混合式選制融合了多數決與比例代表兩種選制的精神，不過關於此種選制兩票架構對投票的影響，有兩種截然不同的主張:一是認為兩票各自獨立運作，其影響可以獨立估算;另一派則主張兩票抉擇會相互影響，亦即具有所謂連動效果(interaction effects，或稱感染效果 contamination effects)的存在，因此必須綜合考量兩票架構的互動關係。而在日本施行的混合選制，又因允許候選人在兩種選票間重複提名，更使得其兩票間的連動關係益形複雜。
本文以日本自民黨為背景，透過日本選舉研究(JES)的民調資料，採雙變數機率單元模型(bivariate probit model)分析 1996 至 2005 年間四屆眾議員選舉中的兩票效應。結果顯示，除傳統的政黨認同與保革意識形態會影響選民投票行為外;更重要的是，日本新制下的候選人重複提名制度，確實會影響選民兩票決策的互動性。依據本文的分析，自民黨若針對選區候選人採重複提名策略，若有助於選民把單一選區(SMD)票投給自民黨的話，也能提升選民對於政黨比例代表(PR)的投票支持(即一致投票的效果);但此一正面之間接效果尚不足以抵銷重複提名對自民黨政黨比例代表票本身的負面直接效果。至於自民黨之所以採重複提名策略，似乎常是考量該選區候選人較居弱勢，因此希望藉由重複提名以提升政黨比例代表票。
The Mixed Electoral System combines the spirits of plurality and proportional representation. The way two-ballot structure influencing the voting behavior however is not settled. Of the two distinctive schools, one believed that two ballots work separately and therefore, their influences should be estimated independently. The other group advocated the existence of contamination effects (or interaction effects), and believed that it is necessary to consider the interaction between two ballots. As for the Mixed- Member Majoritarian (MMM) Electoral System in Japan, it is actually more complicated because of the addition of dual candidacy.
Based on the survey collected by the Japanese Election Studies (JES) between 1996 and 2005, we took the Liberal Democratic Party (LDP) as an example and analyzed the interaction between their two ballots. The bivariate probit result shows that, in addition to the party identification and political ideology, the dual candidacy in the new electoral system did influence the interaction between the two ballots. According to our findings, the dual candidacy helped those who vote for LDP candidates in single member district (SMD) to continue their support with the party votes (a so-called straight-ticket voting scenario). However, this indirect effect did not offset the negative direct effect that dual candidacy brought to the LDP party votes. To explain the contradictory direct and indirect effect, we find that the dual candidacy was a nomination strategy for LDP. They are more likely to adopt dual candidacy on the weak candidates, in order to attract more party votes in the districts.
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