For clarity of exposition, the theory of comparative advantage is usually first outlined as though only two countries and only two commodities were involved, although the principles are by no means limited to such cases. Again for clarity, the cost of production is usually measured only in terms of labour time and effort; the cost of a unit of cloth, for example, might be given as two hours of work. The two countries will be called A and B; and the two commodities produced, wine and cloth. The labour time required to produce a unit of either commodity in either country is as follows:

As compared with country A, country B is productively inefficient. Its workers need more time to turn out a unit of wine or a unit of cloth. This relative inefficiency may result from differences in climate, in worker training or skill, in the amount of available tools and equipment, or from numerous other reasons. Ricardo took it for granted that such differences do exist, and he was not concerned with their origins.

Country A is said to have an absolute advantage in the production of both wine and cloth because it is more efficient in the production of both goods. Accordingly, A’s absolute advantage seemingly invites the conclusion that country B could not possibly compete with country A, and indeed that if trade were to be opened up between them, country B would be competitively overwhelmed. Ricardo, who focused chiefly on labour costs, insisted that this conclusion is false. The critical factor is that country B’s disadvantage is less pronounced in wine production, in which its workers require only twice as much time for a single unit as do the workers in A, than it is in cloth production, in which the required time is three times as great. This means, Ricardo pointed out, that country B will have a comparative advantage in wine production. Both countries will profit, in terms of the real income they enjoy, if country B specializes in wine production, exporting part of its output to country A, and if country A specializes in cloth production, exporting part of its output to country B. Paradoxical though it may seem, it is preferable for country A to leave wine production to country B, despite the fact that A’s workers can produce wine of equal quality in half the time that B’s workers can do so.

The incentive to export and to import can be explained in price terms. In country A (before international trade), the price of cloth ought to be twice that of wine, since a unit of cloth requires twice as much labour effort. If this price ratio is not satisfied, one of the two commodities will be overpriced and the other underpriced. Labour will then move out of the underpriced occupation and into the other, until the resulting shortage of the underpriced commodity drives up its price. In country B (again, before trade), a cloth unit should cost three times as much as a wine unit, since a unit of cloth requires three times as much labour effort. Hence, a typical before-trade price relationship, matching the underlying real cost ratio in each country, might be as follows:

The absolute levels of price do not matter. All that is necessary is that in each country the ratio of the two prices should match the labour–cost ratio.

As soon as the opportunity for exchange between the two countries is opened up, the difference between the wine–cloth price ratio in country A (namely, 5:10, or 1:2) and that in country B (which is 1:3) provides the opportunity of a trading profit. Cloth will begin to move from A to B, and wine from B to A. As an illustration, a trader in A, starting with an initial investment of $10, would buy a unit of cloth, sell it in B for £3, buy 3 units of B’s wine with the proceeds, and sell this in A for $15. (This example assumes, for simplicity, that costs of transporting goods are negligible or zero. The introduction of transport costs complicates the analysis somewhat, but it does not change the conclusions, unless these costs are so high as to make trade impossible.)

So long as the ratio of prices in country A differs from that in country B, the flow of goods between the two countries will steadily increase as traders become increasingly aware of the profit to be obtained by moving goods between the two countries. Prices, however, will be affected by these changing flows of goods. The wine price in country A, for example, can be expected to fall as larger and larger supplies of imported wine become available. Thus A’s wine–cloth price ratio of 1:2 will fall. For comparable reasons, B’s price ratio of 1:3 will rise. When the two ratios meet, at some intermediate level (in the example earlier, at 1:21 2), the flow of goods will stabilize.

At a later stage in the history of comparative-advantage theory, English philosopher and political economist John Stuart Mill showed that the determination of the exact after-trade price ratio was a supply-and-demand problem. At each possible intermediate ratio (within the range of 1:2 and 1:3), country A would want to import a particular quantity of wine and export a particular quantity of cloth. At that same possible ratio, country B would also wish to import and export particular amounts of cloth and of wine. For any intermediate ratio taken at random, however, A’s export-import quantities are unlikely to match those of B. Ordinarily, there will be just one intermediate ratio at which the quantities correspond; that is the final trading ratio at which quantities exchanged will stabilize. Indeed, once they have stabilized, there is no further profit in exchanging goods. Even with such profits eliminated, however, there is no reason why A producers should want to stop selling part of their cloth in B, since the return there is as good as that obtained from domestic sales. Furthermore, any falloff in the amounts exported and imported would reintroduce profit opportunities.

In this simple example, based on labour costs, the result is complete (and unrealistic) specialization: country A’s entire labour force will move to cloth production and country B’s to wine production. More elaborate comparative-advantage models recognize production costs other than labour (that is, the costs of land and of capital). In such models, part of country A’s wine industry may survive and compete effectively against imports, as may also part of B’s cloth industry. The models can be expanded in other ways—for example, by involving more than two countries or products, by adding transport costs, or by accommodating a number of other variables such as labour conditions and product quality. The essential conclusions, however, come from the elementary model used above, so that this model, despite its simplicity, still provides a workable outline of the theory. (It should be noted that even the most elaborate comparative-advantage models continue to rely on certain simplifying assumptions without which the basic conclusions do not necessarily hold. These assumptions are discussed below.)

As noted earlier, the effect of this analysis is to correct any false first impression that low-productivity countries are at a hopeless disadvantage in trading with high-productivity ones. The impression is false, that is, if one assumes, as comparative-advantage theory does, that international trade is an exchange of goods between countries. It is pointless for country A to sell goods to country B, whatever its labour-cost advantages, if there is nothing that it can profitably take back in exchange for its sales. With one exception, there will always be at least one commodity that a low-productivity country such as B can successfully export. Country B must of course pay a price for its low productivity, as compared with A; but that price is a lower per capita domestic income and not a disadvantage in international trading. For trading purposes, absolute productivity levels are unimportant; country B will always find one or more commodities in which it enjoys a comparative advantage (that is, a commodity in the production of which its absolute disadvantage is least). The one exception is that case in which productivity ratios, and consequently pretrade price ratios, happen to match one another in two countries. This would have been the case had country B required four labour hours (instead of six) to produce a unit of cloth. In such a circumstance, there would be no incentive for either country to engage in trade, nor would there be any gain from trading. In a two-commodity example such as that employed, it might not be unusual to find matching productivity and price ratios. But as soon as one moves on to cases of three and more commodities, the statistical probability of encountering precisely equal ratios becomes very small indeed.

The major purpose of the theory of comparative advantage is to illustrate the gains from international trade. Each country benefits by specializing in those occupations in which it is relatively efficient; each should export part of that production and take, in exchange, those goods in whose production it is, for whatever reason, at a comparative disadvantage. The theory of comparative advantage thus provides a strong argument for free trade—and indeed for more of a laissez-faire attitude with respect to trade. Based on this uncomplicated example, the supporting argument is simple: specialization and free exchange among nations yield higher real income for the participants.

The fact that a country will enjoy higher real income as a consequence of the opening up of trade does not mean, of course, that every family or individual within the country will share in that benefit. Producer groups affected by import competition obviously will suffer, to at least some degree. Individuals are at risk of losing their jobs if the items they make can be produced more cheaply elsewhere. Comparative-advantage theorists concede that free trade would affect the relative income position of such groups—and perhaps even their absolute income level. But they insist that the special interests of these groups clash with the total national interest, and the most that comparative-advantage proponents are usually willing to concede is the possible need for temporary protection against import competition (i.e., to allow those who lose their jobs to international competition to find new occupations).

Nations do, of course, maintain tariffs and other barriers to imports. For discussion of the reasons for this seeming clash between actual policies and the lessons of the theory of comparative advantage, see State interference in international trade.