所得稅分配不均及累進程度一直以來都是所得稅研究熱門重點，過去對臺灣地區所得分配之實證研究不在少數，惟均僅對某特定年度或數個年度來研究。本研究為了解歷年臺灣地區課徵綜合所得稅後之累進程度變化情形，乃以長達33年之資料為研究對象。又本研究對於各縣市綜合所得稅金額變化及課徵所得稅後之累進程度變化情形有著相當濃厚興趣，進而欲了解臺灣23個縣市綜合所得稅金額及課徵所得稅後之累進程度變化情形。 本研究主要係透過所得差距倍數、所得份額及所得稅負擔比率，並以1979至2011年行政院主計處統計之「中華民國臺灣地區家庭收支調查報告」為資料來源，觀察臺灣地區課稅前所得在最高20%之家庭與所得在最低20%之家庭與課稅後之所得分配情形變化。 本研究結果發現：綜合所得稅在所得均化功能上扮演重要角色，量能課稅效果顯著，觀察累進程度效果，累進力道則顯得些薄弱。

本文以Sandmo引進勞動供給的租稅逃漏模型為基礎，以是否具有其他所得來區別社會上的富人或窮人，富人可以在勞動市場中投入勞動供給賺取勤勞所得，或是將勞動供給投注在以其他所得做投資的投資行為上，賺取資本利得；但是窮人只能將其勞動供給投注在勞動市場中賺取勤勞所得，在雙方決定勞動供給以追求效用極大化下，可決定彼此的所得差距。 由於所得稅具有所得重分配的功能，因此我們首先比較在不同所得稅制下的所得差距，以得知所得稅是否達成其重分配功能。結果是：僅課徵勤勞所得稅時的所得差距會大於未課稅時的所得差距，勤勞所得稅反而使所得情形惡化，因為富人可藉由移轉勞動供給的方式規避稅負；而同時課徵勤勞所得稅及資本利得稅後之所得差距會小於僅課徵勤勞所得稅時的所得差距，代表資本利得稅可有效改善所得分配惡化的現象。 政府除了可以用所得稅的手段改善所得分配外，也可以以移轉性支出的方式達到相同目的。第一個情況是假設目前僅課徵勤勞所得稅，所得差距情形惡化，當政府決定將全國所收取的勤勞所得稅做為移轉性支出平均分發給窮人時，會得到不一定可以改善所得差距惡化現象的結果，必須視窮人對消費和休閒的替代彈性，以及替代效果與所得效果的大小而定。第二個情況是假設目前同時課徵勤勞所得稅及資本利得稅，並以富人所支付的資本利得稅做為移轉性支出平均分發給窮人，此時的移轉性支出能夠進一步改善所得差距惡化的情形。由以上兩段可知，若政府要有效地改善所得差距惡化的情形，最好的辦法是課徵資本利得稅，並將資本利得稅做為移轉性支出的財源。 / In this paper we use a Sandmo’s (1981) type tax evasion model to discuss the effects of proportional income taxation on income distribution among the riches and the poor. The riches are different from the poor in that only the former have exogenous income which can be invested to obtain capital gains. Therefore, while the riches choose between allocating their time to make labor income or to make capital gains, the poor can only attribute their time to making labor income. The tax system treats labor income and capital gains differently. The income distribution among the riches and the poor is determined by the difference in total incomes. We first show that when only labor incomes are taxed, the income tax worsens the income distribution as compared to the income distribution when there is no income tax. In other words, an income tax system with preferential treatments on capital gains is detrimental to income distribution. This result is intuitive because the riches can switch their labor supply from making labor income to making capital gains for tax avoidance, while the poor do not get to do so. An income tax system is beneficial to income distribution when capital gains are taxed at the same rate as labor income. Second we discuss the role of income transfer program on the income distribution and show that if the government uses labor income tax as the only financial source for the transfer, the effect of such income transfers on income distribution is ambiguous. It is decided by the magnitude of the elasticity of substitution between consumption and leisure, and the net effect of the substitution and the income effects. To improve income distribution, the best scenario is to use tax revenues from capital gains as the financial source of income transfer.