Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I even say who wrote them. If you have more recent summaries to add to this collection, send them my way I guess. Sorry for the ads; they cover the costs of keeping this online.
Shugart and Haggard. 2001. Institutions and Public Policy in Presidential Systems. In Presidents and Parliaments, Haggard and McCubbins, eds. Cambridge: Cambridge University Press.
Presidentialism has separation of power, but varies when it comes to separation of purpose (see below for definitions). Having both separation of power and separation of purpose introduces the potential for deadlock, instability, and balkanization; there are many veto players, so any compromises reached in this case must be based on broad consensus, making the status quo very difficult to change. However, if there is extreme unity of purpose in presidentialism, then it can be highly decisive (perhaps dangerously so) as the branches of government (divided in power but united in purpose) enact radical changes to policy.
Hypothesis: few presidential regimes are located at either of these two extremes, yet the occurrence of the two extremes "has fueled a substantial literature claiming that presidentialism is dysfunctional or even 'perilous'." Most presidential regimes are between these extremes, thus tame.
See Cox and McCubbins (2001, from the same volume), "The Institutional Determinants of Economic Policy Outcomes." It is the chapter immediately preceding this one, and it defines "separation of power" and "separation of purpose" in a more general sense before this chapter applies the same distinction to analyze presidential policy more closely.
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