# Gains from international trade

> IB Economics Higher Level · Unit 4: The Global Economy
> Source: https://www.owlsprep.com/study/ib-economics-hl-u4-gains-from-international-trade/

This module explains why nations engage in international trade, and how both trading partners can achieve net welfare gains even when one country is more productive at producing all goods. We cover core concepts, calculations and graphical analysis for IB HL exams.

**Prerequisites:** [Opportunity cost and production possibility frontiers (PPFs)](https://www.owlsprep.com/study/ib-economics-hl-micro-opportunity-cost-ppf/)

## Learning objectives

- Distinguish between absolute and comparative advantage
- Calculate opportunity cost to identify comparative advantage
- Derive the range of mutually beneficial terms of trade
- Illustrate gains from trade using production possibility frontiers

## Absolute vs Comparative Advantage

**Absolute Advantage** — A country has an absolute advantage in producing a good if it can produce more output with the same quantity of inputs, or the same output with fewer inputs, than another country.

*Example:* If 1 worker produces 10 cars in Country A and 5 cars in Country B, Country A has absolute advantage in car production.

**Comparative Advantage** — A country has a comparative advantage in producing a good if it can produce that good at a lower opportunity cost than another country. This is the core determinant of mutually beneficial trade.

*Example:* Even if Country A has absolute advantage in both goods, it will still have comparative advantage in only one good.

> **info**
>
> A country can have absolute advantage in all goods, but it **cannot** have comparative advantage in all goods in a two-good model, due to the reciprocal nature of opportunity cost.

**Worked example:** Two countries (Domestic and Foreign) produce wheat and cloth. Output per worker: Domestic = 10 wheat, 5 cloth; Foreign = 6 wheat, 4 cloth. Identify absolute advantage for each good.

1. Compare output per worker for each good across countries:
2. Wheat: Domestic produces 10 units per worker, Foreign produces 6 units. Domestic has higher output.
3. Cloth: Domestic produces 5 units per worker, Foreign produces 4 units. Domestic also has higher output.
4. Conclusion: Domestic has absolute advantage in both wheat and cloth.

> **Exam tip:** Always explicitly label the country and good when stating advantage, to avoid ambiguous answers that lose marks.

## Calculating Comparative Advantage via Opportunity Cost

Comparative advantage is determined by comparing the opportunity cost of producing each good. The opportunity cost of 1 unit of good X is the amount of good Y you must give up to produce it.

**Opportunity Cost Calculation** — For output per worker model: $OC_{X} = \frac{\text{Quantity of Y given up}}{\text{Quantity of X gained}}$

**Worked example:** Use the same output data: Domestic: 10 wheat, 5 cloth per worker; Foreign: 6 wheat, 4 cloth per worker. Calculate opportunity cost and identify comparative advantage.

1. Calculate opportunity cost of 1 unit of wheat for each country:
2. $$\text{Domestic: } OC_{wheat} = \frac{5\ cloth}{10\ wheat} = 0.5\ cloth$$
3. $$\text{Foreign: } OC_{wheat} = \frac{4\ cloth}{6\ wheat} \approx 0.67\ cloth$$
4. Domestic has lower OC for wheat, so it has comparative advantage in wheat.
5. Calculate opportunity cost of 1 unit of cloth for each country:
6. $$\text{Domestic: } OC_{cloth} = \frac{10\ wheat}{5\ cloth} = 2\ wheat$$
7. $$\text{Foreign: } OC_{cloth} = \frac{6\ wheat}{4\ cloth} = 1.5\ wheat$$
8. Foreign has lower OC for cloth, so it has comparative advantage in cloth.

**Check your understanding**

Test your understanding: Which statement is correct?

1. If Country A has absolute advantage in both goods, which is true?

   - Country A should not trade with Country B
   - Country A will import both goods
   - Country A should trade based on comparative advantage
   - Country B cannot gain from trade

   *Why:* Correct. Even with absolute advantage in all goods, comparative advantage means both countries can gain from specialization and trade.

## Mutually Beneficial Terms of Trade

For trade to be mutually beneficial for both countries, the terms of trade (the exchange rate between the two goods) must fall between the opportunity costs of the two countries. Each country specializes in the good it has comparative advantage in, then trades, and both end up better off.

**Range of Mutually Beneficial Terms of Trade** — For good X (exported by Country A), $OC_{X,A} < ToT_X < OC_{X,B}$ where Country B imports X.

**Worked example:** Using the wheat and cloth example above: what range of terms of trade for 1 unit of wheat is mutually beneficial?

1. Recall that Domestic exports wheat, and its opportunity cost of 1 wheat is 0.5 cloth. Foreign imports wheat, and its opportunity cost of 1 wheat is 0.67 cloth.
2. To be mutually beneficial, the terms of trade must lie between these two values:
3. $$0.5\ cloth < 1\ wheat < 0.67\ cloth$$
4. If ToT is 1 wheat = 0.6 cloth: Domestic gains 0.6 cloth per wheat (more than the 0.5 it gives up producing domestically). Foreign pays 0.6 cloth per wheat (less than the 0.67 it costs to produce domestically). Both gain.

> **Exam tip:** Always show the full range of terms of trade, do not just give a single value unless explicitly asked by the question.

## Illustrating Gains from Trade on a PPF

Gains from trade can be shown graphically: without trade, a country's consumption is limited to points on or inside its production possibility frontier (PPF). After specialization and trade, the country can reach a consumption point outside its original PPF, meaning higher total welfare.

**Worked example:** Domestic has a maximum PPF output of 100 wheat or 50 cloth. It specializes completely in wheat (its comparative advantage), and terms of trade are 1 wheat = 0.6 cloth. If Domestic consumes 50 wheat, calculate the gain from trade.

1. Without trade: if Domestic produces and consumes 50 wheat, it can produce and consume 25 cloth (from its PPF: 50% of maximum wheat means 50% of maximum cloth).
2. With trade: Domestic produces 100 wheat (full specialization), keeps 50 wheat for consumption, and exports the remaining 50 wheat.
3. At ToT of 0.6 cloth per wheat: 50 exported wheat buys $50 \times 0.6 = 30$ cloth.
4. Domestic consumes 50 wheat + 30 cloth, which is 5 more cloth than without trade. This consumption point lies outside the original PPF, representing the net gain from trade.

## Common pitfalls

- **Wrong:** Confusing absolute advantage with comparative advantage as the basis for trade
  - Why it fails: Absolute advantage reflects raw productivity, but comparative advantage (opportunity cost) determines mutually beneficial trade patterns
  - Correct: Always use opportunity cost to identify comparative advantage, regardless of absolute advantage
- **Wrong:** Inverting the opportunity cost calculation (OC = X/Y instead of Y/X)
  - Why it fails: This flips the opportunity cost, leading to wrong identification of comparative advantage
  - Correct: Remember: OC of 1 X = (Y given up)/(X gained) = Y/X
- **Wrong:** Claiming one country can have comparative advantage in both goods
  - Why it fails: Opportunity cost is reciprocal: lower OC for X means higher OC for Y by definition
  - Correct: In a two-good model, each country will always have comparative advantage in exactly one good
- **Wrong:** Claiming all terms of trade are mutually beneficial
  - Why it fails: Only terms of trade between the two countries' opportunity costs benefit both parties
  - Correct: Always derive and state the valid range of terms of trade for mutual gain
- **Wrong:** Claiming all groups in both countries gain from trade
  - Why it fails: Gains from trade are aggregate: some domestic groups (e.g. import-competing producers) lose
  - Correct: Distinguish between total national gain and distributional impacts of trade in exam answers

## Cheatsheet

| Concept | Key Rule | Exam Check |
| --- | --- | --- |
| Absolute Advantage | Higher output per input | Compare raw output across countries |
| Comparative Advantage | Lower opportunity cost | Calculate OC = Y/X for good X |
| Mutually Beneficial ToT | $OC_{export} < ToT < OC_{import}$ | ToT must fall between OCs |
| Gains from Trade Outcome | Consumption beyond original PPF | Aggregate gain, not all groups gain |

## What's next

Understanding the core theory of gains from trade is the foundation for all international trade topics in IB Economics HL. This framework explains why economists generally support trade liberalization, and allows you to evaluate the impacts of trade barriers, exchange rate policies, and globalization on national welfare. You will build on this to analyze why governments impose trade protection, who benefits and loses from protectionist policies, and how trade agreements shape global economic integration. You will also extend this basic model to consider real-world complications like factor immobility, economies of scale, and imperfect competition that modify the core gains from trade result.

- [Economic Integration](https://www.owlsprep.com/study/ib-economics-hl-u4-economic-integration/)
- [Comparative advantage theory](https://www.owlsprep.com/study/ib-economics-hl-u4-comparative-advantage-theory/)

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