9.2 Mundell-Fleming model

The Mundell-Fleming model extends the IS-LM framework to a small open economy, showing how fiscal and monetary policy work under different exchange rate regimes. Its central finding is a sharp trade-off: fiscal policy is powerful under fixed exchange rates but ineffective under flexible rates, while monetary policy is the mirror image. Understanding this trade-off is essential for open-economy macroeconomics.

The Mundell-Fleming Model

Components of the Mundell-Fleming Model

The model starts with a small open economy, meaning the country is too small to influence world interest rates or world prices. Two baseline assumptions hold throughout:

• The price level is fixed (short-run analysis only).
• The economy operates below full employment, so output can rise without hitting capacity constraints.

Perfect capital mobility is the other critical assumption. Domestic and foreign bonds are perfect substitutes, so investors move funds freely across borders until returns equalize. This gives us the interest rate parity condition: $i = i^*$, where $i$ is the domestic interest rate and $i^*$ is the foreign (world) interest rate. Any gap between the two triggers instant capital flows that close it.

Goods market equilibrium (IS curve) requires that total output equals total spending:

$Y = C + I + G + NX$

• $C$ = household consumption
• $I$ = firm investment (sensitive to the interest rate)
• $G$ = government spending
• $NX$ = net exports (exports minus imports), which depends on the real exchange rate and foreign income

Money market equilibrium (LM curve) requires that the real money supply equals money demand:

$\frac{M}{P} = L(Y, i)$

The central bank sets the nominal money supply $M$ exogenously. Money demand $L$ rises with income $Y$ (more transactions) and falls with higher interest rates $i$ (higher opportunity cost of holding money).

The key difference from a closed-economy IS-LM model is the addition of net exports and the constraint that $i = i^*$. The exchange rate becomes the variable that adjusts to maintain equilibrium, and its behavior depends entirely on the exchange rate regime.

Fiscal vs. Monetary Policy Effects

Fiscal Policy Under Fixed Exchange Rates: Effective

1. The government increases spending (or cuts taxes), shifting the IS curve right.
2. This puts upward pressure on the domestic interest rate, pushing it above $i^*$.
3. Capital inflows rush in, creating pressure for the domestic currency to appreciate.
4. To defend the fixed exchange rate, the central bank buys foreign currency and sells domestic currency, expanding the domestic money supply.
5. The LM curve shifts right to accommodate the higher output.
6. Result: output rises with no crowding out, because the interest rate stays at $i^*$ and the money supply expands automatically.

Fiscal Policy Under Flexible Exchange Rates: Ineffective

1. The government increases spending, shifting the IS curve right.
2. Upward pressure on the interest rate attracts capital inflows.
3. Increased demand for the domestic currency causes it to appreciate.
4. Appreciation makes exports more expensive abroad and imports cheaper at home, so $NX$ falls.
5. The decline in net exports shifts the IS curve back to the left, fully offsetting the original fiscal expansion.
6. Result: output is unchanged. Government spending simply crowds out net exports.

Monetary Policy Under Fixed Exchange Rates: Ineffective

1. The central bank increases the money supply, shifting the LM curve right.
2. The domestic interest rate falls below $i^*$, triggering capital outflows.
3. Outflows put depreciation pressure on the domestic currency.
4. To defend the peg, the central bank sells foreign reserves and buys domestic currency, shrinking the money supply back.
5. The LM curve shifts back to its original position.
6. Result: output is unchanged. The central bank cannot independently control the money supply under a fixed rate; it becomes endogenous, dictated by the need to defend the peg.

Monetary Policy Under Flexible Exchange Rates: Effective

1. The central bank increases the money supply, shifting the LM curve right.
2. The domestic interest rate falls below $i^*$, triggering capital outflows.
3. Outflows cause the domestic currency to depreciate.
4. Depreciation makes exports cheaper and imports more expensive, so $NX$ rises.
5. Higher net exports shift the IS curve right, boosting aggregate demand.
6. Result: output rises. The exchange rate channel amplifies the effect of monetary policy.

Foreign Interest Rate Impacts

A rise in the foreign interest rate $i^*$ creates pressure for capital to flow out of the domestic economy. The consequences depend on the exchange rate regime.

Under fixed exchange rates:

1. Higher $i^*$ draws capital abroad, putting depreciation pressure on the domestic currency.
2. The central bank defends the peg by selling foreign reserves and buying domestic currency.
3. This contracts the domestic money supply, pushing the domestic interest rate up toward the new $i^*$.
4. Result: output falls. The economy effectively "imports" the tighter monetary conditions from abroad.

Under flexible exchange rates:

1. Higher $i^*$ triggers capital outflows and the domestic currency depreciates.
2. Depreciation boosts net exports, shifting the IS curve right.
3. Result: output rises. The economy benefits from improved competitiveness, even though the shock originated abroad.

This contrast highlights a key insight: fixed exchange rate regimes transmit foreign monetary shocks directly into the domestic economy, while flexible rates provide a buffer through exchange rate adjustment.

Limitations of the Mundell-Fleming Model

• Small open economy assumption. The model doesn't apply well to large economies like the United States or China, whose policies move world interest rates and exchange rates.
• Perfect capital mobility. Many countries impose capital controls, and domestic and foreign assets are rarely perfect substitutes due to differences in risk, liquidity, and regulation.
• Fixed price level. By holding prices constant, the model ignores inflation dynamics and real exchange rate adjustments that matter over longer horizons.
• Short-run focus only. It captures immediate policy impacts but not the longer-run adjustments in prices, wages, expectations, or debt accumulation.
• No role for expectations. Agents don't anticipate future policy changes or exchange rate movements, which in reality heavily influence capital flows and investment decisions.
• Simplified financial markets. The model omits banking sector dynamics, financial frictions, and the possibility of speculative attacks on fixed exchange rates.

Despite these limitations, the Mundell-Fleming model remains the standard framework for thinking about the short-run trade-offs between exchange rate regimes and policy effectiveness. Its core lesson, that you cannot simultaneously have free capital mobility, a fixed exchange rate, and independent monetary policy (the "impossible trinity"), is one of the most important results in international economics.