「制度有效乎（institution matters)?」這個命題是晚近政治學界針對政治選擇與政治行爲持續所探討的重點之一。大多數的制度論者往往 都會假定，選舉制度對於政黨體系之影響有著直接的關聯性。本文乃著 重於「個人選擇」與「制度設計」之間的層際連結，以「跨層次推論分 析」的硏究途徑探討選舉方式的制度性抉擇與政黨體系之形成與否的相 關性。究竟是什麼選舉規則本身難以改變的「機械因素」之制約，從而影響選民投票的「心理因素」，造成所謂「策略投票」或「複雜投票」 的政治現象？政黨、候選人與選民是否會根據不同選舉制度間的差異性 直接或間接決定其選舉策略之操作，最終牽引著不同類型的「區域層次」或「國家層次」政黨體系之形塑？因此，本文欲對於世界主要民主 國家所採取不同的選舉制度——「單一選區相對多數決制」、「單記非 讓渡投票制」、「比例代表制」——在台灣的應用進行檢證，並進一步 分析探討這三個制度變數對依變數「政黨體系」之影響。本文將以中央 選舉委員會選舉資料庫所提供台灣第九任至第十一任總統選舉、第九屆 至第十四屆縣市長選舉、第四屆至第六屆立法委員選舉，及二〇〇五年 剛舉辦的任務型國大選舉之資料，以有效政黨數的計算公式爲測量指 標，進行實證分析，並藉以說明「選舉制度對政黨體系之影響」。

“Do institutions matter?” This question has been important to the study of political science, in recent years especially to the study of the relation between political behavior and its outcomes. Most scholars tend to regard the institutional organization and practice as influential factors directly relevant to the operation of the electoral system as well as that of the party system. To clarify this general presupposition, this paper focuses on the linkage between “individual choice” and “institutional design” and adopts a “cross-level analysis” to examine the relationship between institutional choices of the electoral system and the formation of different types of party system. Moreover, this paper attempts to show that different kinds of electoral systems will lead to distinctive outcomes, which could influence the formation of the party system as well as individual voting behavior.To demonstrate the influences of the electoral system exerted on the individual decision, this paper further explores how the “mechanical factor” regulated by the electoral system constrains the “psychological factor” of the voter’s mind and eventually leads to the so-called “strategic voting” or “complex voting.” In addition, to infer possible reasons for the influences of the electoral system on the partysystem, this paper employs the number of valid parties as an indicator of the fragmentation of the party system. This number is the analytical result of the empirical data from Taiwan Presidential Election, County Magistrate and Mayoral Election, Legislator Election, and Missionary National Assembly Representative Election. The effective number of parties is utilized to analyze the electoral results of three electoral systems—plurality with single-member-district system, single non-transferable vote, and proportional representation. Through the analysis, this paper intends to show that these three electoral systems do respectively lead to quite different consequences in the distinctive forms of the district-level and the national-level party system.