Like Day and Night? Party Unity in Legislative Voting in Parliamentarianism and Presidentialism

Using statistical methods (e.g. Praise-Winsten regression and logistic regression) common hypotheses about the party unity in parliamentarian and presidential regimes are tested. For this purpose an extensive compilation of data on party unity in democracies is used. Parliamentarianism proves to be a (nearly) sufficient condition for high party unity. Presidentialism turns out to be a necessary condition for low party unity. But to suppose that there is always no or little party unity in presidentialism is wrong. It is hard to imagine modern democracies without parties and in modern democracies agenda-setting in legislatures is dominated by the governments. Still the condition “presidentialism vs. non-presidentialism” has by far the biggest explanatory power for the differences of party unity in different countries. The absence/presence of a competition between candidates of the same party during national elections (whereby the authority of party leadership is constricted) explains the large variation of party unity in presidential democracies. In parliamentary democracies this factor has only a marginal effect as a non-presidential government nearly guarantees a high level of party unity.