本文旨在探討台北縣選民在2005年縣長與鄉鎮市長兩個不同層級選舉中的一致與分裂投票行為。本研究的重點在強調：欲瞭解選民於不同層級選舉中的投票行為，不但要關注各個層級選舉影響因素的異同，更需考量到兩項公職同時選舉時可能產生的相互影響。因此在本文中，除概述台北縣選民在這兩種選舉中的一致與分裂投票情形之外，亦嘗試以隨機效用(random utility)理論，推演兼顧上述兩個性的雙變數「聯立式機率單元模型」(simultaneous equations probit model)，以此檢證影響選民一致與分裂投票的重要因素，並說明由下而上的「基層動員力」、或者由上而下的「上層加持力」，何者才是影響選民投票抉擇的關鍵。
研究結果顯示，2005年台北縣選民有近八成採取一致投票，且「藍藍型一致投票」高於「綠綠型一致投票」，而分裂投票部分則是「綠藍型分裂投票」稍高於「藍綠型分裂投票」。聯立式機率單元模型估計的結果則顯示，台北縣的選情其實較接近來自縣長選舉「上層加持」的觀點，而加持的力道主要來自泛藍陣營在縣長競選策略上強力主打「換黨執政」議題，相當程度強化了選民對候選人的主觀評價與對藍綠陣營的偏好態度。 This article examines the first ever simultaneous local elections of county and township magistrates in Taiwan in December, 2005. We argue that in order to fully understand the voting choices in simultaneous inter-level elections scholars need to take account not only those factors affecting each level's election but also the interactions between different levels. There are two competing theories concerning the flow of causal influences, i.e., the bottom-up mass mobilization hypothesis and the top-down coat-tail effect hypothesis. We therefore construct a random utility model of joint choices and then test it with the bivariate simultaneous equations probit model.
Empirical evidence indicates that in this particular election nearly 80% of voters in the Taipei County cast their votes straightly, although the pan Blue (i.e., KMT and PFP) seems to enjoy a higher straight-ticket rate than the pan Green (i.e., DDP and TSU). As to the inter-level ticket splitting rates, we find that the Green-Blue type (i.e., those who vote for the DDP candidate of county magistrate and yet support the KMT/PFP candidate of township magistrate) is slightly higher than the Blue-Green type (i.e., those who vote for the KMT candidate of county magistrate and yet support the DDP/TSU candidate of township magistrate). Our test of the simultaneous equations probit model also shows that in the 2005 Taipei County election, campaign for the county magistrate exerts much greater influence on voters' choice on township magistrates than the other way around. This top-down influence mainly comes from the pan-Blue camp's campaign strategy chanting for a shift of ruling party for the Taipei County government, a strategy that further reinforces candidate images and voters' party preferences.