有鑑於歐洲各國的福利政策與福利支出相較於其他國家高，歐洲國家多會提供窮困與殘障的人民最基本的社會保障，一般人民亦可享受到基礎醫療保障，在失業時也可以領取失業補助；歐洲各政府甚至會收購即將倒閉的企業或者提供補助使企業能夠繼續經營。然而在持續延燒的歐債危機下，為維持歐元區普遍的薪資和福利水平，歐元區內國家開始採取了國家借貸的做法，但由於國際經濟形勢不佳，歐元區經濟增量未達預期，許多國家原有債券陸續到期，若無法借到新貸款，國家將面臨倒閉危機。因此本論文主要以動態隨機一般均衡模型(DSGE)，探討一個封閉經濟體系下，政府支出進入家計單位效用函數後，換言之政府支出變動將會影響家計單位消費的邊際效用，試著尋找能夠極大化社會福利的政府公債占國內生產毛額的最適比例。我們發現在提高政府公債占國內生產毛額比例時：家計單位將預期未來稅賦繳納之金額更高，因而減少消費、增加儲蓄，所以會排擠掉部分私人消費；同時政府必須提撥部分費用以支付債券利息，所以將排擠掉部分政府支出。另外隨政府公債占國內生產毛額比率上升，整體社會福利水準會逐漸下降；因此，我們認為最適政府公債占國內生產毛額比率應為零。 關鍵字：動態隨機一般均衡模型、政府支出、邊際效用、公債、國內生產毛額、社會福利水準 / In Euro zone, large spending obligations needed to support the welfare state and redistribute wealth in an effort to gain greater equality. Most European countries provide liberal social security benefits to the poor, disabled, basic medical needs and very liberal unemployment benefits. They also own and run large public companies. Under the ongoing Euro debt crisis, European governments figure out the way to maintain the high welfare level by increasing the public debt they hold. In our paper, we would like to investigate the optimal ratio of public debt to GDP by constructing a micro-based dynamic stochastic general equilibrium model (DSGE) under a closed economy. We focus on discussing the optimal ratio of public debt to GDP which maximizes social welfare in economy while putting the government expenditure into households’ utility. That is to say, the government expenditure will influence the marginal utilities of households. We find out that as a government decides to raise the debt to GDP ratio; it will crowd out both private consumption and government expenditure. Because households will expect to pay more tax in the future, they will decrease their consumption and increase their saving; high debt ratio means government should have paid more interest payment in the future. Also as the ratio of public debt to GDP rises, the social welfare becomes lower. Thus, in our findings, the optimal debt ratio to GDP should be 0. Keywords: DSGE, Government expenditure, Marginal utility, Public debt, GDP, Social welfare

思及當前捉襟見肘的財政窘境，無可避免地，債台高築的臺灣實陷入飲鴆止渴般以債養債之無限迴圈中，導致政府政策效能不彰、社會福利運作生弊亦無可厚非；於『公共債務法』之財政規範下，臺灣業已瀕臨法定舉債門檻，故不論是對短期政府支出之排擠、扭曲性稅率之稽徵抑或對長期經濟成長的斲傷，皆是身為中華民國國民真正惶悚不安之所在。 職是之故，本研究係採用一納入政府財政部門及貨幣當局之擴充『實質景氣循環模型』，藉以Sidrauski（1967）所提出的貨幣效用函數為出發點，將實質餘額引進理論模型，並透過計量操作捕捉實證期間起於西元1971年第一季迄至2007年第四季之政府政策函數，過程中，我們不難發現政府購買性支出及稅率皆存在相當的持續性，且對政府未償公債餘額之高低作出某種程度的反應。亦即，若政府實施公債融通政策，俾使期初公債餘額較高之際，則本期甚或往後各期的政府支出將遭受抑制和排擠，尤有甚者，政府勢必擬以提高未來稅率以茲挹注該債務之還本付息所造成的財政缺口；是以，本研究著眼於引進公債餘額對政府支出及稅率存在反饋作用下，財政政策與貨幣政策之總體效果及各總體變數之動態調整過程的風貌。即便公債發行或賒借為政府提供一財務週轉工具以裨益財政政策保有更靈活之彈性，然據模型所產生的結果顯示，就長期而論，政府必須維持一穩定之未償公債餘額，即公債水準具備『均數復歸』性質，而該財政目標係透過削減未來政府支出、調整扭曲性稅率及鑄幣稅融通政策方得以達成預算平衡，準此，該設定將造成公債融通之減稅政策對經濟體系具有實質效果，『公債融通』管道亦『非中立性政策』，從而傳統『李嘉圖等值定理』於本模型中無法成立。 就政策面層次而言，本研究試圖放寬『反饋法則』與政策係數之設定，以檢視透過不同程度之政府支出、稅率甚至貨幣供給途徑的改變來平衡因增加公債發行所造成的財政赤字，對經濟體系之長短期效果有何迥異處；是文亦藉由衝擊反應函數分別探討於政府支出增加、減稅措施及貨幣擴張之下，政策的傳遞機制與各總體變數之動態性質，顯然地，就高債務比率前提下，當政府戮力於刺激景氣而欲積極實施立竿見影的總體經濟政策之際，卻常因狃於急效而欲速不達，非但政策效果有限，亦可能使體系落入更為不景氣的田地，從而，財政惡化不啻為經濟危機的導火線也就不言而喻。再者，貨幣政策對體系之實質變數具有一定程度的作用，是故，本模型於短期內無法一窺『貨幣中立性』之堂奧，唯長期始得以復見。總括言之，政府亟須奉『健全財政』為圭臬，擬定政策時更得戒慎恐懼，並適切權衡利弊得失，以茲裨益有更具信心的經濟表現。 此外，本研究亦透過『效準』實驗以評估模型『配適度』之良窳，即便於反覆疊代法下，該模擬表現係瑕瑜互見而不盡完美，卻也大抵符合景氣循環之『典型化特徵』；然就實質景氣循環模型所為人詬病之勞動市場一隅而論，引進公債之反饋法則下的財政政策操作，無疑地改善了傳統工時與工資率動輒高度正相關之本質，從而獲致相對較低之理論相關係數，亦朝實證資料所呈現工時與工資率存在幾近零相關甚或低度負相關之表徵更邁進一大步。 / With current financial difficulties beyond government capability, it is inevitable that the already deep-in-debt Taiwan opted for momentary relief by paying debt through debt financing and ended up in an infinite loop, causing spiral-down performances in government policies and faulty operations of social welfare instruments. Taiwan has been on the verge of reaching the statutory upper limit of debt financing according to “The Public Debt Act” regulations and all nationals are becoming anxious about such impacts as crowding out of short-run government spending, levying of distorting taxes, and damages on long-run economic growth. To better understand the debt’s impacts, this research uses the “Real Business Cycle Model” extended by taking government treasury agency and monetary institution into account. Starting with Money In Utility Function (MIUF) as proposed by Sidrauski (1967) to introduce real money balance into the theoretical model and, in the process of econometric manipulation, to detect empirical governmental policy functions in the period between the first quarter, 1971 and the fourth quarter, 2007, it is not hard to discover that there are considerable persistence in both government purchases and tax rates, with manifestation of certain degree of responses to the total amount of outstanding bonds the government has yet to pay. In other words, a governmental bond financing policy designed to render high initial bonds outstanding tends to cause suppression and crowding out of government spending in current and even later periods. Furthermore, the government is bound to plan on raising taxes in the future in order to cut financial deficit gap caused by paying back the principles and interests of the debt. Therefore, this study focuses on presenting the macroeconomic effects of fiscal policies and monetary policies, as well as the dynamic adjustment processes of macroeconomic variables based on the impact of feedback effect of bonds outstanding on government spending and tax rates. Even thought public bonds issuance or debt financing serves as a governmental fiscal instrument for financial turnover to ensure flexibility of fiscal policies, our model shows that the government should, from a long-run perspective, maintain a stable amount of bonds outstanding. Put in a different way, the level of bonds outstanding shows “mean-reverting” characteristics which rely on future government spending cut, distorting tax adjustment and seigniorage financing policy to achieve balance of budget. As a result, such setup would cause the bond-financing backed tax deduction policies to create practical effects on economies and, as the bond financing instruments are “Non-Neutrality” policies, would render the “Ricardian Equivalence Theorem” invalid in our model. In the policy aspect, this study tries to relax both “feedback rules” and setup of policy parameters for investigating the differences between long-run and short-run effects on the economy by different degrees of changes in government spending, tax rates and even money supply channels which are used to balance the fiscal deficit caused by increased bond issuance. This article also studies, through the impulse response function, the policy propagation mechanism and the dynamics of key macroeconomic variables under the situation of government spending increase, tax deduction and monetary expansion. It is obvious that the government, in the case of high debt ratios and when making all endeavors to spur economy by implementing macroeconomic policies aimed for instant results, is accustomed to seeking quick fixes only to achieve very limited effects, sometimes even to drive the economy into further recession. It is therefore evident that fiscal degradation could lead to economic disaster. Moreover, as the monetary policies have certain degrees of influence on real variables of the economy, this model will not be able to clearly analyze the “neutrality of money” in such a short period of time. The effect will only reveal in the long run. In summary, the government should keep “sound finance” as the highest guiding principle and be extremely cautious in formulating policies in order to weigh all pros and cons discreetly, thus help to achieve a benefiting economic performance that generates more confidence. Furthermore, this study assesses “goodness of fit” of the model through a “calibration” experiment. Although the simulation results show, under recursive method, intermingled good and poor occasions that are beyond satisfaction, they generally agree with the “typical characteristics” of business cycles. However, in the aspect of long-criticized labor market of the real business cycle model, the fiscal policy operation under feedback rules with introduction of public debts for sure has greatly improved on the conventional intrinsic property of high correlation between labor hours and real wage rates, by delivering a relatively low theoretical correlation coefficient, which is a big step towards the empirical results of almost zero or even weakly negative correlation between labor hours and real wage rates.