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The first past the post voting system tends to favour a two-party system.

Suppose an election is held using a proportional representation voting system. Then suppose that the two largest parties, between them, command a majority. Why wouldn't they form a grand coalition and convert the voting system to first past the post for their future benefit?

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    This question is too broad because it does not pertain to a single country. The reasons for staying on proportional representation may vary by country. I haven't researched enough countries to know, but I suspect in a significant amount of countries this would require a referendum or amendment which cannot be passed by a simple majority. Further in some cases the second most popular party may not agree to switch the representation because first-past only benefits the most popular. – Braydon Oct 8 '17 at 18:29
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    The fact that two parties may be the largest in one election does not guarantee that they'll be the two largest parties in the next election. – phoog Oct 8 '17 at 19:14
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    The question is not too broad. I guess it doesn't happen so often, because the premises (two parties with a strong majority, constitution allows changing the voting system) are not given that often and otherwise if a voting system is changed it is rather abolished or at all. The question should mention more cases where this potentially could have happened. – Trilarion Oct 8 '17 at 21:28
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    It's understandable to have the cynicism reflected in this question regarding the motivations of politicians, and at the same time, not all politicians are completely self-serving. – Todd Wilcox Oct 8 '17 at 23:22
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    This is similar to the question, "A single party state favours the ruling party, why don't voting systems evolve towards a single party state?" Of course, this does happen sometimes, but I wouldn't call it evolution. – Jodrell Oct 9 '17 at 11:20
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  1. Voters may not like it. Particularly voters who prefer one of the other parties. Note how German voters went for AfD in areas where CSU wasn't available. This could be seen as a reaction against the lack of more moderate options, as polling suggests that many of those voters preferred CSU.

  2. The politicians may of course feel that the current system is more representative.

  3. First-past-the-post also changes how politicians are selected as representatives of the party. So the politicians that currently represent the party may not continue to represent them after the switch to first-past-the-post. In particular, first-past-the-post blocks two politicians from the same geographic district.

  4. The coalitions change. For example, Ronald Reagan defined the Republican party as favoring economic freedom (primarily low taxes), morality (e.g. opposition to abortion), and a strong military. But prior to that, the economic freedom folks weren't against abortion or for a strong military. Many of the old candidates would later lose, as their positions didn't match the new party.

  5. It may shift the balance. Of the two largest parties, it is likely that one is in the current coalition and the other is not. Shifting the coalition building from the legislature to the voters may cause that to shift. The dominant party may lose its coalition.

  6. Politicians tend to be cautious about such things. They won under the old system. They probably want to keep it.

In general, voters tend to react negatively to decisions by politicians that look to help the politicians more than voters. This gives political opponents an easy issue without improving relations with one's own voters at all.

Even if the switch does help the party (and it may not), politicians may still oppose it if it hurts them.

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    "Note how German voters went for AfD in areas where CSU wasn't available." In areas where CSU was available, they did so too in comparable intensity. – Trilarion Oct 8 '17 at 21:18
  • @Trilarion This does not invalid the point made. – NotTelling Oct 9 '17 at 9:56
  • @NotTelling Really? I think it indeed invalidates point 1. Of course not the whole answer. Point 1 seems to suggest that AfD voters would prefer CSU instead. However, results in areas where CSU are available indicate this is not the case. AfD voters seem to prefer AfD itself. – Trilarion Oct 9 '17 at 10:40
  • @Trilarion As stated in the answer, I remember a poll, which was made on the voting day, which suggests that about 70% of AfD voters did vote for the AfD, because CSU was not available. – NotTelling Oct 9 '17 at 10:52
  • @NotTelling Somehow I doubt the results of that poll are right. In the area where the CSU was available, AfD got a quarter of the votes of the CSU (only slightly less than in neighboring federal states). This is is quite incompatible with the results of that poll. – Trilarion Oct 9 '17 at 11:20
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The assumption in the question would seem to be that the two largest parties in a proportion representational parliament would become the two largest parties in a first past the post parliament. While this is quite logical there are many things that might mean this is not the case.

For the top two parties in PR to come out at the top in FPTP they would need a majority of seats this might not reflect well how their votes are currently distributed i.e. 20% across a country doesn't necessarily main 20% in all seats or in one specific seat and 20% in one seat might not be enough to win it anyway

The tendency in FPTP for the voters to choose the top two parties/candidates arises from the perception that other parties aren't likely to win in their seat. This is a position that is likely to take time to set in and while this is happening other parties may maneuver themselves into leading in the most seats.

other points to consider include:

  • Candidates elected under one system are not likely to wish to abolish it in case the replacement system leads to them losing their position
  • Some countries require either a referendum or larger then usual majority for such major constitutional change. The two larges parties may not be able to produce the majority
  • FPTP is/seems/may be perceived to be less democratic and therefore a backwards step
  • This is a significant change in organisation terms for political parties and perhaps in bureaucratic terms (depending on how it would be implemented)
  • This would be a massive sweeping change requiring lots of explaining to the public lots of parliamentary time and so forth - the two main parties may feel there are more important things to do
  • Popularity - if this isn't popular with the electorate this could cause a swing against the parties leading to lost seats

TLDR; They don't do it because it's a lot of effort and the two biggest parties might not win afterwards

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    "FPTP is/seems/may be perceived to be less democratic and therefore a backwards step" Looking at the world at large, I feel like backwards steps are not so uncommon. Sometimes they happen. My hope is that on average there is still movement forward, but on that probably history will have a say. – Trilarion Oct 8 '17 at 21:31
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According to Duverger's Law, the first past the post voting system favours the two largest parties in an election.

No, it doesn't.

Per Wikipedia:

In political science, Duverger's law holds that plurality-rule elections (such as first past the post) structured within single-member districts tend to favor a two-party system

There's a world of difference between "favors a two-party system" and "favors the two largest parties". Also:

  1. Parties would be able to institute FPTS only if they're winning under PR, and if they're winning under PR, why would they want to change the rules of the game?
  2. This sort of drastic change generally requires a supermajority.
  3. They'll go into the next election having to deal with charges of trying to steal democracy.
  4. Politicians generally like to be able to pretend, if only to themselves, that they're working for the greater good, not just cynically manipulating the system for their own benefit.
  5. If politicians are just cynically working for their own benefit, why do they care what's good for the party?
  • Thanks for pointing this out @Acccumulation, and I've now edited my question. However, I still don't see how 'favors a two-party system' is so different from 'favors the two largest parties'. When I click on 'two-party system' I get to the Wikipedia page that reads 'A two-party system is a party system where two major political parties[1] dominate the government'. – Martin Oct 9 '17 at 5:54
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    @mlm: because the largest parties under PR may not be the largest under FPTP. See Germany 2017 election: CSU had 5% of votes and would get 5% in FPTP (i.e. they won all their districts), FDP had 10% in PR but would get nothing in FPTP. – kat0r Oct 9 '17 at 8:28

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