Why trade at all?
In Topic 1.2 you saw that a producer with scarce resources faces a trade-off. To make more of one good, it must give up some of another. That sacrifice is the opportunity cost.
Trade changes the picture. If two producers have different opportunity costs, each can concentrate on the good it makes most cheaply, in terms of what it gives up, and swap for the other. Both can then end up with more than they could make on their own. This idea is usually credited to the economist David Ricardo (1817). It applies to individuals, businesses and countries.
Key terms:
- Specialisation: using your resources to produce mainly one good (or a few goods) instead of everything you consume.
- Exchange (trade): giving some of your output in return for another producer’s output.
- Terms of trade: the rate at which one good is exchanged for another, for example “1 solar panel for 2.5 tonnes of rice”.
- Gains from trade: the extra goods each side can consume because it specialises and trades.
Absolute advantage and comparative advantage
Absolute advantage: a producer has an absolute advantage in a good if it can produce more of that good than another producer with the same quantity of resources. Put the other way round, it needs fewer resources (such as hours of labour) to make one unit.
Comparative advantage: a producer has a comparative advantage in a good if it can produce that good at a lower opportunity cost than another producer.
The two ideas answer different questions. Absolute advantage asks, “Who is more productive?” Comparative advantage asks, “Who gives up less to make this good?” Only comparative advantage decides who should specialise in what.
Two facts follow from the definitions:
- One producer can have an absolute advantage in both goods.
- With two goods and two producers, one producer cannot have a comparative advantage in both goods. If your opportunity cost of good A is lower, your opportunity cost of good B (which is the reciprocal) must be higher. The only exception is equal opportunity costs, where neither side has a comparative advantage.
Finding opportunity costs from a table or a PPC
Exam data come in two forms. Read the column headings carefully before you divide.
Output data tell you how much of each good a producer can make with all its resources (or in a fixed time). The intercepts of a straight-line PPC give you the same information.
- Opportunity cost of 1 unit of X = maximum output of Y ÷ maximum output of X.
- The producer with the larger output of a good has the absolute advantage in it.
Input data tell you how many resources (often hours of labour) one unit of each good needs.
- Opportunity cost of 1 unit of X = hours per unit of X ÷ hours per unit of Y.
- The producer that needs fewer hours per unit has the absolute advantage.
Notice the switch. With output data, the other good goes on top. With input data, the same good goes on top. If you are unsure, test it with a quick sentence: “To make one more X, how many Y must I stop making?”
In this topic PPCs are straight lines. That means the opportunity cost is constant along each PPC, and it equals the size of the slope.
Our fictional economies: Valdoria and Kestria
Valdoria and Kestria are fictional countries. Using all of its resources for one month, each can produce the following.
| Country | Rice (tonnes), if it makes only rice | Solar panels, if it makes only panels |
|---|---|---|
| Valdoria | 120 | 40 |
| Kestria | 60 | 30 |
All figures are fictional. Each country has a straight-line PPC, so opportunity costs are constant.
Worked example 1: who has which advantage?
Question. Using the table above, identify (a) which country has the absolute advantage in each good and (b) which country has the comparative advantage in each good.
(a) Absolute advantage. These are output data, so compare the numbers directly. Valdoria can make 120 tonnes of rice against Kestria’s 60, and 40 panels against Kestria’s 30. Valdoria has the absolute advantage in both goods.
(b) Comparative advantage. Calculate the opportunity cost of one unit of each good for each country.
| Country | Opportunity cost of 1 panel | Opportunity cost of 1 tonne of rice |
|---|---|---|
| Valdoria | 120 ÷ 40 = 3 tonnes of rice | 40 ÷ 120 = 1/3 panel |
| Kestria | 60 ÷ 30 = 2 tonnes of rice | 30 ÷ 60 = 1/2 panel |
- Panels: Kestria gives up 2 tonnes of rice per panel; Valdoria gives up 3. Kestria has the comparative advantage in solar panels.
- Rice: Valdoria gives up 1/3 panel per tonne; Kestria gives up 1/2. Valdoria has the comparative advantage in rice.
Check. The rice column is the reciprocal of the panel column for each country (1/3 is the reciprocal of 3). Each country has a comparative advantage in exactly one good, as it must. Kestria is less productive at everything, yet it is the lower-cost producer of panels.
Specialisation and the terms of trade
Each country should specialise in the good where it has the comparative advantage and trade for the other: Valdoria makes rice, Kestria makes panels.
At what price? Think about each side’s alternative.
- Valdoria wants panels. Making a panel itself costs 3 tonnes of rice. It will buy a panel only if it pays less than 3 tonnes.
- Kestria sells panels. Making a panel costs it 2 tonnes of rice. It will sell a panel only if it receives more than 2 tonnes.
So mutually beneficial terms of trade lie between the two opportunity costs:
1 panel for more than 2 but fewer than 3 tonnes of rice.
The same range per tonne of rice is between 1/3 and 1/2 panel. At exactly 2 or exactly 3 tonnes, one side is no better off than producing for itself, so it does not gain. Terms outside the range make one side worse off, so that side refuses. For example, at 3.5 tonnes per panel, Valdoria would pay more than it costs to make a panel at home.
Worked example 2: gains from trade beyond the PPC
Question. Before trade, Valdoria produces and consumes 72 tonnes of rice and 16 panels. Kestria produces and consumes 32 tonnes of rice and 14 panels. Then the countries specialise completely and trade 16 panels for 40 tonnes of rice. (a) Check the terms of trade are mutually beneficial. (b) Find each country’s consumption after trade. (c) Show that each consumes outside its own PPC.
Step 1: check the before-trade points are on the PPCs. Valdoria: 72 tonnes uses 72/120 = 0.6 of its resources and 16 panels use 16/40 = 0.4. Total 1.0, so the point is on the PPC. Kestria: 32/60 + 14/30 = 0.533… + 0.466… = 1.0. Also on the PPC.
(a) 40 tonnes for 16 panels = 40 ÷ 16 = 2.5 tonnes of rice per panel. This is between 2 and 3, so both sides gain.
(b) After specialising, Valdoria produces 120 tonnes of rice and 0 panels; Kestria produces 0 rice and 30 panels.
| Valdoria | Kestria | |
|---|---|---|
| Production after specialising | 120 rice, 0 panels | 0 rice, 30 panels |
| Trade | exports 40 rice, imports 16 panels | exports 16 panels, imports 40 rice |
| Consumption after trade | 80 rice, 16 panels | 40 rice, 14 panels |
| Consumption before trade | 72 rice, 16 panels | 32 rice, 14 panels |
| Gain | +8 tonnes of rice | +8 tonnes of rice |
(c) On its own PPC, Valdoria could have at most 120 − 3 × 16 = 72 tonnes of rice with 16 panels. It now has 80. Kestria could have at most 60 − 2 × 14 = 32 tonnes with 14 panels. It now has 40. Both points lie outside their PPCs. Neither country’s resources or technology changed; trade alone made the extra consumption possible.
Check. Total rice consumed rose from 72 + 32 = 104 to 120 tonnes, while total panels stayed at 30. The extra 16 tonnes came from moving production to the lower-cost producer. Each country gains 0.5 tonnes of rice on each of the 16 panels traded (3 − 2.5 for Valdoria, 2.5 − 2 for Kestria): 16 × 0.5 = 8.
Worked example 3: an input table
Question. Two freelance designers in Valdoria make reports and posters. The table shows the hours each needs to make one unit.
| Designer | Hours per report | Hours per poster |
|---|---|---|
| Mira | 3 | 1 |
| Jonah | 6 | 4 |
(a) Who has the absolute advantage in each good? (b) Who has the comparative advantage in each good? (c) Would “1 report for 2 posters” be mutually beneficial?
(a) These are input data, so fewer hours means more productive. Mira needs fewer hours for both goods, so Mira has the absolute advantage in both.
(b) Opportunity cost of 1 report = hours per report ÷ hours per poster.
| Designer | Opportunity cost of 1 report | Opportunity cost of 1 poster |
|---|---|---|
| Mira | 3 ÷ 1 = 3 posters | 1 ÷ 3 = 1/3 report |
| Jonah | 6 ÷ 4 = 1.5 posters | 4 ÷ 6 = 2/3 report |
Jonah has the comparative advantage in reports (1.5 posters < 3). Mira has the comparative advantage in posters (1/3 report < 2/3).
(c) Mutually beneficial terms: 1 report for between 1.5 and 3 posters. Two posters is inside the range, so yes. Jonah gains 0.5 poster on each report he sells (2 − 1.5). Mira gains 1 poster on each report she buys (3 − 2).
Check by converting to output. In 12 hours Mira can make 12 ÷ 3 = 4 reports or 12 ÷ 1 = 12 posters; Jonah can make 2 reports or 3 posters. Using the output method: Mira’s opportunity cost of a report is 12 ÷ 4 = 3 posters and Jonah’s is 3 ÷ 2 = 1.5 posters. Same answers.
Common misconceptions
- “The more productive producer has the comparative advantage.” That is absolute advantage. Comparative advantage depends only on opportunity cost.
- “A country with an absolute advantage in both goods cannot gain from trade.” It gains by specialising where its opportunity cost is lower, as Valdoria does.
- “One producer can have a comparative advantage in both goods.” Not with two goods, unless opportunity costs are equal, and then neither has one.
- Using the output rule on input data. With hours per unit, fewer hours means absolute advantage, and the good itself goes on top when you divide.
- Writing the opportunity cost upside down. Say it as a sentence: “One panel costs Valdoria 3 tonnes of rice.”
- “Any terms of trade work.” Only terms strictly between the two opportunity costs make both sides better off.
- “Trade moves a country’s PPC outward.” No. Resources and technology are unchanged, so the PPC stays where it is. Trade lets the country consume at a point outside it.
- “Gains from trade happen because one side wins and the other loses.” With terms inside the range, both sides gain.
Where this leads
Comparative advantage explains why people and countries specialise and exchange in markets. Next, Topic 1.4 builds the first market model, starting with demand. If the opportunity-cost calculations felt shaky, revisit opportunity cost and the PPC. Try the practice questions now, then use the revision notes and the checklist to consolidate.
Quick questions
Can one country have a comparative advantage in both goods?
No. With two producers and two goods, if one has the lower opportunity cost of good A, the other must have the lower opportunity cost of good B. The only exception is when their opportunity costs are equal; then neither has a comparative advantage and there is nothing to gain from trade.
Is a country with an absolute advantage in both goods better off not trading?
No. It still gains by specialising in the good where its opportunity cost is lower and buying the other good at terms of trade that are cheaper than its own opportunity cost.