提要

一個優良的租稅必須具備三個條件—公平性、效率性與穩定性。個人所得稅的公平性及穩定性一直是國內財稅論文的熱門課題，但是學者每論及效率性時，往往就一筆輕輕帶過。因此從來我們只知道吾國個人所得稅的課征可能對資源配置存有不良的影響，但是其影響的程度究竟有多深，則成了一個謎題。

本文的目的卽在探討這個謎題的解答。筆者個人淺見以為，個人所得稅之所以會產生福利損失，除了「課征」行為外，「補貼」行為更為一項重要的因素。前者導至某種資源供應的減少，後者則導致某種資源的供應過多，凡此都將扭曲資源配置的合理狀態，進而產生福利損失。旣然如此，個人所得稅對勞動所得課稅，就是福利損失產生的原因之一；對沒算房租免稅，就是產生福利損失的原因之二；對某些所得給予優惠課稅，就是產生福利損失的原因之三。本文對這幾個問題都予以詳細的討論，前二個部份外國學者曾經做過研究，因此筆者除重述其模式外，並運用台灣的資料實際予以估算；最後再對其模式略加修改，觀察其對福利損失的影響。至於最後一個部分，係筆者不惴淺薄所自行創設的一個模式，所幸對問題亦能稍作解釋。惟其中所存問題仍多，且筆者學識不足，錯誤疏忽乃在所難免。其次在估算福利損失的過程中，往往涉及彈性的數值，由於彈性估計牽涉過深，困難重重，筆者僅能儘量以合理的假設數值暫代，當然所估得的結果也就有誤差存在了。

根據本文研究結果顯示，對勞動所得課稅的福利損失相對於美國，為數不大。在勞動的補償性供給彈性為0.125，補償性需求彈性為–∞的前提，67.年度吾國這方面所產生的福利損失僅新台幣四千六百萬元左右。若需求彈性的絕對值逐漸減少，福利損失將有縮小的趨勢。其詳細內容參閱本文第三章。

對自有住宅採取租稅補貼所產生的福利損失相對於美國為數亦小。若房屋勞務的補償性供給彈性為1，租屋者及自宅者的補償性需求彈性為–1.5，則所產生的福利損失約為新台幣1.546億元，約占是項租稅補貼額的2. 83%。如果租屋者因政府的房屋租稅策略影響，移出成為自宅者的比例越高，福利損失亦將隨之減少。惟政府對自用住宅租稅補貼所引起的所得重分配效果卻不容我們忽視，因為補貼結果將使所得由經濟上的弱者移往經濟上的強者，嚴重違背了租稅公平的原則。其詳細內容請參閱本文第四章。

對某種所得優惠課稅，其在列舉扣除額所產生的福利損失約為新台幣一億元，占是項租稅補貼總額的30.％左右，其數額相當鉅大。至於其他各項，譬如短期票券利息20％分離課稅，二年期定存利息免稅……等，其福利損失由於缺乏統計資料，無法估出。惟針對估計模式的數學式觀察，大略可知其數值應該極微小，甚至可以予以忽略。其詳細內容請參閱本文第五章。

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