稅制與所得分配不均間因果關係的實證研究在國內外皆相當有限，本文利用合成控制法（synthetic control method, SCM），以台灣兩稅合一制度（integrated income tax system）相較於OECD國家作爲準實驗（quasi-experiment），分析其對於所得分配不均的影響。根據1980-2010年的資料顯示，兩稅合一實施後，台灣前5%及前10%所得者之所得份額逐漸增加，相較於制度實施前，平均分別提升12.32%及9.53%，表示兩稅合一制度使所得分配不均的現象惡化，對所得均等造成不利的影響。

There exists little empirical evidence on the causal relationship between tax policy and income inequality. This paper applies the synthetic control method (SCM) to analyze the effects of Taiwan’s integrated income tax system on income inequality, using the OECD countries as the donor pool. The results show that after the reform, in the period from 1980 to 2010, the top 5% and top 10% income shares increased by 12.32% and 9.53%, respectively. This finding suggests that the integrated income tax system has an adverse effect on the equality of income.