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.
Tsebelis. 2002. Veto Players. Princeton: Princeton University Press.
Veto players (collective or individual actors whose agreement is necessary to change the status quo) can be institutional (created by constitution) or partisan (created by the political game). Adding a veto player can make the win set of the status quo (the field of options that can beat the status quo) smaller or leave it the same (depending whether the new veto player is absorbed by another or not). Similarly, adding a veto player can increase the size of the unanimity core (the field of policies that cannot be beaten by unanimous consent of the veto players) or not. In addition, spreading the veto players out more (ideologically) reduces the size of the winset of the status quo and increases the size of the unanimity core. A smaller win set or a larger unanimity core (essentially the same thing) leads to greater policy stability (see figures on pages 22-23). The agenda setter (a veto player who can make take it or leave it offers to other veto players--in other words, the veto player that decides first) has considerable advantage (see figure on page 34). The significance of agenda setting declines as policy stability increases, but increases as the agenda setter moves towards the center (ideologically) of the other veto players (pg 35-36).
Veto player theory can be used to classify regimes instead of looking at presidential/parliamentary, majoritarian/consociational, multi-party/two-party, and so on. See conclusion (283-289).
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