Mundell-Fleming Model: Negative Money Market Shock Analysis

The IS* curve for an open economy is derived from Y = C(Y-T) + I(r*) + G + NX(E), showing aggregate income equals consumption plus investment (determined by world interest rate) plus government spending plus net exports. Net exports decrease as the exchange rate appreciates because foreign goods become relatively cheaper. The LM* curve derives from M/P = L(r*, Y), becoming vertical since the exchange rate doesn't affect money market equilibrium when r = r*. With perfect capital mobility, domestic interest rates must equal world interest rates. Equilibrium occurs at the intersection of IS* and LM*, determining endogenous variables Y (income) and E (exchange rate), with policy variables including G, T, M, P, and r*.

The Mundell-Fleming model extends the IS-LM framework to analyze small open economies with high trade openness. It examines two exchange rate mechanisms: floating rates (market-determined) and fixed rates (central bank-managed). A key assumption is perfect capital mobility, meaning domestic real interest rates equal world rates. The model focuses on goods market equilibrium (IS curve) and money market equilibrium (LM curve), with the IS curve showing the relationship between exchange rate and output. The LM curve becomes vertical under perfect capital mobility because interest rates are fixed by the world market.

The Mundell-Fleming model extends the IS-LM framework to open economies with five key assumptions: small open economy with perfect capital mobility (r = r*), inclusion of net exports in the goods market, inverse relationship between net exports and exchange rate, price level equality between domestic and foreign economies, and policy effectiveness depending on exchange rate regime. The IS curve incorporates consumption, investment, government spending, and net exports, while the LM curve reflects money market equilibrium with fixed interest rates under perfect capital mobility.

The Mundell-Fleming model extends the IS-LM framework to small open economies with two key assumptions: (1) Domestic interest rate equals the world interest rate (r = r*), meaning the economy cannot influence global rates; (2) Perfect capital mobility, meaning capital flows freely to eliminate interest rate differentials. The IS* curve represents goods market equilibrium including net exports as a function of the exchange rate. The LM* curve represents money market equilibrium and is vertical because the interest rate is fixed at the world level. Equilibrium occurs at the intersection of IS* and LM*, determining both the exchange rate and income level.

The Mundell-Fleming model extends the IS-LM framework to open economies by incorporating net exports and international capital flows. In a small open economy with perfect capital mobility, the domestic interest rate equals the international interest rate (r* = r*), making the LM* curve vertical. The IS* curve shows the relationship between output and nominal exchange rate, with a negative slope because depreciation (lower exchange rate) increases net exports and thus output. Equilibrium occurs at the intersection of IS* and LM* curves, determining equilibrium output and exchange rate.

The LM curve shifts when money supply or money demand changes. An increase in money supply shifts the LM curve to the right (forward shift), while a decrease shifts it to the left (backward shift). A shift in liquidity preference (money demand changes due to factors other than income or interest rate changes) also shifts the LM curve. For example, if people become pessimistic about future bond prices, they may increase money demand, shifting the LM curve to the left. When the LM curve shifts due to money demand changes, equilibrium interest rates change even though income remains unchanged, as interest rate adjustments are needed to restore money market equilibrium.

Liquidity Preference Theory, developed by Keynes, explains interest rate determination through money supply and demand. The supply of real money balances is fixed (M/P = M̄/P̄), while money demand depends on income and interest rates. The LM curve represents all combinations of interest rates and income that equilibrate money supply and demand. It has a positive slope because higher income increases money demand, requiring higher interest rates to restore equilibrium. Central banks control interest rates by adjusting money supply: increasing supply lowers rates, decreasing supply raises rates. The LM curve shifts right with expansionary monetary policy and left with contractionary policy.

When money demand (liquidity preference) increases, the LM curve shifts to the left. This occurs because at any given income level, higher money demand requires a higher interest rate to equilibrate the money market, resulting in a new LM curve positioned to the left of the original.

This section presents Keynes's liquidity preference theory explaining interest rate determination. Real money supply (M/P) is fixed by monetary authorities, while money demand depends on interest rates and income through three motives: transactions, precautionary, and speculative. The speculative motive creates the inverse relationship between interest rates and money demand. Money market equilibrium occurs where M/P = L(r, Y). The LM curve slopes upward because higher income increases transactional money demand, creating excess demand that pushes interest rates upward to restore equilibrium. Changes in money supply shift the LM curve—reducing supply shifts it upward, increasing supply shifts it downward. Non-monetary factors affecting money demand (like credit card fraud increasing cash usage) also shift the LM curve.

The theory of liquidity preference explains that interest rates are determined by money supply and money demand. The LM curve represents all combinations of interest rate and income that equate money supply and demand. It slopes upward because higher income increases money demand, requiring higher interest rates to maintain equilibrium. Expansionary monetary policy (increasing money supply) shifts the LM curve right, lowering interest rates and increasing income. Changes in money demand parameters (such as increased cash usage from credit card fraud) shift the LM curve left, raising interest rates. The model assumes fixed prices in the short term, so equilibrium changes come from curve shifts rather than price adjustments.

Exchange rate regimes determine how domestic currencies are valued relative to other currencies and significantly affect monetary policy autonomy. Fixed exchange rate regimes involve pegging the domestic currency to another currency or basket, requiring central bank intervention to maintain the peg. Floating exchange rate regimes allow market forces to determine currency values. Under fixed rates, monetary policy is constrained because the central bank must maintain the peg, limiting its ability to change money supply. Under floating rates, monetary policy is more effective because the central bank can change money supply without affecting exchange rates. Expansionary monetary policy under floating rates lowers rates, increases output, and depreciates currency. Expansionary policy under fixed rates may be ineffective due to peg constraints. The choice between regimes involves trade-offs between monetary policy autonomy, exchange rate stability, and international competitiveness.

Fixed exchange rates provide predictability for international trade and investment but constrain monetary policy independence. When domestic interest rates rise, capital flows in, requiring central bank intervention to prevent appreciation. Floating rates allow market forces to determine currency values but create uncertainty for international transactions. Dirty floating combines both approaches with occasional central bank intervention to prevent excessive volatility. The choice of regime affects a country's ability to pursue independent monetary policy.

Exchange rate regimes are classified into fixed and flexible systems. Fixed regimes maintain stable currency values, while flexible regimes allow market-driven fluctuations. Historically, fixed rates dominated until the 1970s. Under flexible rates, monetary policy retains autonomy: central banks can adjust interest rates to address inflation, with currency appreciation offsetting expansionary effects. Under fixed rates, monetary policy autonomy is lost: domestic interest rates must equal foreign rates to maintain the fixed exchange rate. Countries like Bulgaria, Hong Kong, and Saudi Arabia use fixed rates, while the US, UK, and Japan use flexible rates. The fundamental trade-off is between exchange rate stability and monetary policy independence.

This section covers the core mechanics of flexible and fixed exchange rate regimes. In flexible regimes with inflation targeting, the long-run depreciation rate equals the inflation differential between countries, while uncovered interest parity requires the same relationship for interest rates. Using South Africa (4.5% inflation target) and the US (2% target) as examples, the 2.5% differential constrains monetary policy autonomy. Countries can adjust rates in the short run to respond to demand shocks but face constraints over time. Fixed exchange rate regimes eliminate monetary policy independence entirely, forcing interest rates to match foreign rates and requiring inflation alignment to prevent real exchange rate changes.

Under floating exchange rates, expansionary monetary policy shifts LM right, lowering interest rates, stimulating investment, and causing currency depreciation that increases trade balance, ultimately raising output. Under fixed exchange rates, monetary policy is not possible: attempting to expand money supply would lower interest rates below the pegged rate, breaking the peg. The central bank must simultaneously reduce money supply to defend the peg, demonstrating that monetary policy autonomy is lost under fixed exchange rate regimes.