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.
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