Lesson 02: Comparative Advantage & Gains from Trade

Warm-Up

LeBron James is (let's stipulate) the best in his neighborhood at both basketball and mowing lawns. Should he mow his own lawn? Almost certainly not — an hour mowing costs him enormous basketball earnings, while it costs a teenager down the street almost nothing. The teenager mows; LeBron plays; both are better off. That's comparative advantage: what matters isn't who's better at a task, but who gives up less to do it. This single idea generates guaranteed points on nearly every AP Micro exam — the calculations are mechanical once you know the drill.

Core Concept

Two kinds of advantage

• Absolute advantage: you can produce more of a good with the same resources (or the same amount with fewer resources). It's about raw productivity.
• Comparative advantage: you can produce a good at a lower opportunity cost. It's about what you sacrifice.

Specialization and trade are driven by comparative advantage, never absolute advantage. One producer can hold absolute advantage in both goods, but (except in the knife-edge case of identical opportunity costs) each producer always has comparative advantage in exactly one of two goods.

Output problems ("Over" method)

An output table shows how much each producer can make with the same resources — bigger numbers are better.

Absolute advantage: Ana in both (20 > 6, 10 > 4).

Opportunity costs (output problems: divide the other good by the good in question — "other over"): - Ana: 1 wheat costs 10/20 = ½ cloth; 1 cloth costs 20/10 = 2 wheat - Ben: 1 wheat costs 4/6 = ⅔ cloth; 1 cloth costs 6/4 = 1½ wheat

Comparative advantage: Ana's wheat cost (½ cloth) < Ben's (⅔ cloth) → Ana specializes in wheat. Ben's cloth cost (1½ wheat) < Ana's (2 wheat) → Ben specializes in cloth. Notice Ana has absolute advantage in both, yet Ben still has comparative advantage in cloth.

Memory hook: Output → Other goes Over (the other good in the numerator).

Input problems (flipped logic)

An input table shows resources needed per unit of output — smaller numbers are better.

Absolute advantage: Carla in shoes (2 < 3 hours); Dev in hats (3 < 4).

Opportunity costs (input problems: divide the good in question by the other — the ratio flips): - Carla: 1 hat takes 4 hours, which could have made 4/2 = 2 shoes → 1 hat costs 2 shoes; 1 shoe costs ½ hat. - Dev: 1 hat takes 3 hours = 3/3 = 1 shoe → 1 hat costs 1 shoe; 1 shoe costs 1 hat.

Comparative advantage: Dev in hats (1 shoe < 2 shoes), Carla in shoes (½ hat < 1 hat).

The most reliable safety net: convert any input table to an output table. Pick a convenient time window (say, 12 hours): Carla can make 6 shoes or 3 hats; Dev can make 4 shoes or 4 hats. Then apply the output method as usual.

Terms of trade

Terms of trade = the exchange ratio at which the two parties trade. A trade benefits both parties when the price of each good lies between the two producers' opportunity costs.

From the Ana/Ben output example: Ana's cloth costs 2 wheat to make herself; Ben's cloth costs him 1½ wheat. If they trade cloth for wheat at any rate between 1½ and 2 wheat per cloth — say 1¾ — both gain: Ana gets cloth cheaper than making it (1¾ < 2), and Ben gets more wheat per cloth than his own cost (1¾ > 1½).

• Offered rate outside the range → at least one party refuses.
• Rate exactly equal to someone's opportunity cost → that party is indifferent (no gain).

Why trade creates gains

When each producer specializes in their comparative-advantage good, total output of both goods rises with the same total resources — the world effectively moves beyond individual PPCs. Graphically: with trade, each producer can consume at a point outside their own PPC, though they still produce on it.

[GRAPH: Two straight-line PPCs side by side. Left — Ana: X-axis "Cloth" (0–10), Y-axis "Wheat" (0–20), straight line between intercepts. Right — Ben: X-axis "Cloth" (0–4), Y-axis "Wheat" (0–6), straight line. After specialization and trade at 1¾ wheat per cloth, each consumes at a point outside their own line (dashed consumption-possibility line steeper than Ben's PPC, flatter than Ana's).]

Worked Examples

Example 1 (easy): Output table

With one day of labor: Nation X makes 30 phones or 15 drones; Nation Y makes 8 phones or 8 drones. Find all advantages.

Solution: - Absolute: X in both. - X: 1 drone costs 30/15 = 2 phones; 1 phone costs ½ drone. - Y: 1 drone costs 8/8 = 1 phone; 1 phone costs 1 drone. - Comparative: Y in drones (1 < 2 phones), X in phones (½ < 1 drone).

Interpretation: X should export phones, Y should export drones. Absolute advantage in both ≠ specialize in both.

Example 2 (medium): Input table

Hours per unit: Pat needs 5 hours per table, 10 per bookshelf. Quinn needs 6 per table, 8 per bookshelf.

Solution (convert to output per 120 hours): Pat: 24 tables or 12 bookshelves. Quinn: 20 tables or 15 bookshelves. - Pat: 1 bookshelf costs 24/12 = 2 tables. Quinn: 1 bookshelf costs 20/15 = 4/3 tables. - Quinn → bookshelves (4/3 < 2), Pat → tables (Pat's table costs ½ bookshelf < Quinn's ¾).

Interpretation: Input problems reward the convert-to-output reflex. Note the absolute advantages split here too: Pat in tables (5 < 6 h), Quinn in bookshelves (8 < 10 h) — matching comparative advantage this time, but only the opportunity-cost comparison proves it.

Example 3 (AP-style): Terms of trade

From Example 1, X and Y agree to trade phones for drones. (i) Which country exports drones? (ii) Would 1.5 phones per drone benefit both? (iii) Would 2.5 phones per drone?

Solution: - (i) Y (comparative advantage in drones). - (ii) Drone prices must lie between the opportunity costs: 1 phone (Y's cost) and 2 phones (X's cost). 1.5 is in (1, 2) → both gain. - (iii) 2.5 > 2: X would pay more phones per drone than it costs X to make its own drones → X refuses.

Interpretation: State the acceptable range explicitly — FRQ rubrics award a point for the comparison, not just the yes/no.

Common Mistakes

1. Using absolute advantage to assign specialization. The exam builds tables where one producer dominates both goods precisely to punish this.
2. Flipping the ratio the wrong way. Output → other good over. Input → the ratio inverts. When in doubt, convert inputs to outputs.
3. Claiming one producer has comparative advantage in both goods. Impossible with different opportunity costs — if you compute it, you've made an arithmetic error. Recheck.
4. Testing terms of trade against only one party's cost. A mutually beneficial rate must beat both opportunity costs (strictly between them).
5. Saying trade shifts the PPC. Trade lets countries consume beyond the PPC; it does not move the production frontier itself.

Practice Problems

Use this table for questions 1–4. Output per worker per day:

9. (FRQ-style) With equal resources, Ferland can produce 60 apples or 20 pears. Gorland can produce 30 apples or 15 pears. (a) Which country has an absolute advantage in pears? Explain. (b) Calculate each country's opportunity cost of 1 pear. (c) Which country should specialize in pears? Explain using your answer to (b). (d) The countries consider trading 1 pear for 2.5 apples. Would both countries accept? Explain.