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Marshall-Lerner Condition and the J-Curve Effect
IB DP Economics — Unit 4: The Global Economy | HL | 4.6 Balance of Payments [HL Extensions]
📋 What You Will Learn
- Explain why a currency depreciation does not automatically improve a country's current account balance
- State and apply the Marshall-Lerner condition to determine when depreciation improves the trade balance
- Analyse the J-curve effect and explain why the current account worsens before it improves following depreciation
- Apply the concepts of price elasticity of demand for exports and imports to real-world trade scenarios
- Evaluate the effectiveness of exchange rate depreciation as a policy tool for correcting a current account deficit
🔍 Guiding Question If a country deliberately weakens its currency to boost exports, under what conditions will this actually improve its trade balance — and why might the situation get worse before it gets better?
🌍 Why This Matters In 2022–2023, the Egyptian pound lost over 50% of its value against the US dollar following IMF-mandated devaluations. Rather than immediately boosting Egypt's trade balance, the current account deficit initially widened — import bills surged in local currency terms while export revenues took months to respond. This is precisely the dynamic the Marshall-Lerner condition and J-curve effect predict.
The Trade Balance and Exchange Rates — The Core Problem
A country running a persistent current account deficit may consider depreciating or devaluing its currency as a corrective policy. The logic seems straightforward: a weaker currency makes exports cheaper in foreign markets and imports more expensive domestically, so export volumes should rise and import volumes should fall — improving the trade balance.
But this intuition is incomplete. Whether the trade balance actually improves depends critically on how responsive export and import quantities are to the price changes caused by depreciation. This is where elasticity enters the picture.
📖 Definition — Trade Balance (Balance of Trade in Goods and Services) The trade balance is the difference between the value of a country's exports and the value of its imports: Trade Balance = Export Revenue − Import Expenditure. A trade deficit exists when import expenditure exceeds export revenue.
📖 Definition — Currency Depreciation Depreciation is a fall in the market value of a currency relative to other currencies under a floating exchange rate system. Devaluation refers to a deliberate reduction in a currency's fixed exchange rate by government or central bank action.
After depreciation, the price of exports in foreign currency falls (foreigners can buy more with the same amount of their currency), and the price of imports in domestic currency rises (residents must pay more in local currency for the same imported goods). The effect on the trade balance depends on whether the volume changes are large enough to outweigh the price changes.
The Marshall-Lerner Condition
The Marshall-Lerner condition, developed independently by economists Alfred Marshall and Abba Lerner, provides the precise elasticity threshold that determines whether a currency depreciation will improve the trade balance.
📖 Definition — The Marshall-Lerner Condition The Marshall-Lerner condition states that a depreciation of the exchange rate will improve a country's trade balance (current account) if and only if the sum of the price elasticity of demand for exports (PED X ) and the price elasticity of demand for imports (PED M ) is greater than one (in absolute value terms): |PED X | + |PED M | > 1.
Marshall-Lerner Condition
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