9.1 IS-LM-BP model and policy implications

IS-LM-BP Model and Policy Analysis

The IS-LM-BP model combines the goods market, money market, and foreign exchange market into a single framework for analyzing macroeconomic policy in open economies. It reveals how fiscal and monetary policies affect output, interest rates, and the balance of payments depending on the exchange rate regime and the degree of capital mobility. The model also underpins the "impossible trinity," which explains the fundamental trade-offs policymakers face in a globalized economy.

Interaction in the IS-LM-BP Model

This framework brings together three curves, each representing equilibrium in a different market:

• IS curve (goods market): Shows all combinations of interest rates and output where planned investment equals saving. Shifts right with expansionary fiscal policy (higher government spending or tax cuts) and left with contractionary fiscal policy.
• LM curve (money market): Shows all combinations of interest rates and output where money supply equals money demand. Shifts right with expansionary monetary policy (increased money supply) and left with contractionary monetary policy.
• BP curve (balance of payments): Shows all combinations of interest rates and output where the balance of payments equals zero. Points above the BP curve indicate a surplus; points below indicate a deficit.

In a closed economy, equilibrium is simply where IS and LM intersect. Opening the economy adds the BP curve, which captures the effects of international trade and capital flows.

General equilibrium occurs where all three curves intersect simultaneously. At that point, the goods market, money market, and foreign exchange market are all in balance. The position and slope of the BP curve depend on two things: the exchange rate regime (fixed vs. floating) and the degree of capital mobility.

Policy Effectiveness Across Exchange Rate Regimes

The effectiveness of fiscal and monetary policy flips depending on whether a country uses a fixed or flexible exchange rate. This is one of the most important results of the model.

Fixed Exchange Rate Regime

Under fixed rates, the central bank commits to maintaining a specific exchange rate by buying or selling foreign reserves.

• Fiscal policy is effective. Expansionary fiscal policy shifts IS right, raising output and interest rates. Higher interest rates attract foreign capital, creating upward pressure on the currency. To prevent appreciation, the central bank buys foreign reserves and sells domestic currency, which expands the money supply and shifts LM right. The result: output increases further, and equilibrium is restored at a higher level of output.
• Monetary policy is ineffective. Expansionary monetary policy shifts LM right, lowering interest rates. Lower rates cause capital outflows, putting downward pressure on the currency. To defend the fixed rate, the central bank sells foreign reserves and buys domestic currency, shrinking the money supply and shifting LM back to its original position. The net effect on output is zero.

Flexible Exchange Rate Regime

Under flexible rates, the exchange rate adjusts freely to balance the foreign exchange market, and the central bank does not intervene.

• Fiscal policy is less effective. Expansionary fiscal policy shifts IS right, raising interest rates. Higher rates attract capital inflows, which appreciate the domestic currency. A stronger currency makes exports more expensive and imports cheaper, reducing net exports. This partially offsets the initial fiscal expansion by shifting IS back to the left. Output still rises, but by less than under fixed rates.
• Monetary policy is effective. Expansionary monetary policy shifts LM right, lowering interest rates. Lower rates cause capital outflows, which depreciate the domestic currency. A weaker currency boosts net exports, shifting IS right and reinforcing the initial monetary expansion. Output increases substantially.

Capital Mobility in the IS-LM-BP Model

Capital mobility refers to how easily financial assets flow across borders in response to interest rate differentials. It directly determines the slope of the BP curve.

• Perfect capital mobility: The BP curve is horizontal. Even a tiny change in the domestic interest rate relative to the world rate triggers massive capital flows. This is the standard assumption in the Mundell-Fleming model.
• Low (imperfect) capital mobility: The BP curve is steeper than the LM curve. Larger interest rate changes are needed to induce enough capital flows to restore balance of payments equilibrium.
• Zero capital mobility: The BP curve is vertical. The balance of payments depends entirely on the trade balance, with no role for capital flows.

How capital mobility affects policy outcomes:

Under perfect capital mobility with fixed rates:

• Fiscal policy is highly effective because the horizontal BP curve means capital flows quickly finance any payments imbalance, and the central bank's reserve operations amplify the output effect.
• Monetary policy is completely ineffective because any change in the money supply is immediately reversed by capital flows and central bank intervention.

Under perfect capital mobility with flexible rates:

• Fiscal policy is weakened (and fully ineffective in the extreme case) because currency appreciation crowds out net exports by exactly the amount of the fiscal stimulus.
• Monetary policy is highly effective because currency depreciation reinforces the expansion through higher net exports.

Under low capital mobility, results fall between the extremes. The steeper BP curve gives policymakers somewhat more autonomy because capital doesn't flow as freely in response to interest rate changes. Both fiscal and monetary policy retain some effectiveness regardless of the exchange rate regime, though the relative advantage still follows the same pattern (fiscal works better under fixed rates, monetary works better under flexible rates).

The Impossible Trinity and Its Implications

The impossible trinity (also called the trilemma) states that a country cannot simultaneously maintain all three of the following:

1. A fixed exchange rate
2. Free capital mobility
3. An independent monetary policy

A country can achieve any two, but pursuing all three at once is unsustainable. The logic follows directly from the model: if capital flows freely and the exchange rate is fixed, the central bank must use monetary policy to defend the peg, sacrificing independence.

Each combination of two goals implies a specific sacrifice:

Understanding the trilemma is essential because it frames the core policy choice every open economy faces. There's no way around it: gaining control in one area always means giving it up in another.