ECON6023 · International Trade
World Trade and the Idea of Comparative Advantage
Week 1 opens ECON6023 with the central puzzle of the unit — why a country imports goods it could make itself — and answers it with comparative rather than absolute advantage, the gains from trade and the production-possibility frontier. The comparative-advantage table and the world-price band recur in the mid-semester test and as the opening moves of the exam's Ricardian theory question, so getting the opportunity-cost logic exact here pays off all semester.
What this chapter covers
- 01Scale and pattern of world trade; the gravity relationship (trade rises with size, falls with distance)
- 02Absolute advantage (lower unit labour requirement aᵢ) vs comparative advantage (lower opportunity cost)
- 03Opportunity cost of good 1 in terms of good 2 = a₁/a₂ = slope of the PPF = autarky relative price
- 04Reading a unit-labour-requirement table: assigning AA and CA; a country can have AA in both yet CA in only one
- 05The world relative-price band: the free-trade price must lie between the two autarky relative prices
- 06The straight-line Ricardian PPF and constant opportunity cost
- 07CES and Cobb-Douglas preferences: the demand side that pins down the equilibrium price
- 08Gains from trade: opening to trade creates both winners and losers (the distributional theme of the unit)
Comparative advantage from a unit-labour-requirement table
- +1Absolute advantage = the lower unit labour requirement. Home has the lower aᵢ in BOTH goods (4 < 8 in cloth, 6 < 9 in wine), so Home has the absolute advantage in cloth and in wine. This is the classic case where absolute advantage cannot decide the pattern of trade.
- +1Autarky relative price of cloth = a_cloth/a_wine (price equals unit labour cost, so the wage cancels). Home: 4/6 = 0.667 wine per cloth. Foreign: 8/9 = 0.889 wine per cloth.
- +1Comparative advantage = lower opportunity cost of cloth. Home gives up 0.667 wine per cloth, Foreign gives up 0.889 wine per cloth, so Home has the comparative advantage in cloth and Foreign has the comparative advantage in wine — even though Home is absolutely more productive at both.
- +1The free-trade world relative price of cloth must lie between the two autarky ratios, 0.667 ≤ (p_cloth/p_wine)^world ≤ 0.889. Outside this band one country would be worse off than in autarky and would refuse to trade.
Key terms
- Unit labour requirement (aᵢ)
- Hours of labour to make one unit of good i. Its reciprocal is the marginal product of labour, MPLᵢ = 1/aᵢ. The good with the lower aᵢ is the one a country has an absolute advantage in.
- Absolute advantage
- Producing a good with fewer resources (a lower unit labour requirement / higher MPL) than the other country. It does not determine who exports what — comparative advantage does.
- Comparative advantage
- Producing a good at a lower opportunity cost than the other country. Home has it in good 1 iff a₁/a₂ < a₁*/a₂*. Every country has a comparative advantage in some good.
- Opportunity cost
- Units of good 2 forgone to make one more unit of good 1, equal to a₁/a₂ in the Ricardian model. It is the slope of the straight-line PPF and, under perfect competition, the autarky relative price.
- World relative-price band
- The interval between the two countries' autarky relative prices. The free-trade world relative price must fall inside it; at the boundary the diversified country stays partly unspecialised.
- Gravity relationship
- The empirical regularity that bilateral trade rises with the trading partners' economic size and falls with the distance between them — the first-pass description of the pattern of world trade.
World Trade and the Idea of Comparative Advantage FAQ
Why can a country that is worse at making everything still gain from trade?
Because trade runs on comparative, not absolute, advantage. Even a country with a higher unit labour requirement in both goods has a lower opportunity cost in one of them; specialising in that good and trading for the other lets both countries consume outside their production-possibility frontiers. Absolute productivity levels only affect wages, not the pattern of trade.
How do I tell absolute from comparative advantage on a table?
Absolute advantage is read down a column: whoever has the lower unit labour requirement (or higher MPL) in a good has the absolute advantage in it. Comparative advantage is read across ratios: form each country's a₁/a₂, and the country with the smaller ratio has the comparative advantage in good 1. They can point to different countries — that is the whole lesson.
Can AI help me with comparative advantage in ECON6023?
Yes, as a study aid. Sia can build the opportunity-cost table with you, check which country has the comparative advantage, and derive the world-price band step by step, then set fresh practice with new numbers. It explains the method and checks your reasoning; it does not do graded work for you, and the University of Sydney academic-integrity policy applies. Confirm assessment details on Canvas.
Does the world price always sit exactly in the middle of the band?
No — the band only tells you the limits. Where inside it the price actually settles depends on the demand side (the CES or Cobb-Douglas preferences) and relative country sizes, which you solve in the Ricardian and DFS chapters. At a boundary of the band one country only partly specialises.
Exam move
Make the opportunity-cost table your reflex: given any 2×2 unit-labour-requirement (or MPL) grid, write each country's a₁/a₂, assign comparative advantage to the lower ratio, and state the world-price band as an inequality — this is worth easy marks at the start of the exam's Ricardian question and in the Week 5 quiz. Practise the trap case where one country has the absolute advantage in both goods but the comparative advantage in only one. Sketch the straight-line PPF and mark its slope as the opportunity cost. Keep the demand side in view: the band gives limits, and CES/Cobb-Douglas preferences pin the exact price later. Confirm the quiz and exam timing on Canvas.
Working through World Trade and the Idea of Comparative Advantage in ECON6023? Sia is AskSia’s AI Economics tutor — ask any ECON6023 World Trade and the Idea of Comparative Advantage question and get a clear, step-by-step explanation grounded in how ECON6023 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
