Marshall-Lerner Condition

It seems obvious that a cheaper currency should boost exports and shrink imports, improving the trade balance. But economists know it isn't automatic. The Marshall-Lerner condition spells out exactly when a devaluation actually helps: only if the sum of the price elasticities of demand for exports and imports is greater than 1.

The Logic

When a currency weakens, exports become cheaper abroad and imports dearer at home. Whether the trade balance improves depends on how *responsive* buyers are to those price changes. If foreign demand for your exports and domestic demand for imports are both elastic — quantities react strongly to price — the volume effects dominate and the trade balance improves. If demand is inelastic (people keep buying roughly the same amount despite higher prices), a weaker currency can actually *worsen* the trade balance, because you pay more for the same imports.

The J-Curve Twist

In practice, the trade balance often follows a J-curve after a devaluation: it gets worse first, then better. Right after the rupee falls, contracts are locked and consumers are slow to switch, so the higher import bill bites immediately while export volumes take time to respond. Only later, once buyers adjust, does the Marshall-Lerner condition kick in and the balance improves.

Relevance to India

This matters for India because of its structural import dependence. A large share of imports — crude oil, gold, electronics and fertiliser — is relatively price-inelastic in the short run; India needs the oil regardless of the rupee's level. So a depreciating rupee doesn't quickly fix the trade deficit and can worsen the current account by inflating the oil import bill. That's one reason the RBI manages rupee volatility rather than relying on devaluation to correct external imbalances. For anyone analysing the rupee, the current account or India's trade data, the Marshall-Lerner condition is the theory explaining why a weaker currency is no magic cure.