OK, that question is probably way too ambitious for a blogpost. There are dozens of types of democracy, so let’s just look at two types: presidential democracy and parliamentary democracy (PrD and PaD for short in what remains). And that means not only limiting the scope of the investigation but also simplifying it: there are many different types of PrD or PaD (the Westminster model is one form of PaD, the US system is one form of PrD). But that’s what you have to do if you want to keep your blogposts relatively short and readable.
Moreover, separation of powers is an enormously complex topic as well, so again I’ll have to simplify. I’ll focus on two of the three powers that are traditionally distinguished: the executive and the legislative powers and ask how separation between these two powers is compatible with PrD and PaD.
First, why is separation between these two powers an important value? For the same reason that separation of powers in general is important: to create checks and balances and to pit different powers of the state against each other so that there is less risk of tyrannical government and collusion of different powers against the people. The executive power, which normally executes the laws voted by the legislative power, usually also has a veto power against certain acts of legislation in order to limit the risk of oppressive or unjust legislation. Sometimes, when it gets very bad, the executive can also disband the legislative power and provoke new elections. Conversely, the legislative power often has the power to demand accountability and transparency from the executive power. If the legislative believes that the executive power acts in impermissible ways it can vote laws that make those acts illegal. And so on.
What are the main differences between PrD and PaD? In a PrD – where you have of course also a parliament – the executive power is elected directly by popular vote. People elect a president and this president selects her government. The people also elect members of parliament in separate elections.
In a PaD, the executive isn’t elected directly by the people. The people elect only the members of parliament. The political party (or parties) that manage to get a majority of elected members of parliament then form a government (often after coalition negotiations between parties when there isn’t one party that has managed to acquire a majority of representatives in parliament).
A PrD seems better able to respect the separation between the executive and legislative powers. A president doesn’t sit in parliament and doesn’t rely on the approval of the legislative for her political survival and hence she is unlikely to always have the same views as the legislative majority. Checks and balances can work. She has an independent mandate from the people and she can have a view that’s different from the view of the parliamentary majority. In PaD, the executive is a product of a parliamentary majority. It’s often even composed of some members of the parliamentary majority who sit both in the government and in parliament. Therefore, it isn’t common in a PaD for the executive to counteract the legislative or vice versa. In a PaD, these two powers are more or less the same. The executive is the parliamentary majority and parliament as such is systematically in agreement with the executive. It’s only the parliamentary minority that can voice opposition. But that’s it: it has a voice but it can’t effectively block executive initiatives, since it’s merely a minority. The act of legislation often originates in the executive that in fact has the power to enact whatever legislation it wants since it automatically has the support of the parliamentary majority. Why is that the case? Members of the parliamentary majority who aren’t part of the executive are often second rate party members who are easily persuaded to approve the legislative initiatives of the executive because their political career depends on the support from the senior party members who make up the executive. For the same reasons, the accountability and transparency requirements are often sidestepped because the parliamentary majority doesn’t want to embarrass the executive.
So, PaD abandons part of the separation of powers in order to gain efficiency. Parliamentary systems, compared to presidential systems, can act in a more decisive and a quicker manner (in theory at least). In a PrD, the president can block legislation coming from parliament or can have her legislative proposals voted down by a parliamentary majority.
However, this efficiency advantage of PaD compared to PrD is often lost when coalitions are necessary. This is why some parliamentary systems avoid proportional representation – which tends to produce more than two political parties with representatives in parliament – and use some kind of district system combined with first-past-the-post elections – which tends to lead to two party systems and hence avoids the need for coalitions.
Of course, this doesn’t mean that PaD doesn’t have any separation of powers at all. It usually has an independent judiciary that can act as a counterweight and that can use for example judicial review to invalidate laws that are incompatible with the constitution. So it really isn’t easy to say which system is preferable. Efficiency is perhaps just as important as separation. Yet the direct election of the executive, which is typical of PrD and also the basis of many of the advantages of PrD compared to PaD, generates more popular control and hence more democracy, and that is important as well. And finally. PrDs are more stable. So on balance I think I prefer PrD.