[QUOTE=t-bonham@scc.net]
The major factor producing a two-party system is a structural one: the fact that we elect the Executive branch in a separate election from the Legislative branch. Thus we have a President elected separately (even for a different term of office) than other countries, where they have a Prime Minister chosen from the winning party (or coalition of parties) in the Legislative branch.
This means that there is no structural force supporting third parties; indeed, this government structure works to make them irrelevant. So any third parties that arise either die off eventually, or they replace the weaker one of the existing 2 parties (like the Republicans replacing the Whigs).
All the countries that have third (and more) parties have a Prime Minister form of Executive. (They might have an office titled President, but it is basically a figurehead.)
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No – it’s the other way round, oddly enough. When you have a parliamentary system, in which the legislature chooses the prime minister, there is far more party discipline in the legislature, and the legislature polarises into government versus opposition. Even if there are multiple parties, in the longer term they teld to form stable coalitions.
And if you look at the US Congress, even though there are nominally two parties, in fact both houses are full of people acting pretty independently, with coalitions forming from time to time around various issues. The only time when party discipline becomes important is when electing various office-holders (e.g., the Speaker and the chairs of the committees).
There are whole organisations devoted to tracking the voting records of members of the US Congress. No one (except perhaps the party whips) tracks the voting records of the British House of Commons, because practically all the time they vote rigidly on party lines.
If you changed the voting system for the US Congress to bring in instant run-off voting or proportional representation, you’d find members of the US Congress behaving even more independently than they do now, as well as minor parties splitting off in every direction.