First-past-the-post tend to lead to two-party systems while proportional representation tend to lead to multi-party systems. But you can’t have proportional representation in presidental elections since only one candidate can win. Countries with multi-party systems tend to have parlimentary systems.
And yet the French somehow manage to have a multi-party system while citizens directly vote for the president. And they also have a parliament, as does the USA.
> And they also have a parliament, as does the USA.
A "parliamentary system" is not about just having a parliament (an elected legislature), it is a name for a system where the elected parliament is the paramount power in government, including choosing the head of government. The French have something close to that, the US does not.
The French have a semi-presidential system where, as in a parliamentary system, the head of government is the PM elected by the parliament and formally appointed by the President who is the head of state, the US has a (very strong) Presidential system, where the President is head of government (as well as having a Presidential election system that, even ignoring the role of the President, inherently favors duopoly more than the two-round French system.)
