Quantity responses must be large enough to offset the price effect.
Under the usual Marshall-Lerner assumptions, including an initially balanced trade position, a depreciation improves the trade balance when the sum of the absolute price elasticities of demand for exports and imports exceeds one. Export and import quantities must respond sufficiently to the changed relative prices. This is a conditional statement about the trade balance, not a guarantee of higher living standards. Contracts, adjustment time, pass-through and imported production costs matter.
- Demand responsiveness
- Price elasticity of demand measures how strongly quantity demanded responds to a percentage price change. Its absolute value is the magnitude, ignoring the minus sign. Here the relevant responses are demand for exports and imports.
- Marshall-Lerner condition
- Under the standard assumptions, including initially balanced trade, depreciation improves the trade balance when the two absolute demand elasticities add to more than one. The trade balance compares export earnings with import spending, so quantities must respond enough to offset the price effects.
Use the condition within its boundary
Standard assumptions
The textbook result assumes a small exchange-rate change from initially balanced trade. Prices stay fixed in sellers' currencies, the currency change reaches buyers' prices, and producers can supply the quantities demanded. Different initial imbalances or pricing behaviour require care.
Absolute elasticities
Use the magnitudes of export and import demand elasticities. The rule concerns their combined responsiveness, not the negative sign convention of demand elasticity.
Time
Existing contracts and slow switching can delay quantity responses; a later improvement after initial deterioration is often called a J-curve pattern. It is a possible pattern, not a guarantee.
Assessment boundary
Understand the condition and explain its relevance. Deriving the formula or calculating it is not required by the stated 2026 H2 syllabus.
Worked example: Import costs change before orders can adjust
A currency depreciates, making exports cheaper to foreign buyers and imports dearer in domestic currency. Existing orders are fixed initially, but buyers can change suppliers and quantities later.
- With quantities slow to adjust, the import bill can rise before sufficient export and import responses occur.
- Over time, more responsive quantities may allow the trade balance to improve under the condition's assumptions.
- Higher imported-input costs and weak foreign demand can still limit output or welfare gains. A sufficient elasticity condition is not a complete policy evaluation.
Watch out for this
Cheaper exports mean the trade balance must improve immediately.
The trade balance compares values, not only quantities. Import prices and the timing of quantity responses also matter.
Check your understanding
Why may the trade balance initially worsen after depreciation?
- Contracted quantities change slowly while imports cost more in domestic currency.
- Depreciation always makes imports cheaper in domestic currency.
- The condition guarantees improvement under every starting balance and every pricing assumption.