考慮Tobin稅之通貨替代模型──實質匯率動態與匯率穩定

TOBIN TAX EFFECTS UNDER CURRENCY SUBSTITUTION: REAL EXCHANGE RATE DYNAMICS AND STABILITY

本文沿用曹添旺（2011）的模型，並納入了Tobin稅，探討Tobin稅提高對實質匯率的長期均衡以及動態調整行為的影響，並討論Tobin稅如何與擴張性質貨幣政策進行協調，以達到穩定匯率的政策目標。分析發現：（1）Tobin稅提高長期下造成實質匯率下降、產出增加、實質外國貨幣餘額（本國商品計）減少；當通貨替代下，實質本國貨幣餘額（本國商品計）會增加，通貨互補時，則須視互補程度的大小而定。（2）Tobin稅提高對實質匯率的衝擊效果，則隨其外國通貨膨脹率大小，以及各種模型的結構參數不同，呈現過度調整、調整不足或錯向跳動等多樣化走勢。Tobin稅提高瞬間，實質匯率會跳升或驟降，取決於期初外國通貨膨脹率的大小而定；而實質匯率瞬間變動幅度的大小，取決於稅率提高對外國貨幣存量（以外國商品為單位）的影響幅度大小，以及各模型參數的相對大小。（3）貨幣增長率提高的同時，Tobin稅應提高較小幅度的稅率，幅度的大小，取決於兩通貨間的成本彈性與本國民眾之國外通貨需求成本彈性的大小而定。通貨毛替代程度愈大（愈小），則Tobin稅提高的幅度愈接近（遠離）貨幣增長率提高的幅度；國外通貨需求成本彈性愈大（愈小），則Tobin稅提高的幅度愈遠離（接近）貨幣增長率提高的幅度。

This paper extends the model of Tsaur (2011) to include a Tobin tax. It proposes to examine how the real exchange rate will be affected by an increase in the Tobin tax, and investigate how the monetary expansion and the Tobin tax coordinate to maintain the real exchange rate stability. The major findings are as follows: (1) In the long run, the rise of the Tobin tax rate would cause the real exchange rate to decrease, output to increase, and real foreign currency balances to decrease, but whether the real balances increase or decrease depends on the magnitude between the currency substitution elasticities. (2) In the short run, the magnitude of foreign inflation rates is crucial in influencing the behavior of real exchange rates, (i.e., real exchange rates may overshoot, undershoot or misjump around their equilibrium values) while the magnitude of the effect of the Tobin tax on the stock of foreign currency and other structure parameters in the model are key elements to determine the degree of real exchange rate movements. (3) The range of an increase in the rate of the Tobin tax should be smaller than monetary expansion in order to maintain real exchange rate stability. It is shown that if the degree of currency substitution is higher, the range of an increase in the rate of the Tobin tax should be bigger; if the cost elasticity of holding foreign currency is higher, the range of an increase in the rate of the Tobin tax should be smaller.