選舉制度影響政黨體系，是廣為人知的因果推論。依此邏輯，若以政黨體系為因，以選舉制度的變化為果，即產生內生性的選制變遷理論。然而，某些研究者仍認為選制變革起於和政黨體系無關的外生因素。本文主張，選制變遷是否受到政黨體系的影響，取決於選制的種類。第一，有利大黨的多數決選制如果始終未出現一黨過半，則國會屬於多黨制，有可能因為小黨組成多數聯盟而改採比例性選制。尤其當現狀為並立式單一選區兩票制時，可能因為小黨的議席多來自政黨名單，而比代表區域主義的多數決選制更容易導致變遷。在此情況下，多黨制和多數決選制的變化有關。第二，有利於多黨制的比例性選制如果產生一黨過半，則小黨即便結盟也不能掌握多數，過半政黨也欠缺改變選制的誘因;如果比例性選制產生多黨制，則只有在席次分布差距很大的情況下，才可能出現政黨結盟改採多數決選制;此時政黨體系的影響力不若外生因素。根據這樣的理論推演，本文檢測最新的全球選制資料庫，得到以下的發現。第一，在多數決選制下，「無黨過半」的確是影響選制變遷的重要變項;對比例性選制的變遷而言，政黨體系的顯著性遠低於外生因素。第二，比例性選制幾乎都產生多黨制，欠缺變遷的動能，而多數決選制也包含了一定數量的多黨制，所以變遷率高於比例性選制。這些發現釐清了相關理論的爭辯：多數決選制的變遷較易受到政黨體系的影響，而比例性選制的變遷則受制於外生因素較多。 It is well known that electoral system shapes party system. By this logic, an endogenous explanation for changes in electoral systems should treat party system as the cause and changes as the consequence, in sharp contrast with theories attributing changes to exogenous factors that are irrelevant to party system. This paper argues that whether changes in electoral system are caused by party system depend on the type of the electoral system. First, if a majority party never emerges from a majoritarian electoral system, minor parties may form a majority coalition to choose a proportional electoral system. In particular, multipartism under a mixed-member majoritarian system is more likely than other majoritarian electoral systems to induce change because minor parties tend to receive their seats from a party-list proportional tier while counterparts under other majoritarian systems may represent dominant district interests. Multipartism under majoritarian systems is thus associated with electoral system change. Second, if a majority party comes out of a proportional system, minor parties cannot form a majority coalition whereas the majority party may lack incentives to change the status quo; if a proportional system creates a multiparty system, electoral reform is unlikely unless some parties receive a large share of seats. Changes in the proportional system are thus affected more by exogenous factors than by party system. Verifying this theory using the most-updated global dataset of electoral systems, we obtained the following results: first, multipartism is indeed an important condition for changes in the majoritarian electoral system, while reforms in the proportional system are mainly affected by exogenous factors rather than its party system. Second, there are more changes in the majoritarian electoral system than in the proportional system because multipartism accompanying both systems can produce divergent outcomes of electoral reform. These findings show that changes in the majoritarian electoral system are induced by multipartism but reforms of the proportional system are susceptible to exogenous factors.