日本選民政黨支持與投票抉擇：小泉執政時期參眾兩院選舉的分析

Party Support and Voting Choices: An Analysis of Japan’s House and Councilor Elections under Koizumi’s Premiership

民主國家的選舉，選民的投票抉擇，長久以來都是學界研究關懷的重要課題。選民投票抉擇的動態變化，不僅攸關不同政黨勢力的起伏，也具體反映出一國政黨體系演變的軌跡。也因為這種演變的狀況對民主發展的模式與軌跡各具不同的意義，國內外學界對此皆具高度濃厚興趣，不論是從理論概念的測量界定、研究設計與分析方法的開拓，抑或是各國實證資料的廣泛蒐集，都累積相當豐碩的成果。本文聚焦於我們東亞鄰國的日本，探討21世紀初期在小泉純一郎首相領導執政時期之下，國會參眾兩院選舉之選民投票抉擇與影響要因。本文評估小泉執政時期的選民投票抉擇，透過混合勝算對數（Mixed Logit）與廣義混合勝算對數模型（Generalized Mixed Logit）等的分析比較，我們發現廣義混合勝算對數模型是較合適的模型，因為該模型能同時考量選民在偏好上的異質性、以及相同政黨偏好在投票抉擇上展現不同幅度的不確定性。該模型的分析顯示：在控制了制度的因素後，政黨情感溫度計、政黨支持、小泉魅力等，都是這期間左右選民投票穩定的關鍵要素。此外，廣義混合勝算對數模型所特有的「尺度異質」更指出，儘管政黨支持是影響日本選民投票抉擇的重要因素，但其影響力又會因人而異，例如同樣是認同自公聯盟者，有人較篤定投給該黨、但也有人較多變。這樣的結果，除了呼應既有國內外選舉文獻之觀點外，我們採用的小泉執政時期完整的跨時序定群追蹤資料，也更延伸並豐富了現有對日本選舉行為的觀察。

Elections and voting behavior have been enduring subjects in studying democracies. Electoral stability and volatility not only shape the faith of individual political parties, but also the evolution of the party system as a whole. Hence topics related to party support and voting choices, both methodological and substantive, have attracted attention from scholars around the world. This study focuses on the voting choices in Japan’s bicameral system under Junichiro Koizumi’s governance in the early 21st century. By employing the panel data from the Japanese Election Study (JES III), we analyze four waves of House and Councilor elections from 2001 to 2005. By comparing Mixed Logit (MXL) and Generalized Mixed Logit (GMXL) models, we find that the GMXL model is the most appropriate tool in analyzing Japanese longitudinal voting behavior because it can simultaneously take into account the “individual’s preference heterogeneity” and “scale heterogeneity across individuals” of party identification. The results of the GMXL model indicate the significance of the party feeling thermometer, party identification, and Koizumi’s leadership in influencing Japanese voting choices while controlling the bicameral institution variable. More importantly, although the GMXL analysis confirms many findings in the previous literature on party identification in influencing voting behavior, such as how the voters behave in supporting the LDP-Koumei Coalition, the GMXL model further captures both preference and scale heterogeneity in the Japanese voters’ utility function, i.e., some voters behave with greater variation while others behave more consistently. These results not only enrich the existing Japanese electoral literature, but also contribute to the methodology of studying voting choices in general.