Comparative Advantage

The Ricardian setup gives each country a fixed output capability for two goods, stated either as units produced per worker (or per hour) or as total output from a fixed labour force. Opportunity cost is the ratio between the two: the cost of one unit of the first good equals the output of the second good divided by the output of the first. If one worker-day yields four units of cloth or two units of wine, one unit of wine costs two units of cloth, and one unit of cloth costs half a unit of wine. The two ratios are reciprocals, which is why in a two-good, two-country world each side must hold comparative advantage in exactly one good unless the ratios are identical.

Because the ratio is dimensionless, the scale of productivity cancels out. Doubling both of a country's output figures leaves its opportunity costs unchanged, which is precisely the reason absolute productivity carries no information about the direction of trade.

Specialisation alone creates nothing; the gain appears only when goods are exchanged at a ratio that neither party could achieve domestically. The feasible range for the terms of trade lies strictly between the two countries' autarky opportunity cost ratios. Inside that band, each side obtains the imported good more cheaply than by producing it at home, and both can consume outside their own production possibility frontier: the classic result that trade relaxes the domestic transformation constraint without any change in technology.

Where within the band the price settles determines the split. A terms-of-trade outcome close to one country's domestic ratio leaves that country with almost no gain and hands most of the surplus to its partner. Relative country size and demand conditions push the world price toward the larger economy's internal ratio, which is why small open economies often capture a disproportionate share of the gains from opening trade.

Ricardo attributes cost differences to technology with a single mobile factor, labour. The Heckscher-Ohlin extension makes the source explicit: with two factors, a country exports the good that uses its relatively abundant factor intensively, because relative factor abundance depresses that factor's relative price. Labour-abundant economies therefore export labour-intensive manufactures, capital- and skill-abundant economies export capital goods, research-intensive pharmaceuticals and advanced semiconductors, and resource-rich economies export commodities.

The framework also explains distributional friction. Trade raises returns to the abundant factor and compresses returns to the scarce one, so the aggregate gain coexists with identifiable losers: the political foundation of tariffs, quotas and voluntary export restraints. Comparative advantage is not static either: capital accumulation, human-capital deepening, exchange-rate shifts and supply-chain fragmentation move cost ratios over time, so the pattern of trade in a given industry can reverse within a generation.

You should be able to take a two-by-two output or input table, compute both opportunity cost ratios, state which country exports which good, and identify a range of exchange ratios at which both parties benefit. You should also be able to demonstrate that absolute productivity differences are irrelevant to that conclusion, and to explain why identical opportunity costs eliminate the basis for trade entirely.

Equally testable are the model's boundaries. The simple Ricardian result assumes constant opportunity costs, two goods, no transport costs or trade barriers, full employment and immobile factors between countries. Relaxing constant costs produces incomplete specialisation, since rising marginal costs eventually erase the cost gap. Adding transport costs, tariffs and adjustment lags narrows or eliminates the gains for marginal goods. The welfare conclusion is aggregate: it says total output rises, not that every household or sector inside each country is better off.