疊代模型中的零利率政策

THE ZERO INTEREST RATE POLICY IN THE OVERLAPPING GENERATIONS MODEL

既存文獻在確定生命期限與外生成長的疊代模型中發現：若政府給予各世代相同的現金移轉，單純的貨幣政策將引發跨代財富的重分配，進而導致名目利率為零的Friedman法則並非最適的貨幣政策；不過，若貨幣政策可搭配著給予各世代不同的現金移轉，進而完全抵銷貨幣政策所形成的跨代財富重分配，零利率政策將為最適。然而，現實社會的狀況是長期經濟會有正成長且民眾的生命期限具不確定性，本文以不確定生命期限與內生成長的疊代模型為架構，並考慮貨幣為交易媒介的功能，得到與既存文獻不同的結論：（1）無論民眾是否收到等額的現金移轉，零利率政策皆非最適；（2）相較於給予各世代等額的現金移轉，政府若可透過給予各世代不同的現金移轉以消除貨幣政策所形成的跨代財富重分配將帶來較高的社會福利；（3）釘住名目利率的政策在一般情況的動態上具有不確定性，顯示無論零利率政策是否為最適，經濟體系未必會隨時停留在長期穩定的均衡上，除非有誠實的政府願意隨時維持實質貨幣餘額的穩定性，名目利率指標在動態上才會具有確定性。

In the study of the overlapping-generation model with a certain lifetime and exogenous growth, it was found that if there is a simple monetary policy where each cohort receives an equal lump-sum monetary transfer from the government, this might result in intergenerational wealth redistribution, and therefore the zero interest rate policy (Friedman rule) fails to be the optimal monetary policy. However, if the government provides different fiscal transfers for each cohort, the redistribution effect could be offset and the zero interest rate policy becomes optimal. Nevertheless, the economic growth rate is positive and the lifetime of a person is uncertain in reality. We consider an uncertain lifetime, endogenous growth, and the role of money as a medium of exchange in an overlapping-generation model. This paper results in different conclusions from the literature: (1) the zero interest rate policy fails to be the optimal monetary policy no matter whether each cohort receives an equal lump-sum monetary transfer from the government; (2) the social welfare from different fiscal transfers to each cohort is larger than that from equal lump-sum monetary transfer; (3) the nominal interest rate target is indeterminacy and then the economic outcome does not always stay at the long-run equilibrium, unless the honest government is willing to maintain the stability of the real money balance, and then the nominal interest rate target can be determinacy