some of them, like part
of supporters of one candidate voting another one, are very hard to
control successfully in large public elections.
> 2. ... last round ... attention from the media
This may have some impact but I don't expect this influence to so big
that it would clearly differentiate the two methods.
Some other observations:
The number of analysed IRV countries was small and many of them have
their own peculiarities. A more detailed analysis would be needed to
make conclusions from them.
The page indicated that the listed top-two runoff countries use that
method for some presidential elections. I guess most of them use
other methods to elect the (multi-party) parliament.
The parliaments (and their "multi-party members") are often elected
in multi-winner elections, not single-winner (like IRV and top-two
runoff). I'd say that the division to countries using single-winner
districts vs. multi-winner districts (+ a proportional method) is a
more important explanation to why some countries become two-party
systems than the election method that is used in the single winner
(president or parliament) elections. I believe this is the situation
in many of the listed multi-party countries.
In the US the president forms his own government (that is typically a
single-party government) but in many other countries the government
is not linked to the presidency (and may be a multi-party
government). Therefore the presidential elections in many cases don't
have much impact on the two-party vs. multi-party question.
The discussed two methods both favour large parties. Favouring large
parties may lead to two party domination but not necessarily. In the
case of the top-two runoff method there could be e.g. 3 or 4 parties
that can make their way to the second round (depends on the country,
its history, and the nature of the election). In countries where the
president forms the government people are probably more loyal to
their favourite big party than in coutries where the presidential
post is more ceremonial or just "one man's post" (and the government
will be elected via other routes). In the second type presidential
elections the personal characteristics of the candidates play a
bigger role (which leads to more parties having a chance).
In summary I'd say that IRV and top-two runoff favour big parties but
having single-winner districts is a more important factor in making
the two-party countries what they are. Condorcet, Approval and Range
may elect centrist small party candidates quite easily but the two
discussed methods tend to eliminate them. The differences between the
two discussed methods are maybe not that radical. Combinations like
single-winner districts + Condorcet would probably lead to compromise
candidates in some districts but major parties (and centrists) would
still have an advantage (when compared to fully proportional multi-
winner methods).
Juho
On Feb 23, 2007, at 13:17 , Jan Kok wrote:
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
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