>How and why though?
Democracy is messy.. it requires deliberation, cooperation and compromise with multiple competing interests in order to effectuate policy. The "direct route" nature of elected executives is very tempting for those who don't have the patience to compromise. We often see this in the US, with vocal partisans calling for unilateral executive branch actions instead of the hard work of passing legislation. This tends to appeal to populists, who are generally motivated by only a few unrefined positions, and who view experienced legislative negotiators as corrupt, elitist, and establishment. Likewise, ambitious politicians find common cause with such homogeneous groups of populists as they offer a source of political support that is easier to shepherd than with coalition building.
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>According to Presidentialism, Parliamentarism, and Democracy by Jose Antonio Cheibub, presidential systems and parliamentary system have very similar survival rates, and that the incentives presented with presidentialism is as conducive to democracy as the ones in parliamentarianism. The reason why presidentialism has tended to fail more is that it as arisen in places where the conditions for democratic institutions were not favorable.
Look, there are a lot of people who refused to take Juan Linz seriously (people like Cheibub) because they would always say "yeah, but the US is a huge counter example". Well, look around... those people spoke too soon. The accelerating degradation of the political system in the US is ample evidence that it too ultimately succumbs to the perils of presidentialism. I've read Cheibub's works and frankly I'm not convinced by his arguments. He has a tendency to wave off the symptoms of presidentialism as being unrelated to presidentialism, and he approaches his comparison as an oversimplified statistical analysis of regime survivorship. Rarely does he go to the trouble of dissecting the actual mechanisms of democratic power sharing and accountability, which I've described in depth above.
I particularly don't buy the argument that presidentialism has merely had the misfortune of being co-located with democratically unfavorable regions. It exists across multiple continents with very few (really only one) example of providing good democratic outcomes. Throughout South/Central America, Africa, Central Asia, etc.. presidentialism is how you dress up dictatorship as a democracy.
I'm also not sure survivorship is the right measurement to be considering, which is very often subject to exogenous effects (proxy wars, etc). Presidential regimes also tend to excel at concealing functional authoritarianism behind nominal democratic survival.
Ultimately though this debate can just be reduced to a question of power mechanics. Representative democracy isn't about simply funneling the electoral will into a single power wielding individual. Democracy is about deliberation and power sharing, things which only a representative legislature can provide but which an elected executive negates the meaningful exercise of.
>an elected single member executive undermines the democratic legitimacy of the elected legislature. This is due to the lone executive being able to claim a parallel source of legitimacy as well as exclusively representing the whole populace while members of the legislature each individually represent some segment of the populace.
This is kind of a subjective point. It isn't a inherent quality of the system itself, but rather a possibility of the people who occupy that system. Also, what is "the parallel source of legitimacy," mentioned?
>an elected single member office creates a false sense of representativeness, as that office holder can only truly represent the interests of their co-partisans, leaving ~50% of the population unrepresented by that office.
Why only 50% though? You can win a election win with more or less than 50% of the population. As well, why "only the interests of their co-partisans?"
>the executive branch is the most physically dangerous branch of government and must be under continuous accountability to a supervisory body (the legislature) that can remove it at will.
Why is the executive branch of government the most physically dangerous branch of government? Also, couldn't the executive be supervised by more than one branch of government, like the judiciary or a separate supervisory branch?
>democratic elections are not an instrument of executive accountability, as they only occur every few years.
If democratic elections held periodically can't hold the executive to account, than what would you say about legislative elections?
>presidential elections invite populist demagoguery.
How and why though?
>empirically, presidential systems across the world have been far more prone to corruption and authoritarianism than parliamentary systems.
According to Presidentialism, Parliamentarism, and Democracy by Jose Antonio Cheibub, presidential systems and parliamentary system have very similar survival rates, and that the incentives presented with presidentialism is as conducive to democracy as the ones in parliamentarianism. The reason why presidentialism has tended to fail more is that it as arisen in places where the conditions for democratic institutions were not favorable.