@freemo why do you call it a myth? From what I understand, the mechanics are such that there are practical risks involved, concerning votes cast to "lesser"/"minor" parties concerning wasted/lost votes when the minor party loses.
Its a myth because it is only the beleif in it that creates the effect at all. If no one thought there was a two party system there wouldnt be one.
Few points of evidence:
* Before people in the USA thought there was a two party system, there wasnt. Despite the voting system not having significiantly changed prior to the last 100 years the primary parties changed all the time. Throughout US history the 2 dominate parties have been replaced 8 times
* Other countries with a first-past-the-post voting system do not show a tendency to a 2-party system. There are countless elections around the world using this approach that dont consistently have the same 2 parties win
* Even if you model out the debunked theory claiming FPTP results in 2 party it doesnt make sense. Under that model it would only produce an illusion of a two party system (where the real support concentrates in the top 2 in votes). Nothing about the model would keep the same 2 parties in power, it would just cause the switching between parties to be abrupt (third parties with little votes in previous years suddenly jumping to 51% support over a single election). So even in pure theory if we accept the myth in that context it is still not real in any meaningful way.
Yes the "there were always 2 parties" is the illusion I spoke of.
In a FPTP system if the underlying real support for parties is red: 40% blue: 39% lib: 21% then the vote will come out as something like red 51%, blue 49%. However in the next election if the real preference shifts than the vote will immediately and drastically change as well to reflect the new party.
In other words, while the votes themselves will give the illusion of a two party system by converting true support to a 2-party vote outcome not reflecting true support. The real underlying support will not reflect a 2-party system and there will be **no** preference for the same 2 parties to win from one election to the next.
We see this with the historic changes in parties, in every election where a new third party becomes dominate within a single election a previously near 50% party shoots completely down to less than 2% and the third party immediately shoots up to near 50%. In other words the numbers move rapidly and shifts in primary parties easily change.
So again this doesnt reflect a 2-party system, just an illusion that reinforces the myth and thus is self fullfilling.