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Starting from something shared on a face book feed I came across a claim that a disadvantage of preferential voting is that it disadvantages (reduces the chance of being elected) minor parties and independents. I've seen this on the website australianpolitics.com and https://votingsystemspreferentialandproportional.weebly.com/preferential-pros-and-cons.html

The claim is repeated on both sites as below

Disadvantages of the Preferential System

...

It promotes a two-party system to the detriment of minor parties and independents.

Is this correct, has it been studied?

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    Disadvantages them compared to what other system? – gerrit Jan 23 at 11:42
  • @gerrit first past the post aka simple majority voting.. in preferential voting you give ordered preferences and if 1 does not get in your vote goes to the two preference. Simple majority is only one vote – user1605665 Jan 23 at 20:12
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    @user1605665 - First past the post is notorious for promoting a two party system. The two linked articles are about Australia's federal elections. Australia got rid of the idea of first past the post for its federal elections over 100 years ago. – David Hammen Jan 24 at 9:13
  • @DavidHammen Wikipedia claims that preferential voting was introduced with the purpose to prevent Labor from winning in electorates where two non-Labor parties were running. en.wikipedia.org/wiki/… – Andrew Grimm Jan 25 at 6:08
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I believe this question is based on a misunderstanding of the poorly-expressed claim (which seems to be plagiarised on many sites).

Compared with the distortions caused by First Past the Post voting, Preferential voting gives an advantage to minority parties:

Dr Peter Chen from the University of Sydney said preferential voting allows for a greater number and diversity of candidates.

"In the Australian system using the numbering preference system it can mean that a less popular candidate can be eliminated and the general preferences that the elector has can be redistributed until one candidate receives 50 per cent plus one for the lower house of the votes," he said.

"And Australia really introduced that with an attempt to, in a sense, facilitate a more diverse range of political parties and candidates to run and it's associated with in a sense the growth of some of the minor parties in Australia."

However, the Weebly site listed in the question isn't comparing First Past the Post and Preferential voting.

It is comparing Proportional representation (in the form of a Single Transferable Vote) used in the Australian Senate to elect all of the Senators, with the Preferential Voting used in the Australian House of Representatives to elect individual representatives.

By example, if in every electorate, 40% of each vote for (i.e. give their first preference to) one major party, 40% vote for a second major party, and 20% vote for a minority party:

  • under preferential voting used in the House of Reps, each candidate will effectively be chosen by the minority party voters' second choice. They will all be one of the two major parties.

  • under proportional voting used in the Senate, the number of each parties' candidates will approximate the 40%:40%:20% split.


While the Preferential Voting system avoids much of the "tactical voting" associated with First Part The Post, the system is not flawless. Indeed, Arrow's impossibility theorem suggests it must necessarily have deficiencies.

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  • It appears I did not include the exact page on the weebly site that specifically refers to preferential vs first past the post votingsystemspreferentialandproportional.weebly.com/… – user1605665 Jan 24 at 6:23
  • Arrow's impossibility theorem, at least in its standard form, applies to single candidate races, not to multiple member districts. – Acccumulation Jan 24 at 6:41
  • @user1605665: That was the page I was referring to - it doesn't mention FPTP, but it does mention (in context of the site) Proportional Voting. I am making a generous interpretation, describing their claim as an unclear comparison to Proportional Voting. If you want to make a less generous interpretation, and suggest they are comparing to FPTP, then the claim is flat wrong, and I think the Peter Chen quote covers it. – Oddthinking Jan 24 at 7:03
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    @user1605665: Those people you are hearing from are... mistaken, and haven't learnt their lesson from Nader/Gore/Bush Jr. In FPTP, a spoiler candidate can split the vote, and make the most disliked candidate the winner, which means minority candidates are pressured not to run at all. – Oddthinking Jan 25 at 1:18
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    Arrow's theorem can be generalized to all kinds of things. For multi-winner races with ranked voting systems, that would be the Duggan–Schwartz theorem. – Tgr Jan 26 at 6:57

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