本文檢視全世界民主國家在不同總統與國會選制的搭配組合下，政黨體系有何不同。本文發現，就國會選制而言，國會選舉採比例代表制的國家， 其國會有效政黨數在整體上明顯多於國會選舉採單一選區相對多數制的國家;而不論是國會選舉採比例代表制或是採單一選區相對多數制的國家，有總統直選制度之國家的國會有效政黨數，明顯少於無總統直選制度的國家。 進一步言，在國會選舉採比例代表制的國家中，若總統選舉採相對多數制，其國會有效政黨數會少於總統選舉採兩輪決選制的國家;不過，在國會選舉採..

This study comprehensively observes the party system under different collocations of presidential and parliamentary electoral system in democracies all over the world. Regarding parliamentary electoral system, it is found that the effective number of parliamentary parties in the countries adopting proportional representation system(PR), overall, is apparently larger than that in countries adopting plurality with single-member-district system(SMD), and that in countries holding direct presidential elections is clearly smaller ..

傳統的研究智慧中，總統制和兩黨制的配套被認為是一個民主國家較容易運作的組合。主要的理由是兩黨制較不可能出現意識形態的極化，以及因為兩個政黨需要贏得中間位置的選票，其可以促成政黨之間較溫和及向心的競爭，因而有助於總統制的運作。然而，近年來，一些兩黨總統制國家發生嚴重的憲政危機，甚至促成民主衰退。為何這些兩黨總統制國家會走上民主衰退的道路，是本文所要探索的研究問題。首先，本文透過對於拉丁美洲兩黨總統制國家的個案分析發現，憲政結構的因素如總統和國會的權力抗衡會影響..

Past research argues that presidentialism and two-party systems are workable combinations that can facilitate democratic stability. The causal mechanisms are that ideological polarization is less possible to appear in a two-party system, that two parties needing to win votes from the center encourages moderation, and that the absence of the extremist parties and the centripetal nature of party competition favor democratic stability. However, in recent years, some presidential countries with a two-party system in Latin America..