本文採用理性選擇制度論對「理性自利行為者」(rational self-interest maximizer)的預設,輔以歷史制度論的「關鍵時刻」(critical juncture)的概念,以及社會學制度論對行為者「脈絡化認知」(contextualized cognition)的強調,用以解釋台灣立委選舉制度變遷「如何發生」、「為什麼在2004年」、「為什麼是MMM制出線」這三個環環相扣的問題。本文主張:立委選制變遷的「關鍵時刻」是「2004年3月的總統大選後」至「同年12月的立委選舉前」這段時間。總統選舉結果帶來的新訊息,使得民進黨與國民黨從消極地維持舊選制轉變為推動選制改革的主要力量;外生壓力與選舉邏輯兩者形成「可信的威脅」(credible threat),徹底屈服了兩個小黨以及個別立委對新選制的抗拒。選舉制度只可能在這關鍵時刻的時段內產生變遷,在2004年總統大選前不可能變遷,過了這段關鍵時刻後變遷也成為不可能。MMM制之所以成為國民黨與民進黨共同接受的新選項,原因在於:國民黨想藉由MMM制扳回一城兼收合併親民黨之效;而民進黨則想藉由MMM制達成完全執政兼收消滅台聯之效。親民黨與台聯最終仍對國會改革法案投下贊成票,乃是忌憚「反改革的代價」之緣故。
This article formulates a synthesized new-institutionalist framework to explain three interlocking questions concerning Taiwanese legislative electoral change, i.e. ”how did it happen?”, ”why was the year 2004?”, and ‘why did MMM system be chosen?’ by adopting the presupposition of ”rational self-interest maximizer” in rational choice institutionalism, the concept of ”critical juncture” in historical institutionalism, and the emphasis of ”contextualized cognition” in sociological institutionalism. The ”critical juncture” of Taiwanese legislative electoral change was the very period from the end of presidential election in March, 2004 to the legislative election in December, 2004. This is the only timing feasible for the electoral change. This electoral reform can not succeed either before the 2004 presidential election or after the 2004 legislative election. The result of 2004 presidential election transformed the preferences of both KMT and DPP for the electoral change from preserving the existing system to promoting a new one. KMT and DPP, as two major parties, shared so similar political logic that MMM system became their common alternative for electoral system. By adopting MMM system, KMT envisaged its victory in the coming legislative elections and the political engineering of merging PFP, while DPP anticipated to gain full power from becoming the majority in Legislative Yuan and to cripple TSU. The convergence of exogenous pressure and electoral logic, serving as a ”credible threat”, dissolved the resistance to the electoral change from two minor parties as well as most legislators. For PFP and TSU (even as well as most legislators in two major parties), the stigma of ”anti-reformer” was unbearable in the coming legislative election. That is why these two minority parties voted for electoral change eventually.