Step 3: Comparative Advantage

Why trade at all? Two flatmates and a puzzle

Two flatmates, Ali and Ben, share the cooking and cleaning. Ali is faster at both. Should Ali do everything? Economists say no: both get more free time if each concentrates on the job where giving up the other costs them least. That surprising result is the idea behind this step.

Countries face the same question. Each country gives something up (its exports) to get something back (its imports). How can both be better off? For centuries, many people believed one country could only gain at another’s expense, an idea called mercantilism. In 1817 David Ricardo showed it was wrong, using simple arithmetic.

Absolute advantage: being the better producer

A country has an absolute advantage in a good if it can produce more of it than another country with the same resources. Saudi Arabia has an absolute advantage in oil. New Zealand’s grasslands give it one in lamb.

Adam Smith, writing in 1776, said each country should specialise in what it is best at and trade for the rest. But what about a country that is not the best at anything? On Smith’s logic it has nothing to offer. Ricardo showed that this is wrong.

Comparative advantage: it is all about opportunity cost

Ricardo’s insight was to compare opportunity costs, not output. Recall from Step 1 that opportunity cost is what you give up to get something.

With a straight-line PPC (Step 2), the opportunity cost of one unit of a good is simply:

The chapter’s example uses two countries with linear PPCs. The USA can make 60 cars or 120 wheat. China can make 100 cars or 50 wheat.

China gives up less wheat per car (0.5 against 2), so China has the comparative advantage in cars. The USA gives up fewer cars per unit of wheat (0.5 against 2), so the USA has the comparative advantage in wheat. Notice that the two opportunity costs for one country are always reciprocals: \( 2 \) and \( \tfrac{1}{2} \).

How specialisation raises world output

Here is Ricardo’s key result. If each country puts its resources into the good where it has the lower opportunity cost, world output of both goods can rise, with no new resources or technology.

Trading beyond the PPC: the terms of trade

Specialisation raises world output, but a country that makes only shirts still wants phones. So the two countries trade. The swap rate is the terms of trade.

For both countries to gain, the rate must lie between their two opportunity costs:

In the example above, 1 phone costs A 5 shirts and costs B 1 shirt. So any rate between 1 and 5 shirts per phone works. Take 2 shirts per phone. A gets a phone for 2 shirts instead of the 5 it would give up making one itself. B gets 2 shirts for each phone instead of only 1 by making them at home. At exactly 1 or 5, one country is indifferent. Outside that range, one country would rather produce everything itself (a situation called autarky).

Now suppose B trades 15 phones to A at 2 shirts each, so it receives 30 shirts:

• B consumes \( 40 – 15 = 25 \) phones and 30 shirts. On its own, B could make 40 shirts or 40 phones, so 30 shirts plus 25 phones is beyond its PPC.
• A consumes \( 100 – 30 = 70 \) shirts and 15 phones. On its own, if A made 70 shirts it could only make 6 phones, so this point is also beyond its PPC.

This is the key picture. The PPC limits what a country can produce. Trade extends what it can consume.

Better at everything, and still gaining

The most surprising part: trade pays even when one country has an absolute advantage in both goods. Imagine the world’s best brain surgeon who is also the world’s fastest typist. Every hour spent typing is an hour not spent on surgery, where their opportunity cost is huge. A slower secretary has a much lower opportunity cost of typing. Each does the job where their own opportunity cost is lowest, and both gain.

Bangladesh shows this in real life. In 1980 it was one of the poorest countries on Earth, with no absolute advantage in any high-value good. But it had a comparative advantage in labour-intensive garments, and by specialising it became the world’s second-largest garment exporter after China. Critics rightly point to low wages and dangerous conditions, but the basic theory has held up.

The model has real limits, and examiners reward students who mention them:

• Transport costs and tariffs can wipe out small differences in opportunity cost.
• Externalities: pollution and health costs are not counted in the opportunity cost.
• Distribution: total output rises, but some groups lose, such as workers in industries that compete with imports.
• It is static: a country stuck in low-skill goods may never build higher-value industries, which is the “infant industry” argument for temporary protection.
• National security: some goods, such as food, medicine and chips, may be too important to rely on imports.

Practise: 6 questions

Try each one on paper first, then check the worked answer.

Check yourself

The short version

• Absolute advantage means producing more; comparative advantage means producing at a lower opportunity cost.
• Calculate the opportunity cost of each good in each country; the lower one wins the comparative advantage.
• Specialising according to comparative advantage can raise world output of both goods with no extra resources.
• Trade lets a country consume beyond its PPC, provided the terms of trade lie between the two opportunity costs.
• Even a country that is better at everything gains from trade, but transport costs, externalities, uneven gains and security concerns limit the simple model.

This guide covers the core of Chapter 3 of Economics Made Simple. The full chapter adds PPC diagrams of trade, the full Bangladesh case study, more on the limits of free trade and a 15-question exam practice set with a full answer key.