Why do proportional representation voting systems not evolve to first past the post systems?

Question

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?

Answer 1

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.

Answer 2

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

Answer 3

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?