商品不可分割性與最適商品稅兼論利益團體遊說的情況

Consumption Indivisibility and Optimal Commodity Taxation: Applicability to Lobbying by Interest Groups

傳統探討最適商品稅制的文獻忽略了商品消費的不可分割性以及利益團體這兩個重要的經濟事實。有鑒於此，本文建構存在不可分割財之二元消費的經濟體系，探討商品稅訂定的效率解與引進利益團體遊說下的政治均衡解。依據我們所獲致的研究結果顯示：基於不可分割財具有高能力者傾向會買，而低能力者傾向不會買的特性。（1）對不可分割財課稅具有差異性定額稅的功能（differential lump-sum tax），使得最適的商品稅應該在高能力者願意購買不可分割財的前提下優先對該財課稅，其稅率為正。至於可分割財則在消費扭曲效果（效率面）、低能力者所得重分配的改善（公平面）、受補償門檻受限效果間取得平衡。（2）由於高、低能力者對於商品稅率的制訂之看法不同，前者對於不可分割財的課徵偏好輕課，後者則偏好重課。這個結果使得考慮利益團體遊說後的商品稅率之制訂，均衡時，不可分割財的稅率將降低、可分割財的稅率則提高。至於社會的福利水準也將因利益團體的存在而下降。

The traditional literature on the optimal commodity tax system ignores two important economic facts: indivisibility of consumption, and interest groups which may lobby for commodity tax policies. In view of this, this paper explores two solutions: the efficiency solution and political equilibrium solution. According to our research, based on the fact that only the high-type taxpayers will buy indivisible goods, (1) the commodity tax on the indivisible goods acts as differential lump-sum taxes, which leads to the conclusion that the optimal commodity tax on the indivisible goods should be given priority for taxation under the premise that high-ability taxpayers will still be willing to purchase indivisible goods. The tax rate of indivisible goods is positive; (2) high-ability taxpayers hope the tax on indivisible goods could be much lighter, while the low-ability taxpayers hope the tax on indivisible goods could be much heavier. This result reveals that the tax rate on indivisible goods in the political equilibrium will be lower than the one in the optimal commodity taxation. As for the social welfare level, it will decline due to the existence of interest groups.