在品質差異與無彈性需求下的從價稅與從量稅比較

Comparing Specific and Ad Valorem Taxes under Price-Inelastic Demand with Quality Differentiation

我們在一個品質差異且缺乏彈性的完全覆蓋之市場下，討論從量稅與從價稅間的優劣。在這樣的模型設定下，我們得到下列兩個主要結果。第一、在短期時從量稅可能優於從價稅。當高品質廠商邊際成本相對較低，或邊際成本雖然相對較高但從價稅率較高時，課徵從量稅下的福利高於課徵從價稅下的福利。第二、在長期時，只要從價稅率夠高，課徵從量稅下的福利不僅較高，從量稅對從價稅還有柏雷圖（Pareto）優勢。這兩個結果顯示，在需求缺乏彈性下，若政府需要較高的稅收，課微從量稅較從價稅好。除此之外我們也做了一些延伸討論。第一、若市場上的兩本國廠商改為一個本國廠商一個外國廠商，上述第二項長期結果依然成立。第二、若市場上的兩民營廠商改為一公營一民營，則無論是課徵從價稅或從量稅，此混合寡佔下的品質水準與產量分配都可達到社會福利最大下的品質水準與產量分配。

We examine the superiority of a specific tax and an ad valorem tax in a fully covered model with vertical differentiation, in which the demand elasticities of the products are inelastic. By focusing on the influences of vertical differentiation and inelastic demand, we show that a specific tax is superior to an ad valorem tax in the short run, when the marginal cost of the high-quality product is relatively low or is relatively high and the ad valorem tax rate is high. We also show that a specific tax may not only be welfare superior to, but may also Pareto dominate an ad valorem tax in the long run, when the ad valorem tax rate is high. These two results imply that the government should impose a specific tax on products with price-inelastic demand if the tax revenue requirement is high. Besides, we obtain that the main result remains unchanged when one of the firms is a foreign firm, and that the firms' equilibrium quality levels and outputs will change to become identical to welfare-maximizing levels regardless of the tax schemes, when one of the firms is a public firm.