COMM1100 Chap.3 Comparative Advantage and Gains from Trade
Comparative Advantage and Gains from Trade
Why do people and countries trade instead of producing everything themselves? The Week 2 lecture answers with two definitions that students often blur.
Absolute advantage is about using fewer resources; comparative advantage is about a lower opportunity cost, and only comparative advantage decides who should specialise in what.
This chapter computes opportunity-cost ratios, finds the price range at which both sides gain, and counts the gains when two producers specialise and trade.
It reads the same story off production possibilities frontiers, and extends it to comparing yourself with a market price and to producing goods that must be combined in a fixed recipe, which is how the Week 2 practice questions test it.
Comparative advantage is also the first place the course shows why voluntary exchange creates value rather than simply moving it around.
That idea becomes economic surplus in Week 3, so the habit of asking who gains from a trade, and by how much, carries straight into the surplus chapter.
What this chapter covers
- 01
Absolute advantage versus comparative advantage
- 02
Opportunity cost ratios for each producer and each good
- 03
Specialisation according to comparative advantage
- 04
The feasible price range between two opportunity costs
- 05
Counting the gains from specialising and trading
- 06
Production possibilities frontiers and their slopes
- 07
Comparing a producer with market prices; only relative prices matter
Worked example · free
Two farmers, wheat and wool
- +1Hana gives up 40/20 = 2 tonnes of wheat per bale of wool; Jo gives up 30/30 = 1 tonne per bale.
- +1Jo has the lower opportunity cost of wool, so Jo has the comparative advantage in wool and Hana in wheat, although Hana grows more wheat in absolute terms.
- +1A price between 1 and 2 tonnes of wheat per bale leaves both better off than producing everything themselves.
Key terms
- Absolute advantage
- Making a good with fewer inputs than someone else, or getting more output from the same inputs.
- Comparative advantage
- The ability to produce a good at a lower opportunity cost than another agent.
- Specialisation
- Concentrating production on the good in which an agent has a comparative advantage, then trading for the rest.
- Terms of trade
- The rate at which one good exchanges for another; mutually beneficial terms lie between the two traders' opportunity costs.
- Production possibilities frontier
- A line showing every combination of two goods a producer can make with fixed time and resources.
Comparative Advantage and Gains from Trade FAQ
Can one person have a comparative advantage in both goods?
No. If a person’s opportunity cost of one good is lower, their opportunity cost of the other good, which is its reciprocal, must be higher. They can have an absolute advantage in both, but a comparative advantage in only one.
Why must the trading price lie between the two opportunity costs?
Below the seller’s opportunity cost, the seller would be better off making the good for themselves; above the buyer’s, the buyer would. Only a price inside the range leaves both parties better off than self-sufficiency.
Does doubling all prices change who should specialise?
No. The Week 2 solutions make the point that only relative prices and productivity matter. If every price doubles, the cost of one good measured in the other is unchanged, so the comparative advantage and the decision stay the same.
How do I handle goods that must be used together, like tables and chairs?
Start each partner on the good they make at lower opportunity cost, check which good is short for the recipe, and move the partner with the lower cost of the short good across until the recipe balances. Joint output beats working alone.
Why do people still trade when one of them is better at everything?
Because what matters is opportunity cost, not raw productivity. The more productive person gives up more of one good to make the other, so both can gain if each specialises where their sacrifice is smaller and they trade at a price between their two opportunity costs.
Assessment move
Make a two-by-two opportunity-cost table the first line of every answer, with units. Then practise three variations: two producers trading with each other, one producer against a market price, and a fixed-recipe product such as a dining set. Check each final answer by confirming that both sides end up with at least what they could have had alone.
This topic sits in the Week 2 material, so expect it in the first in-tutorial test.
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