3. The theory of comparative advantage.

In economics, the law of comparative advantage refers to the ability of a party to produce a particular good or service at a lower marginal and opportunity cost over another. Even if one country is more efficient in the production of all goods (absolute advantage in all goods) than the other, both countries will still gain by trading with each other, as long as they have different relative efficiencies.

For example, if, using machinery, a worker in one country can produce both shoes and shirts at 6 per hour, and a worker in a country with less machinery can produce either 2 shoes or 4 shirts in an hour, each country can gain from trade because their internal trade-offs between shoes and shirts are different. The less-efficient country has a comparative advantage in shirts, so it finds it more efficient to produce shirts and trade them to the more-efficient country for shoes. Without trade, its opportunity cost per shoe was 2 shirts; by trading, its cost per shoe can reduce to as low as 1 shirt depending on how much trade occurs (since the more-efficient country has a 1:1 trade-off). The more-efficient country has a comparative advantage in shoes, so it can gain in efficiency by moving some workers from shirt-production to shoe-production and trading some shoes for shirts. Without trade, its cost to make a shirt was 1 shoe; by trading, its cost per shirt can go as low as 1/2 shoe depending on how much trade occurs.

The net benefits to each country are called the gains from trade. Comparative advantage was first described by David Ricardo who explained it in his 1817 book On the Principles of Political Economy and Taxation in an example involving England and Portugal. In Portugal it is possible to produce both wine and cloth with less labor than it would take to produce the same quantities in England. However the relative costs of producing those two goods are different in the two countries. In England it is very hard to produce wine, and only moderately difficult to produce cloth. In Portugal both are easy to produce. Therefore while it is cheaper to produce cloth in Portugal than England, it is cheaper still for Portugal to produce excess wine, and trade that for English cloth. Conversely England benefits from this trade because its cost for producing cloth has not changed but it can now get wine at a lower price, closer to the cost of cloth. The conclusion drawn is that each country can gain by specializing in the good where it has comparative advantage, and trading that good for the other.

Trade costs, particularly transportation, reduce and may eliminate the benefits from trade, including comparative advantage. Paul Krugman gives the following example. Using Ricardo's classic example:

Unit labor costs
	Cloth	Wine
Britain	100	110
Portugal	90	80

In the absence of transportation costs, it is efficient for Britain to produce cloth, and Portugal to produce wine, as, assuming that these trade at equal price (1 unit of cloth for 1 unit of wine) Britain can then obtain wine at a cost of 100 labor units by producing cloth and trading, rather than 110 units by producing the wine itself, and Portugal can obtain cloth at a cost of 80 units by trade rather than 90 by production.

However, in the presence of trade costs of 15 units of labor to import a good (alternatively a mix of export labor costs and import labor costs, such as 5 units to export and 10 units to import), it then costs Britain 115 units of labor to obtain wine by trade – 100 units for producing the cloth, 15 units for importing the wine, which is more expensive than producing the wine locally, and likewise for Portugal. Thus, if trade costs exceed the production advantage, it is not advantageous to trade.

Krugman proceeds to argue more speculatively that changes in the cost of trade (particularly transportation) relative to the cost of production may be a factor in changes in global patterns of trade: if trade costs decrease, such as with the advent of steam-powered shipping, trade should be expected to increase, as more comparative advantages in production can be realized. Conversely, if trade costs increase, or if production costs decrease faster than trade costs (such as via electrification of factories), then trade should be expected to decrease, as trade costs become a more significant barrier.

Conditions that maximize comparative advantage do not automatically resolve trade deficits. In fact, many real world examples where comparative advantage is attainable may require a trade deficit. For example, the amount of goods produced can be maximized, yet it may involve a net transfer of wealth from one country to the other, often because economic agents have widely different rates of saving.

As the markets change over time, the ratio of goods produced by one country versus another variously changes while maintaining the benefits of comparative advantage. This can cause national currencies to accumulate into bank deposits in foreign countries where a separate currency is used.

Macroeconomic monetary policy is often adapted to address the depletion of a nation's currency from domestic hands by the issuance of more money, leading to a wide range of historical successes and failures.