The Optimal Tax Rates in the Presence of Profit Shifting

過去有關廠商避稅行為的文獻，大多以完全競爭市場為架構，且政府的稅率政策通常為外生。為彌補此項文獻上的缺漏，本文以Singh and Vives (1984)與Slemrod and Wilson (2009)的模型為基礎，並考慮封閉經濟體系下的雙占市場模型，分別以兩階段賽局和三階段賽局模型來檢視在政府制定利潤稅稅率時，廠商的避稅行為所扮演的角色。本文發現：在考慮利潤移轉下，最適稅率的決定會同時受到公共財淨效果與避稅效果所影響，在查緝較寬鬆或利潤移轉的風氣盛行的國家，較可能發生公共財淨效果小於或等於零的情形，故最適稅率容易訂在下界；在查緝較嚴密或利潤移轉的風氣不盛行的國家，最適稅率則須視兩種效果的相對大小而定，而有上界、內部解與下界等不同的情形。此外，就產品市場的結構而言，在不考慮利潤移轉下，利潤稅稅率的決定獨立於產品市場的結構；而考慮利潤移轉下，若保密服務的函數型態不相同，產品市場的結構可能影響利潤稅稅率的訂定。Most of the literatures regarding the firms’ tax avoidance is based on the structure of perfectly competitive markets. Furthermore, the tax rates are generally exogenously determined. To fill the gap in the literature, this thesis follows Singh and Vives (1984) and Slemrod and Wilson (2009), and considers a duopoly in a closed economy. We establish a two-stage game and a three-stage game, respectively, to examine how the firms’ tax avoidance shapes the making of the profit tax rates. In the presence of profit shifting, I find that the optimal tax rate depends on the public-good effect and the tax-avoidance effect. In addition, it is more likely that the public-good effect is less than zero in countries where profit shifting is more prevalent, or the tax avoidance is not strictly audited. In this case, the optimal tax rate will be set at the lower bound. However, in countries where profit shifting is less prevalent, or the tax avoidance is strictly audited, the optimal tax rate depends on the public-good effect and the tax-avoidance effect. Depending on which force is stronger, the optimal profit tax rate can be set at the upper bound, lower bound, or be an interior solution. In terms of the product market structures, the optimal profit tax rate is independent of the product market structures in the absence of profit shifting. However, if the types of the concealment service functions are different, then the optimal profit tax rate is likely to rely on the product market structures