Comparative Advantage and the Gains from Trade

A country that is worse at producing every single good can still make the world richer by producing one of them.

That sentence sounds wrong. It is the most robust result in economics.

Here is why, in twelve lines. Suppose Northland can produce 20 machines or 30 tonnes of rice with one worker-year. Southland, less productive at both, can produce 5 machines or 20 tonnes of rice. Northland beats Southland at machines (20 vs 5) and at rice (30 vs 20). Northland appears to have nothing to gain.

But ask a different question. Not "who is better?" — ask "what does each give up?"

For Northland, one machine costs 1.5 tonnes of rice, because the worker-year spent on that machine could have grown 30 tonnes across 20 machines. For Southland, one machine costs 4 tonnes of rice. Machines are cheap in Northland and expensive in Southland — not in money, but in rice forgone. Rice runs the other way: a tonne of rice costs Northland 0.667 machines and costs Southland only 0.25 machines.

So Northland is the low-cost producer of machines and Southland is the low-cost producer of rice, even though Southland is worse at both in absolute terms. Move machine production toward Northland and rice production toward Southland and total world output of both goods rises. Nothing was invented. No one worked harder. The gain came purely from reallocating existing resources toward whoever sacrifices least to produce each good.

The level of productivity determines how rich a country is. The ratio of productivities determines what it should trade.

Absolute advantage is about levels. Comparative advantage is about ratios. Almost every mistake students make on this topic — and a large share of the mistakes politicians make — comes from confusing the two.

The rest of this chapter builds that result from nothing, tests it to destruction, and converts it into WEC14 marks.

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#1. Prerequisites

You need five things before section 3 will land. If any is shaky, fix it now — the derivations later assume you can do these without thinking.

1. Scarcity. Wants exceed the resources available to satisfy them. This is the founding condition of the subject, not a complaint about poverty. Even a rich economy is scarce, because at full employment producing more of one thing requires producing less of another.

2. Opportunity cost. The value of the next best alternative forgone. Not "the cost." Not "what you paid." The best thing you gave up. If you have one worker-year and spend it on machines, the opportunity cost is the rice that worker-year would have grown.

3. The production possibility frontier (PPF). A curve or line showing the maximum combinations of two goods an economy can produce when all resources are fully and efficiently employed. Points on it are efficient. Points inside are attainable but wasteful. Points outside are unattainable with domestic resources alone — hold on to that phrase, it becomes the whole gains-from-trade argument.

4. The gradient of the PPF is opportunity cost. If the PPF is a straight line running from 1,000 wheat to 400 cloth, the gradient is −1000/400 = −2.5. Moving one unit along the cloth axis costs 2.5 wheat. The magnitude of the gradient is the opportunity cost of the good on the horizontal axis, measured in units of the good on the vertical axis. Every comparative advantage calculation in this chapter is secretly a statement about gradients.

5. Basic index numbers. An index sets a base year to 100 and expresses later values relative to it. If export prices rise from 100 to 112, they have risen 12%. You need this for the terms of trade index in section 6.

Two definitions to keep straight throughout:

• Autarky — a state of no international trade; the economy consumes only what it produces.
• Specialisation — concentrating resources on a narrower range of goods than the economy consumes.

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#2. The problem trade solves

Start with one country, alone. Call it Alpha. It has 100 worker-years of labour, the only factor of production. Each worker-year produces either 10 tonnes of wheat or 4 metres of cloth.

Alpha's choices are bounded. Put all 100 workers on wheat and it gets 1,000 tonnes and no cloth. Put all on cloth and it gets 400 metres and no wheat. Split them and it gets something in between. Those combinations trace Alpha's PPF.

Read the gradient off any two adjacent rows. Moving from row 1 to row 2, Alpha gains 100 cloth and loses 250 wheat. So one metre of cloth costs 2.5 tonnes of wheat. That ratio never changes here — the PPF is a straight line, because every worker is assumed equally good at both jobs. We will destroy that assumption deliberately in section 9.

Now state the problem sharply.

THE AUTARKY CONSTRAINT
Production possibilities = consumption possibilities.
A closed economy cannot consume what it did not make.
Its PPF is also its consumption frontier.
To get more cloth it must lose wheat.
There is no escape from the gradient.

Every point Alpha wants that lies beyond that line is unattainable. Alpha wants (600 wheat, 300 cloth). Check it: 600 wheat needs 60 workers, 300 cloth needs 75 workers, total 135 workers. Alpha has 100. Denied.

So there are exactly three ways for Alpha to consume more of both goods.

1. Increase resources — more labour, more capital. Slow, and usually not available on demand.
2. Increase productivity — better technology. Slow, uncertain, expensive.
3. Trade — acquire goods from outside at a rate of exchange better than the domestic gradient.

Route 3 is the only one that can work this year, with today's technology and today's resources. That is what makes it remarkable. Trade is a technology for turning wheat into cloth without a factory. If Alpha can hand over wheat and receive cloth at any rate cheaper than 2.5 wheat per cloth, it has effectively acquired a superior cloth-making technology — one that consumes wheat as its input.

Trade is not primarily about selling. It is about acquiring goods at a lower opportunity cost than you could produce them.

That is the sentence to hold. Exports are the price you pay for imports. The imports are the point.

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#3. Absolute advantage: the first answer, and why it is not enough

#3.1 Definition and derivation

Adam Smith, in The Wealth of Nations (1776), gave the first systematic answer: countries should produce what they are best at.

ABSOLUTE ADVANTAGE
A country has an absolute advantage in a good
if it can produce more of that good
from the same quantity of resources than another country.
Equivalently: it needs fewer resources per unit.
This is a comparison of LEVELS.

Two equivalent tests. Use whichever the data supports.

• Output test. Higher output per unit of input wins.
• Input test. Lower input per unit of output wins.

They always agree, because one is the reciprocal of the other. Exam data comes in both forms. Read the table headings before calculating — this is the single most common source of inverted answers in Section B.

#3.2 Worked Scenario A — set-up

Alpha and Beta each have 100 worker-years. Output per worker-year:

Apply the output test. Wheat: Alpha 10 > Beta 5, so Alpha has the absolute advantage in wheat. Cloth: Beta 8 > Alpha 4, so Beta has the absolute advantage in cloth.

Now derive the gain rather than assert it. Assume each country initially splits its labour 50/50 — this is the autarky allocation, and we will keep it fixed so the comparison is clean.

Autarky output:

Now let each country specialise completely in its absolute-advantage good.

Specialised output:

World output change: wheat +250 tonnes (+33.3%), cloth +200 metres (+33.3%).

Nothing was added. No new workers, no new machines, no new knowledge. The same 200 worker-years now yield 250 more tonnes of wheat and 200 more metres of cloth, purely because each worker-year was moved to the task at which it was more productive. That extra output is the static gain from specialisation, and it is real output, available to be divided between the two countries.

Verify the arithmetic yourself before moving on. Alpha's 50 workers previously making 200 cloth now make 500 wheat. Beta's 50 workers previously making 250 wheat now make 400 cloth. The world loses 200 cloth from Alpha and gains 400 from Beta: net +200. The world loses 250 wheat from Beta and gains 500 from Alpha: net +250. That is the whole mechanism.

#3.3 Boundary case: where absolute advantage breaks

Absolute advantage answers the question only in the symmetric case, where each country happens to be best at one good. Change one number and the theory falls silent.

Northland has the absolute advantage in both goods. On Smith's rule, Northland should produce both and Southland should produce neither. But Southland's workers cannot be un-employed into nothing; they exist and must do something. And Northland cannot produce everything the world wants — it has only 100 worker-years.

Smith's theory offers no guidance here. Worse, the intuition it plants — "we should make what we're best at, and buy the rest from countries that beat us" — implies that a country with no absolute advantage anywhere has nothing to sell and cannot trade at all. Extend that reasoning and low-productivity economies should be permanently excluded from the world trading system. That prediction is spectacularly false: Bangladesh, whose labour productivity is far below the EU's in every manufacturing category, exported US$48.28 billion in FY2024-25, of which US$39.35 billion (81.49%) was readymade garments (BGMEA, compiled from Export Promotion Bureau data, FY2024-25). A theory that says this cannot happen needs replacing.

LIMITATION OF ABSOLUTE ADVANTAGE
It is a comparison of levels.
Levels are silent when one country wins everything.
It also cannot rank a country's own options.
It cannot say which good to specialise in when you lead in both.
It is a special case, not the general rule.

The replacement is comparative advantage. It is more general: it always gives an answer, and where absolute advantage also gives an answer, the two agree.

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#4. Comparative advantage: full derivation

#4.1 The change of question

Absolute advantage asks: between countries, who produces more of good X?

Comparative advantage asks: within a country, what does producing good X cost in terms of good Y? — and then compares those internal costs across countries.

The first question compares productivity. The second compares opportunity cost. The switch is the whole intellectual move, made by David Ricardo in On the Principles of Political Economy and Taxation (1817), chapter 7.

COMPARATIVE ADVANTAGE
A country has a comparative advantage in a good
if it produces that good at a lower opportunity cost
than the other country.
Opportunity cost is measured in units of the other good.
This is a comparison of RATIOS.

#4.2 Deriving the opportunity cost formula

Take one country with output per worker-year of a units of good A and b units of good B. One worker-year makes either a units of A or b units of B.

To produce one extra unit of A, the country must free up 1/a of a worker-year. That fraction of a worker-year would have produced (1/a) × b units of B.

OPPORTUNITY COST FROM OUTPUT DATA
OC of 1 unit of A = b / a   (measured in units of B)
OC of 1 unit of B = a / b   (measured in units of A)
The two are reciprocals. Their product is 1.
Rule: divide the OTHER good by THIS good.

The reciprocal property is a free error-check. If you compute the opportunity cost of wheat as 0.4 cloth, then the opportunity cost of cloth must be 1/0.4 = 2.5 wheat. If your two answers do not multiply to 1, you have made an arithmetic slip. Use this on every question.

If the data are given as input requirements instead — hours needed per unit — the formula inverts. Let h_A be hours per unit of A and h_B be hours per unit of B.

OPPORTUNITY COST FROM INPUT DATA
OC of 1 unit of A = h_A / h_B   (units of B)
OC of 1 unit of B = h_B / h_A   (units of A)
Rule: divide THIS good by the OTHER good.
Input data flips the rule. Check the column headings first.

Ricardo's original figures are input data, and it is worth doing them once. He supposed England needed 100 men for a year to make a unit of cloth and 120 men for a unit of wine; Portugal needed 90 men for cloth and 80 for wine (Ricardo, 1817, ch. 7). Portugal has the absolute advantage in both. But England's opportunity cost of cloth is 100/120 = 0.83 units of wine, while Portugal's is 90/80 = 1.125 units of wine. Cloth is cheaper in England, in the only sense that matters. England has the comparative advantage in cloth despite being absolutely worse at making it.

#4.3 Scenario A worked through comparative advantage

Return to Alpha and Beta.

Apply OC of A = b/a.

Alpha, opportunity cost of 1 tonne of wheat = cloth per worker ÷ wheat per worker = 4/10 = 0.4 metres of cloth. Alpha, opportunity cost of 1 metre of cloth = 10/4 = 2.5 tonnes of wheat. (Check: 0.4 × 2.5 = 1 ✓)

Beta, opportunity cost of 1 tonne of wheat = 8/5 = 1.6 metres of cloth. Beta, opportunity cost of 1 metre of cloth = 5/8 = 0.625 tonnes of wheat. (Check: 1.6 × 0.625 = 1 ✓)

Assemble the comparison table. This table is the deliverable in an exam — draw it every time.

Alpha sacrifices 0.4 metres of cloth per tonne of wheat; Beta sacrifices 1.6. Alpha is the cheap producer of wheat. Beta sacrifices 0.625 tonnes of wheat per metre of cloth; Alpha sacrifices 2.5. Beta is the cheap producer of cloth.

Conclusion: Alpha has the comparative advantage in wheat; Beta has the comparative advantage in cloth.

Notice this matches the absolute advantage answer from section 3.2. That is not a coincidence and it is not a redundancy — it is the general theory reproducing the special case. When each country has an absolute advantage in one good, comparative and absolute advantage always point the same way.

Notice also a structural fact you can exploit. The comparative advantages must be in different goods. If Alpha's opportunity cost of wheat is lower than Beta's, then Beta's opportunity cost of cloth is necessarily lower than Alpha's, because the two costs are reciprocals. Formally: if b_A/a_A < b_B/a_B then taking reciprocals reverses the inequality, giving a_A/b_A > a_B/b_B. It is arithmetically impossible for one country to have the comparative advantage in both goods. Any exam answer claiming otherwise contains an error — go back and check.

THE MIRROR RULE
Comparative advantage is always split.
One country gets one good, the other gets the other.
This follows from reciprocals, not from fairness.
The only exception is a tie.
Use it to check your work.

#4.4 Scenario B — the counterintuitive case, worked in full

This is the case Smith could not handle and the case examiners love. Northland is absolutely better at everything.

Northland has 100 worker-years. Southland has 400 worker-years — larger population, lower productivity, a deliberately realistic asymmetry.

Step 1 — Absolute advantage. Northland: 20 > 5 machines, 30 > 20 rice. Northland has the absolute advantage in both. Southland has none. On Smith's logic there is no basis for trade.

Step 2 — Opportunity costs. Apply OC of A = b/a.

Northland, 1 machine = 30/20 = 1.5 tonnes of rice. Northland, 1 tonne of rice = 20/30 = 0.667 machines. (1.5 × 0.667 ≈ 1 ✓)

Southland, 1 machine = 20/5 = 4 tonnes of rice. Southland, 1 tonne of rice = 5/20 = 0.25 machines. (4 × 0.25 = 1 ✓)

Northland has the comparative advantage in machines. Southland has the comparative advantage in rice — despite being absolutely worse at growing it.

Sit with that. Southland's workers grow 20 tonnes of rice where Northland's grow 30. Southland is a worse farmer. Yet the world should move rice production toward Southland. Why? Because when Northland grows a tonne of rice it destroys 0.667 machines' worth of capability, and when Southland grows a tonne of rice it destroys only 0.25 machines' worth. The real cost of Southland's rice — measured in the machines the world doesn't get — is less than a third of Northland's.

The economic intuition, stated without algebra: Northland's workers are so extraordinarily good at making machines that using them to grow rice is a waste, even though they are also better at growing rice. A surgeon may be a faster typist than her secretary. She should still not type her own notes, because every minute typing is a minute not operating. Her comparative advantage lies in surgery; the secretary's lies in typing. Absolute skill is irrelevant to the allocation.

Step 3 — Autarky output. Suppose Northland devotes 60 worker-years to machines and 40 to rice. Southland devotes 100 worker-years to machines and 300 to rice. (Different mixes, reflecting different preferences and income levels — nothing hinges on the specific split, but we must fix one to measure gains against.)

Step 4 — Complete specialisation.

World gain: +300 machines and +800 tonnes of rice. Both goods rise. The same 500 worker-years produce more of everything.

Step 5 — Does each country actually gain? World output rising is not sufficient. Each country must end up consuming more than its autarky bundle, or it will refuse to trade. That requires a rate of exchange, which is the subject of the next section — but let us complete the example so the payoff is visible now.

Suppose Northland exports 600 machines and receives 3 tonnes of rice per machine, so it imports 1,800 tonnes of rice.

Check the columns close: 1,400 + 600 = 2,000 machines ✓. 1,800 + 6,200 = 8,000 rice ✓. Every unit produced is consumed; nothing appears from nowhere.

Both countries consume more of both goods. Northland — richer, more productive at everything, with nothing to learn from Southland technologically — is unambiguously better off for having traded with a country it outperforms in every dimension. Southland, with no absolute advantage anywhere, is also better off.

This is the result. It is not a curiosity. It is the reason trade between rich and poor countries is not charity in one direction or exploitation in the other, but a positive-sum reallocation. It is also why "we can't compete with low-wage countries" and "they can't compete with our productivity" are both wrong for the same reason: neither statement is about ratios.

#4.5 Two ways to spot comparative advantage fast

Method 1 — Opportunity cost table. Compute all four opportunity costs, tabulate, pick the lower in each row. Slower but transparent, and it produces the ratios you need later for the terms of trade. Use this in exams. The table itself earns application marks.

Method 2 — Ratio comparison. Compare the ratio of the two countries' productivities in each good. In Scenario B, Northland/Southland is 20/5 = 4 in machines and 30/20 = 1.5 in rice. Northland's advantage is proportionately larger in machines (4× versus 1.5×), so Northland specialises in machines and Southland in rice. Faster, but it gives you no opportunity cost figures, so you cannot proceed to the terms of trade with it.

FAST CHECK
Find where the productivity gap is biggest.
The leader takes that good.
The laggard takes the good where it is least behind.
Then verify with opportunity costs.
Never submit Method 2 alone.

#4.6 Boundary case: identical opportunity cost ratios

Every model must be tested where it fails. Comparative advantage fails, cleanly and completely, when opportunity cost ratios are identical.

Gamma is exactly twice as productive as Alpha at everything. It has a large absolute advantage in both goods.

Opportunity costs: Alpha's wheat costs 4/10 = 0.4 cloth. Gamma's wheat costs 8/20 = 0.4 cloth. Identical. Alpha's cloth costs 2.5 wheat; Gamma's cloth costs 2.5 wheat. Identical.

Neither country has a comparative advantage in anything. The mirror rule's exception has bitten.

Demonstrate the consequence numerically. Autarky, 50/50 splits:

Specialise completely: Alpha makes 1,000 wheat, Gamma makes 800 cloth. World total: 1,000 wheat, 800 cloth. World wheat has fallen by 500 while cloth rose by 200 — and 500 wheat is worth exactly 200 cloth at the ratio 2.5, which both countries share. The world has moved along a straight world PPF, not outward. No output has been created.

Now attempt to trade. The only conceivable exchange rate is 0.4 cloth per wheat — the rate both countries already face at home. Suppose Alpha ships 500 wheat and receives 200 cloth. Alpha ends with 500 wheat and 200 cloth: precisely its autarky bundle. Gamma ends with 500 wheat and 600 cloth. Was that worth it? Gamma's own PPF from (1,000 wheat, 400 cloth) says that gaining 200 cloth costs 500 wheat — landing exactly on (500, 600). Gamma could have reached that point alone. Trade achieved nothing.

NO-GAINS CONDITION
Gains require different opportunity cost ratios.
Identical ratios mean identical PPF gradients.
Parallel frontiers cannot be pushed outward by exchange.
Absolute advantage is neither necessary nor sufficient.
Difference in ratios is both.

This is the sharpest statement of the theory. Gamma's crushing absolute superiority bought it nothing in trade terms. What generates gains is not being better — it is being differently better. Trade is powered by divergence in relative costs, and by nothing else.

#The real-world corollary matters for evaluation. Two economies with very similar factor endowments, similar technology and similar relative costs — say, two neighbouring high-income European economies producing similar manufactures — have weak Ricardian reasons to trade. Yet they trade enormously. That anomaly is real, and section 11 resolves it. Hold the question.

#5. Terms of trade: deriving the feasible range

Comparative advantage tells you who should specialise in what. It does not tell you at what rate goods exchange, and therefore it does not tell you who captures the gains. That is a separate question with a separate answer.

#5.1 Definition

TERMS OF TRADE
The rate at which one good exchanges for another.
In the model: units of imports per unit of exports.
In the data: an index of export prices over import prices.
It divides the gains. It does not create them.

Two senses of the same idea. The barter (real) terms of trade is the physical exchange ratio inside the model — how many metres of cloth a tonne of wheat buys. The terms of trade index is the empirical measure used in national statistics, built from price indices. Section 6 handles the index. This section derives the range.

#5.2 Deriving the range from participation constraints

Nobody trades at a loss. Each country has an outside option: produce the good itself at home, at its own opportunity cost. That outside option sets a limit on what it will accept.

Return to Scenario A. Alpha's opportunity cost of wheat is 0.4 cloth; Beta's is 1.6 cloth. Alpha will export wheat and import cloth.

Alpha's constraint. If Alpha gives up a tonne of wheat at home, it gets 0.4 metres of cloth. It will only export a tonne of wheat if the world gives it more than 0.4 metres in return. Otherwise it produces the cloth itself and saves the shipping.

Alpha requires: terms of trade > 0.4 cloth per tonne of wheat.

Beta's constraint. If Beta wants a tonne of wheat, it can grow it at home by giving up 1.6 metres of cloth. It will only import a tonne of wheat if it can obtain it for less than 1.6 metres of cloth.

Beta requires: terms of trade < 1.6 cloth per tonne of wheat.

Combine.

THE FEASIBLE RANGE
0.4 < terms of trade < 1.6   (cloth per tonne of wheat)
Lower bound = the exporter's opportunity cost.
Upper bound = the importer's opportunity cost.
Outside this range one country refuses.
Inside it, both gain.

The terms of trade must lie strictly between the two countries' opportunity cost ratios. This is not a convention; it is a consequence of each country's ability to walk away.

Two immediate corollaries fall out.

At a boundary, one country gains nothing. If the terms of trade settle at exactly 0.4, Alpha is indifferent — trading gives it precisely what home production gave it. All the gains accrue to Beta. Symmetrically at 1.6, all gains accrue to Alpha. The boundaries are the limiting cases of a one-sided division, not points of "fairness".

If the ratios coincide, the range is empty. Return to Alpha and Gamma in section 4.6, both with an opportunity cost of 0.4. The condition becomes 0.4 < ToT < 0.4, which no number satisfies. The empty range is the formal statement of "no gains from trade". This is why section 4.6's result and this section's result are the same theorem seen from two angles.

The same range can be written in the reciprocal units. Alpha's cloth costs 2.5 wheat, Beta's costs 0.625 wheat, so the terms of trade in wheat per metre of cloth must satisfy 0.625 < ToT < 2.5. Confirm consistency: 1/1.6 = 0.625 ✓ and 1/0.4 = 2.5 ✓. Both statements describe the same set of exchange rates. State your units every time. An answer of "1.2" is meaningless without "metres of cloth per tonne of wheat".

#5.3 Scenario B's range, for practice

Northland's opportunity cost of a machine is 1.5 tonnes of rice. Southland's is 4 tonnes.

Northland exports machines, so it needs more than 1.5 tonnes of rice per machine. Southland imports machines, so it will pay less than 4 tonnes per machine.

Feasible range: 1.5 < terms of trade < 4 tonnes of rice per machine.

In section 4.4 we assumed 3 tonnes of rice per machine. Confirm: 1.5 < 3 < 4 ✓. That is why both countries gained. Had we assumed 5 tonnes of rice per machine, Southland would have refused — it can make its own machines for 4 tonnes each. Had we assumed 1 tonne, Northland would have refused.

#5.4 What determines where inside the range the rate settles?

The Ricardian model as stated does not pin down a point. It gives a range and stops. Determining the actual rate requires demand, which the model omits. Four forces do the work in practice.

Relative demand. John Stuart Mill's extension (1848) introduced reciprocal demand: the terms of trade settle where the quantity each country wants to import equals the quantity the other wants to export. If world demand for wheat is strong relative to cloth, the rate moves toward the top of the range and the wheat exporter captures more.

Relative country size. A small country trading with a large one typically finds the terms of trade settling near the large country's opportunity cost ratio, because the large country's domestic ratio dominates the world market. Counter-intuitively this favours the small country, which captures nearly all the gains. Singapore and Hong Kong SAR illustrate the small-open-economy case: they take world prices as given and gain heavily relative to domestic size.

Market power. Where sellers are concentrated, they can push the rate toward their preferred boundary. OPEC's function is precisely to move the terms of trade for crude oil toward the importers' opportunity cost.

Price elasticity of demand and supply. Where demand for a country's exports is price-inelastic and demand for its imports is price-elastic, price movements are less punishing. Where the reverse holds — typical of primary commodity exporters — adverse price movements bite hard. Section 10 develops this into the Prebisch–Singer argument.

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#6. How the terms of trade divide the gains — three worked splits

The most misunderstood point on this topic: the terms of trade determine the distribution of the gains, not their size. We can prove that.

Hold everything constant except the exchange rate. Scenario A: Alpha specialises fully in wheat (1,000 tonnes), Beta fully in cloth (800 metres). Alpha exports 400 tonnes of wheat in each case. Only the cloth it receives varies.

Autarky benchmarks: Alpha (500 wheat, 200 cloth); Beta (250 wheat, 400 cloth).

#Split 1 — Terms of trade = 0.5 cloth per tonne of wheat (near Alpha's floor of 0.4)

Alpha exports 400 wheat and imports 400 × 0.5 = 200 cloth.

Alpha barely benefits. The rate sits close to Alpha's own opportunity cost, so trading is only marginally better than self-sufficiency.

#Split 2 — Terms of trade = 0.75 cloth per tonne of wheat (mid-range)

Alpha exports 400 wheat and imports 300 cloth.

#Split 3 — Terms of trade = 1.0 cloth per tonne of wheat (toward Beta's ceiling of 1.6)

Alpha exports 400 wheat and imports 400 cloth.

#The distributive result

Look down the "World" row in all three tables. It never changes: +250 wheat and +200 cloth, every time. The total gain is created by specialisation and is fixed by the production technologies. The terms of trade only decides who gets it.

To see the split as a single number, value every gain at one fixed reference rate. Use 1 metre of cloth = 1.333 tonnes of wheat throughout (the mid-range rate), so the yardstick does not move between comparisons. Total world gain = 250 + (200 × 1.333) = 516.7 wheat-equivalents in all three splits.

Working for row 1: Alpha gains 100 wheat and 0 cloth → 100 + 0 = 100.0. Beta gains 150 wheat and 200 cloth → 150 + (200 × 1.333) = 150 + 266.7 = 416.7. Sum 516.7 ✓.

Working for row 3: Alpha gains 100 wheat and 200 cloth → 100 + 266.7 = 366.7. Beta gains 150 wheat and 0 cloth → 150.0. Sum 516.7 ✓.

THE DISTRIBUTIVE RULE
Specialisation creates the gain.
The terms of trade splits the gain.
A rate near your own opportunity cost is bad for you.
A rate near your partner's opportunity cost is good for you.
Total gains stay constant. Only shares move.

This is the analytical foundation of every real dispute about trade. Nobody argues about whether trade creates output. They argue about the terms. When a developing economy complains that trade is not delivering, the complaint is almost never "specialisation is inefficient" — it is "the terms of trade sit too close to our own opportunity cost, so we capture too little." That is a coherent, model-consistent objection, and it earns evaluation marks precisely because it accepts the theory and attacks the distribution.

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#7. The terms of trade index: measurement

#7.1 The formula

Real economies do not trade two goods at a barter ratio. They trade thousands of goods for money. The empirical counterpart of the barter ratio is an index.

TERMS OF TRADE INDEX
ToT = (average export price index / average import price index) × 100
Rise = improvement. Fall = deterioration.
Improvement means exports buy more imports.
It says nothing about volumes.
It says nothing about the trade balance.

The base year is set to 100 for both price indices, so the ToT index starts at 100.

#7.2 Worked calculation — deterioration

A country's price indices, base year 2015 = 100:

Interpretation, and this is the part that earns application marks. In 2015 a basket of exports bought one basket of imports. By 2025 the same physical volume of exports buys only 0.80 of that import basket. The country must export 25% more in volume to import the same quantity — because 1/0.80 = 1.25.

Note why it deteriorated: export prices rose 12% but import prices rose 40%. Both rose. A terms of trade deterioration does not require falling export prices; it requires export prices rising more slowly than import prices. Students routinely get this wrong.

#7.3 Worked calculation — improvement

A 5.9% improvement. Each unit exported now buys 5.9% more imports. Real national income rises even if output volumes are unchanged — this is a pure price effect on purchasing power, sometimes called the terms of trade effect on real gross domestic income.

#7.4 The crucial caveat for evaluation

An improvement in the terms of trade is not automatically good.

Suppose export prices rise 20% because the currency appreciated. The ToT index improves. But if foreign demand for exports is price-elastic, export volumes fall by more than 20% in percentage terms and total export revenue falls. The current account worsens while the terms of trade improve. Whether an improvement raises welfare therefore depends on the price elasticity of demand for exports and imports — the same elasticity condition that underlies the Marshall–Lerner analysis of devaluation.

EVALUATION LINE — TERMS OF TRADE
An improvement raises purchasing power per unit.
It may reduce the number of units sold.
The net effect depends on price elasticity of demand.
Inelastic demand: improvement raises revenue.
Elastic demand: improvement can cut revenue.

This distinction — price effect versus volume effect — is a reliable route to Level 3 evaluation in WEC14, because it recognises different viewpoints and supports them with a chain of reasoning.

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#8. Gains from trade: consuming beyond the frontier

We have shown countries gain. Now show where the gain shows up on a diagram, because WEC14 4.3.2.1 explicitly requires the theory "numerical and diagrammatic".

#8.1 Deriving the consumption possibility frontier

Alpha's PPF runs from (1,000 wheat, 0 cloth) to (0 wheat, 400 cloth). Its equation: for any wheat quantity W, maximum cloth = 400 − 0.4W.

Now open trade. Alpha specialises completely: it produces 1,000 tonnes of wheat and no cloth. It then exchanges wheat for cloth at the world rate of 0.75 metres of cloth per tonne of wheat. Alpha's consumption options are now every combination reachable by exporting some quantity of that 1,000 tonnes.

Every row except the first shows Alpha consuming a bundle it could not have produced. The final row is the headline: Alpha's maximum attainable cloth rises from 400 metres to 750 metres — an 87.5% increase — with no change whatsoever in its technology, labour force or capital stock.

This new line, running from (1,000 wheat, 0 cloth) to (0 wheat, 750 cloth), is the consumption possibility frontier (CPF). Its gradient is the terms of trade. It lies outside the PPF everywhere except at the specialisation point where they touch.

Beta's CPF is derived identically. Beta produces 800 metres of cloth. At 0.75 cloth per wheat, one metre of cloth buys 1/0.75 = 1.333 tonnes of wheat, so 800 metres buys 1,066.7 tonnes. Beta's CPF runs from (0 wheat, 800 cloth) to (1,066.7 wheat, 0 cloth), against a PPF whose wheat intercept is only 500 tonnes. Beta's maximum attainable wheat rises from 500 to 1,066.7 tonnes, a 113% increase.

#8.2 The diagram, described precisely

For each country, draw one set of axes with wheat on one axis and cloth on the other. Plot:

1. The PPF as a straight line between the two autarky intercepts.
2. The autarky consumption point on that line.
3. The specialisation point at the relevant axis intercept — the corner where the country produces only its comparative advantage good.
4. The CPF as a straight line from the specialisation point, with gradient equal to the terms of trade, cutting across and lying outside the PPF.
5. The post-trade consumption point on the CPF, strictly north-east of the autarky point.

Label the gap between the autarky point and the post-trade point as the gains from trade. Two arrows — one showing more wheat, one showing more cloth — convert an assertion into a demonstration and reliably earn application marks.

#8.3 The claim that "trade shifts the PPF outward" — stated correctly

Textbooks frequently write that trade shifts the PPF outward. In the static Ricardian model that is false, and knowing why is worth marks.

The PPF describes production possibilities, determined by resources and technology. Trade changes neither. Alpha still has 100 worker-years and still produces 10 tonnes of wheat or 4 metres of cloth per worker-year. Its PPF is exactly where it was. What moves outward is the consumption possibility frontier. The correct statement is:

STATIC RESULT
Trade does not move the production frontier.
Trade moves the consumption frontier outward.
Production stays on the PPF, at a corner.
Consumption moves beyond the PPF.
Say "consumption possibility frontier" and be precise.

There is a separate, longer-run argument in which the PPF genuinely does shift outward, and it belongs in evaluation rather than analysis because it depends on conditions that may not hold.

Dynamic gains from trade — mechanisms by which the PPF itself can shift outward:

• Capital goods imports. Export revenue funds imported machinery, raising the capital stock. This is a genuine outward shift of the PPF, and it is the primary channel through which trade raises long-run growth in developing economies.
• Technology transfer. Contact with foreign firms, inward foreign direct investment and imported intermediate inputs diffuse production techniques.
• Economies of scale. A larger market allows longer production runs, lowering long-run average cost and raising output from given resources. This is central to intra-industry trade (section 11).
• Competitive pressure. Import competition erodes domestic monopoly power, reducing X-inefficiency and forcing cost discipline.
• Increased variety of inputs. Access to a wider range of intermediate goods raises total factor productivity.

Each is contestable, which is exactly why each is useful in evaluation. Technology transfer requires domestic absorptive capacity — human capital, infrastructure, institutions. Economies of scale require the domestic firm to reach efficient scale before foreign competition eliminates it. Competitive pressure raises efficiency only if domestic firms survive long enough to respond.

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#9. Assumptions: what each one carries, and what breaks without it

The Ricardian model is a machine with named parts. Removing a part does not always stop the machine — sometimes it just changes the output. Knowing which conclusion each assumption supports is the difference between a Level 2 and a Level 4 answer, because it lets you evaluate precisely rather than listing generic criticisms.

For each assumption below: what it is, which conclusion it is load-bearing for, and what happens when it is relaxed.

#9.1 Two countries, two goods

States: the world contains exactly two trading economies and two products.

Load-bearing for: the simple "each country specialises in one good" conclusion, and the two-sided terms of trade range.

Relaxed: with many goods, comparative advantage still operates but by ranking. Line up all goods by the ratio of home to foreign opportunity cost; a country produces those at the top of its ranking and imports the rest, with the cut-off determined by relative wages. With many countries, a country's comparative advantage is defined against the world price, not against a single partner. The core result survives fully — this assumption is a simplification for exposition, not a foundation. Do not use "the real world has more than two countries" as an evaluation point on its own; it is true and worthless. Use it only to reach a consequence: with many countries, a nation may lose a market to a third country rather than to its trading partner, so specialisation is riskier than the two-country model suggests.

#9.2 Constant returns to scale and constant opportunity costs

States: doubling inputs doubles outputs, and the opportunity cost of a good does not change as you produce more of it. This is what makes the PPF a straight line.

Load-bearing for: complete specialisation. This is the most consequential assumption in the model.

Relaxed — the concave PPF. In reality, factors of production are not equally suited to all uses. Land ideal for wheat is poor for cotton. As Alpha shifts resources from wheat to cloth, it moves progressively less suitable resources — starting with those best suited to cloth and ending with those worst suited. Each additional metre of cloth therefore costs more wheat than the last. Opportunity cost rises. The PPF becomes concave to the origin (bowed outward).

Now suppose the world terms of trade are 1.2 tonnes of wheat per metre of cloth. Where does Alpha stop?

Alpha expands cloth production only while producing cloth at home costs less than buying it. The first 100 metres cost 0.5 wheat each — cheaper than the world price of 1.2, so produce them. The next 100 cost 1.0 each — still cheaper than 1.2, so produce them. The next 100 cost 1.5 each — more expensive than 1.2, so stop and import instead.

Alpha produces 200 metres of cloth and 850 tonnes of wheat. It does not specialise completely. It imports the rest of its cloth.

INCREASING COSTS RESULT
Rising opportunity cost stops specialisation early.
Production halts where domestic OC equals the world price.
Specialisation becomes partial, not complete.
Gains from trade shrink but do not vanish.
This is what real economies do.

This single relaxation explains one of the most obvious facts about the world: no country specialises completely. The United States is a net importer of manufactures and still manufactures. Bangladesh exports garments and still grows rice. Complete specialisation is an artefact of the straight-line PPF, not a prediction about reality. Stating this — with the mechanism, not just the observation — is a strong evaluation move.

#9.3 No transport costs

States: moving goods between countries is free.

Load-bearing for: the claim that any terms of trade strictly inside the range generates gains for both parties.

Relaxed: transport, insurance, tariffs and non-tariff barriers all drive a wedge between the price the exporter receives and the price the importer pays. Suppose transport costs 0.3 metres of cloth per tonne of wheat shipped. The effective range narrows: the exporter still needs to net more than 0.4, and the importer still refuses to pay more than 1.6, but the 0.3 must come out of the middle. The feasible band shrinks from (0.4, 1.6) — a width of 1.2 — to an effective width of 0.9.

If costs exceed the width of the range, the range closes entirely and mutually beneficial trade becomes impossible even though comparative advantage exists. This is why comparative advantage predicts trade in high-value, low-weight goods (pharmaceuticals, semiconductors, aircraft parts, financial services) far better than in low-value, high-weight goods (sand, bricks, fresh milk). It is also why falling transport and communication costs — containerisation from the 1950s, digitisation of services since the 1990s — expanded trade so dramatically: they widened every range simultaneously.

A useful measured example: services trade is now the UK's dominant export category precisely because services carry near-zero transport cost. In the three months to May 2026 the UK exported £140.7 billion of services against £100.7 billion of goods, running a services surplus of £51.8 billion against a goods deficit of £60.9 billion (Office for National Statistics, UK trade, May 2026 bulletin).

#9.4 Perfect factor mobility within a country

States: labour and capital can move instantly and costlessly between industries inside a country.

Load-bearing for: the assumption that resources released from the contracting industry are immediately absorbed by the expanding one — and therefore for the claim that specialisation raises output without raising unemployment.

Relaxed — and this is the assumption that fails hardest in practice. A textile worker in a declining industry is not automatically a software engineer. Reallocation requires retraining, geographic relocation, and time. In the interim there is structural unemployment: the mismatch between the skills and location of the unemployed and the requirements of available jobs.

Structural unemployment is not a footnote. Autor, Dorn and Hanson estimate that rising import competition from China reduced US employment by 2.4 million workers between 1999 and 2011, once input–output linkages and aggregate demand effects are included (Autor, Dorn and Hanson, "The China Shock", Annual Review of Economics, 2016; the direct import-exposure channel alone accounts for roughly 560,000). The aggregate gains from that trade were real and are not disputed by those authors; the point is that the losses were concentrated in specific commuting zones and persisted for a decade or more.

FACTOR IMMOBILITY RESULT
Gains from trade are aggregate and diffuse.
Losses are concentrated and visible.
Adjustment is not instant.
Occupational and geographic immobility create structural unemployment.
This is a distribution problem, not a refutation of the theory.

That last line matters. Factor immobility does not show that comparative advantage is wrong. It shows that the net gain conceals a large gross loss for an identifiable group — which is a political problem of the first order and an evaluation point of the first order.

#9.5 Factor immobility between countries

States: labour and capital cannot move across borders, so productivity differences persist.

Load-bearing for: the existence of persistent comparative advantage at all.

Relaxed: if capital could move freely and instantly, it would flow to wherever returns are highest, equalising capital–labour ratios and eroding the cost differences that generate comparative advantage. Foreign direct investment does exactly this, partially. When a firm relocates production to a low-wage economy it is substituting factor mobility for goods mobility. In the limit, perfect factor mobility makes trade in goods unnecessary — you move the factory instead of the output.

Reality sits between. Capital is highly mobile; labour is not. The result is that comparative advantage is increasingly created by investment rather than inherited from endowments, which is the foundation of the dynamic comparative advantage argument in section 10.

#9.6 Full employment of resources

States: every economy operates on its PPF before trade.

Load-bearing for: the claim that expanding one industry requires contracting another — i.e., that opportunity cost is positive at all.

Relaxed: if an economy starts inside its PPF with unemployed resources, it can expand output of one good without sacrificing the other. Opportunity cost falls toward zero in the short run. This has two consequences. First, the measured gains from trade calculated above are overstated, because part of the observed output increase would have been available without trade simply by employing idle resources. Second — and more importantly for policy — a country with mass unemployment has a weaker case for the immediate reallocation the model prescribes and a stronger case for demand management first.

#9.7 Perfect competition and prices equal to opportunity costs

States: market prices accurately signal relative opportunity costs, so private decisions produce the socially optimal specialisation.

Load-bearing for: the conclusion that free trade delivers the gains automatically, without planning.

Relaxed: where there are externalities, prices lie. If garment production imposes uncosted environmental damage, the market price understates true social opportunity cost and the country over-specialises in garments. If an industry generates positive external benefits — a knowledge-spillover-rich sector such as semiconductors — the market price overstates its cost and the country under-specialises. Comparative advantage as computed from market prices is not necessarily comparative advantage as computed from social costs. This is the analytically rigorous version of the infant industry argument, and it is far stronger than the usual "new industries need protection" phrasing.

#9.8 No trade barriers

States: no tariffs, quotas, subsidies or regulatory obstacles.

Load-bearing for: the terms of trade reflecting relative costs rather than policy.

Relaxed: a tariff drives a wedge between world and domestic prices, causing the importing country to produce more of the good than comparative advantage warrants. A subsidy to an export industry can manufacture an apparent comparative advantage that does not reflect real cost differences. Both reduce world output below the specialisation optimum. Both may nonetheless be rational for the country imposing them if it is large enough to shift the terms of trade in its favour — the optimal tariff argument.

#9.9 Fixed technology and endowments

States: the productivity numbers in the table are permanent.

Load-bearing for: the recommendation to specialise permanently in the current comparative advantage good.

Relaxed: comparative advantage moves. It has moved decisively and repeatedly. Textile production ran from Britain to Japan to Hong Kong SAR to mainland China to Bangladesh and Vietnam within a century and a half. A country that specialises in its current comparative advantage may be entrenching a low-value position while a competitor invests its way into a higher-value one. This is the static versus dynamic comparative advantage distinction, developed fully in section 10, and it is the single most powerful evaluation line available on this topic.

#9.10 The assumption summary table

#The three "High" entries are where your evaluation should live. They are the assumptions whose failure changes the policy conclusion, not merely the arithmetic.

#10. Limitations and criticisms

Section 9 removed assumptions one at a time. This section takes the six criticisms that carry the most weight in WEC14 evaluation and develops each to a conclusion. Each is presented as a mechanism, not a slogan, because examiners reward chains of reasoning and ignore lists.

#10.1 Where does comparative advantage come from? Heckscher–Ohlin and the Leontief paradox

Ricardo's model tells you what to do given the productivity table. It does not explain where the table comes from. Ricardo simply assumed technology differs.

Eli Heckscher and Bertil Ohlin supplied the missing explanation in the 1920s and 1930s. Their argument: countries differ in factor endowments, and goods differ in factor intensity.

HECKSCHER-OHLIN THEOREM
Countries export goods that use their abundant factor intensively.
Capital-abundant countries export capital-intensive goods.
Labour-abundant countries export labour-intensive goods.
Abundance lowers a factor's relative price.
Low factor price lowers the cost of goods using it heavily.

The logic is a supply-and-demand argument applied to factor markets. If a country has a great deal of labour relative to capital, the wage is low relative to the return on capital. Goods that use a lot of labour per unit of output are therefore relatively cheap to make. This is a derivation of comparative advantage from something more primitive, and it explains real patterns: Bangladesh's garment specialisation, Germany's specialisation in capital- and skill-intensive machinery (motor vehicles and parts were Germany's largest export category at 16.2% of total exports in 2025, per the Federal Statistical Office, Destatis), Saudi Arabia's specialisation in crude oil.

Then the theory hit the data and lost.

Wassily Leontief, using his own input–output tables, tested Heckscher–Ohlin on US trade data for 1947. The United States in 1947 was overwhelmingly the most capital-abundant economy in the world. Heckscher–Ohlin therefore predicted US exports would be more capital-intensive than the domestic goods that would replace US imports.

Leontief found the opposite. Capital per worker-year embodied in US exports was approximately US$13,991, while capital per worker-year embodied in US import replacements was approximately US$18,184 (Leontief, "Domestic Production and Foreign Trade: The American Capital Position Re-Examined", Proceedings of the American Philosophical Society, 1953). US exports were about 23% less capital-intensive than its import-competing goods. The most capital-rich nation on earth was exporting labour-intensive goods.

This is the Leontief paradox. It was replicated: Robert Baldwin, using 1962 US trade data, found US imports were 27% more capital-intensive than US exports.

Four resolutions have been offered, and each is an evaluation point in its own right.

1. Labour is not homogeneous. Leontief measured worker-years, treating a US engineer and a subsistence farmer as the same unit. If US labour is skill-abundant, then US exports are intensive in human capital, which Leontief's method classified as labour. Add human capital to the capital measure and much of the paradox dissolves.
2. Technology differs between countries, contrary to Heckscher–Ohlin's assumption of identical production functions. This is a return to Ricardo's original position.
3. Natural resources were omitted as a third factor. Many US imports (minerals, ores) are resource-intensive and, in extraction, also capital-intensive.
4. Trade policy distorted the pattern. US protection in 1947 fell disproportionately on labour-intensive imports, suppressing exactly the trade the theory predicted.

The important lesson is methodological. A theory that predicts the direction of trade can be tested, and Heckscher–Ohlin failed its first serious test. This does not overturn comparative advantage — Leontief's finding is entirely consistent with the US having a comparative advantage in skill-intensive goods. It overturns the simplest account of where comparative advantage comes from. Cite it when a question asks about the limitations of trade theory; it demonstrates that you know the theory has an empirical record and not merely a set of assumptions.

#10.2 The Prebisch–Singer hypothesis: the terms of trade as a development problem

Raúl Prebisch and Hans Singer, independently in 1950, advanced a claim that has shaped development policy ever since: the terms of trade of primary commodity exporters tend to decline over the long run against manufactured goods.

The mechanism has four parts, and reproducing the mechanism rather than the conclusion is what earns marks.

1. Income elasticity of demand differs. Engel's Law: as incomes rise, the proportion of income spent on food falls. Primary commodities generally have income elasticity of demand below 1; manufactured and high-technology goods have income elasticity above 1. As world income grows, demand for manufactures grows faster than demand for commodities. Relative prices move against commodities.

2. Technical progress is distributed asymmetrically. In developed economies, productivity gains in manufacturing are captured by producers and workers as higher profits and wages, sustained by market power and trade unions. In developing economies, productivity gains in commodity production are competed away into lower prices, because commodity markets are closer to perfect competition and labour is abundant. The benefits of productivity growth in commodities therefore flow to the importing country as lower prices.

3. Substitution. Synthetic substitutes progressively displace natural commodities — synthetic rubber for natural rubber, synthetic fibres for cotton and jute, fibre optics for copper wire. Each substitution reduces demand and price.

4. Price volatility. Commodity supply is price-inelastic in the short run (crops take a season; mines take years) and demand is price-inelastic too. Small shifts in either curve produce large price swings, making export revenue unpredictable and investment planning difficult.

The empirical record is mixed, and saying so is a strength, not a hedge. The most careful long-run study, using price series stretching back centuries, finds support that is real but partial: across 25 relative commodity prices over 1650–2005, the majority of estimated trends are negative, with 41 of 80 slope estimates negative and statistically significant (Harvey, Kellard, Madsen and Wohar, "The Prebisch–Singer Hypothesis: Four Centuries of Evidence", Review of Economics and Statistics, 2010; extended in IMF Working Paper 13/180, 2013). But 21 of 80 were positive and significant. The decline is not universal, and it is punctuated by long commodity booms — the 2003–2014 super-cycle raised the terms of trade of commodity exporters substantially for over a decade.

The policy implication is the contested part. If Prebisch–Singer holds, then specialising according to current comparative advantage in commodities locks a country into a slowly deteriorating position: it captures the static gains from trade while its share of world income shrinks. This became the intellectual case for import-substituting industrialisation across Latin America and parts of Africa from the 1950s. Those programmes largely underperformed the export-oriented strategies pursued in East Asia, which is itself an evaluation point — the diagnosis may be right while the prescription is wrong.

Connect it to the concentration data. In Zambia, traditional exports — overwhelmingly refined copper — accounted for 74.3% of total export earnings in January 2026 (Zambia Statistics Agency, Acting Statistician General's briefing, 25 February 2026). A country in that position has its entire external position determined by one price it does not control. That is the Prebisch–Singer concern stated as a fact about exposure rather than a claim about long-run trends.

#10.3 Comparative advantage does not mean everyone within a country gains

This is the most important limitation and the one most often stated as a vague worry rather than a mechanism. State the mechanism.

The Ricardian model treats a country as a single agent with a single consumption bundle. Real countries contain owners of different factors of production, and trade changes the relative prices of factors, not just of goods.

STOLPER-SAMUELSON RESULT
Trade raises the price of the export good.
That raises demand for the factor used intensively in exports.
The abundant factor's real return rises.
The scarce factor's real return falls.
Some groups lose in absolute terms, not just relative terms.

Work it through for a capital-abundant, skill-abundant developed economy. Trade opens; it specialises in capital- and skill-intensive goods and imports labour-intensive manufactures. Demand for capital and skilled labour rises, so returns to capital and skilled wages rise. Demand for domestic unskilled labour falls, so unskilled real wages fall. The country gains in aggregate. Unskilled workers lose absolutely. The gains are sufficient to compensate the losers — that is the compensation principle — but compensation does not happen automatically, and in practice frequently does not happen at all.

Add factor immobility from section 9.4 and the picture sharpens. In the short run the relevant model is the specific-factors (Ricardo–Viner) model: factors are stuck in their current industry. Everyone in the contracting industry loses, whatever factor they own; everyone in the expanding industry gains. Over the long run, as factors reallocate, the Stolper–Samuelson pattern reasserts itself. The 2.4 million US jobs figure in section 9.4 is a measurement of exactly this adjustment cost.

Three consequences to deploy in evaluation.

• Aggregate gains are compatible with rising inequality. Trade is a plausible contributor to the widening skill premium in developed economies, though technological change is the larger contributor in most estimates.
• The political economy is asymmetric. Gains are spread thinly across all consumers as slightly lower prices; losses are concentrated on identifiable workers in identifiable towns. The losers organise; the winners do not notice. Protectionist policy is therefore politically favoured even when it is economically inefficient.
• Compensation policy — active labour market policy, retraining, regional development funding — is the theoretically correct response, and its adequacy is an empirical question you can take a position on.

#10.4 The infant industry argument

The strongest theoretical case against following current comparative advantage.

INFANT INDUSTRY ARGUMENT
A new industry has high initial unit costs.
Costs fall with cumulative output: scale and learning-by-doing.
Free trade kills the industry before costs fall.
Temporary protection allows it to reach competitive scale.
The country then acquires a comparative advantage it lacked.

This is a dynamic-efficiency argument against static efficiency. It is analytically respectable because it identifies a genuine market failure: a capital market failure. If future profitability is certain, private finance should fund the loss-making start-up period without protection. Protection is justified only where capital markets will not lend against future learning gains — a real problem in economies with underdeveloped financial systems.

The four standard objections, each an evaluation line.

1. Government failure in selection. Identifying which infants will grow requires information governments rarely have. Backing the wrong sector wastes resources permanently.
2. Protection becomes permanent. Protected industries acquire a lobbying interest in remaining protected. The "temporary" tariff outlives the infancy.
3. Removal of competitive pressure. The mechanism that is supposed to drive cost reduction — competition — is exactly what protection removes. X-inefficiency may rise instead of falling.
4. Consumer cost. Domestic consumers pay higher prices for the protection period, a regressive transfer if the good is a necessity.

The empirical record is genuinely mixed, which is why it evaluates well. South Korea's heavy and chemical industries drive of the 1970s and the subsequent global position of Korean shipbuilding, steel and electronics is the usual supporting case. The failure of many Latin American import-substitution programmes is the usual counter-case. The distinguishing variable most often identified is whether protection was conditioned on export performance — a discipline that preserves competitive pressure — rather than granted unconditionally.

#10.5 Static versus dynamic comparative advantage

The single highest-yield evaluation concept on this topic.

Static comparative advantage is what the current productivity table says. Dynamic comparative advantage is the advantage a country can acquire through deliberate investment in human capital, infrastructure, research and institutions.

The critique of the Ricardian prescription is this: following static comparative advantage is optimal in the current period and may be badly sub-optimal over decades. A country whose static advantage lies in low-skill assembly, and which specialises accordingly, accumulates low-skill capabilities. A country that invests against its static advantage may acquire a higher-value one.

The historical record shows advantage moving constantly. Vietnam had no meaningful electronics sector in 1990. In the first ten and a half months of 2025 it exported approximately US$143 billion of electronics — including US$92 billion of computers and components and US$50.8 billion of phones and parts — out of US$410.3 billion of total exports (Vietnamese Department of Customs, Ministry of Finance, reported November 2025). That advantage was built by policy and foreign direct investment, not inherited from endowments.

DYNAMIC ADVANTAGE RULE
Static advantage is a snapshot.
Dynamic advantage is a choice.
Specialising today shapes what you can do tomorrow.
Lock-in is the risk of obedience to the static model.
Judgement depends on the time horizon.

The phrase "depends on the time horizon" is worth its weight in marks. Over one year, follow static comparative advantage. Over thirty, ask what capabilities the specialisation builds.

#10.6 Additional limitations, stated compactly

Trade deficits and macroeconomic imbalance. The Ricardian model assumes trade balances — exports pay for imports. It has nothing to say about persistent deficits, exchange rates or capital flows. A country can follow its comparative advantage precisely and still run an unsustainable current account deficit.

Environmental externalities. Specialisation increases transport-related emissions, and a comparative advantage derived from lax environmental regulation is a comparative advantage in externalising costs. This is the "pollution haven" concern, whose empirical support is weaker than commonly assumed but which is analytically sound where regulation genuinely differs.

Strategic and food security. Comparative advantage says a country should import food if food is expensive to grow domestically. It ignores the option value of domestic capacity during supply disruption. The 2020–2022 disruptions to semiconductor, pharmaceutical and grain supply chains gave this argument unusual salience. The economically precise version is that resilience is a public good, under-supplied by markets that price only expected cost, not variance.

Distributional effects between countries. If the terms of trade sit persistently close to the developing country's opportunity cost — because of buyer market power in global value chains — that country receives a small share of a real gain. It is better off than in autarky, and it is capturing far less than half the surplus. Both statements are true simultaneously.

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#11. Modern reality: what the model still explains, and what it does not

#11.1 The anomaly: most trade is between similar countries in similar goods

Ricardo predicts trade between different countries in different goods. Much of world trade is between similar countries in similar goods. Germany exports cars and imports cars. France exports cheese to Italy and imports cheese from Italy. This is intra-industry trade, and by volume it dominates trade among developed economies.

Ricardo cannot explain it. Two countries with near-identical relative costs have, by section 4.6, no gains from trade. Yet the trade happens and is enormous.

Three mechanisms resolve it.

Product differentiation and consumer preference for variety. Cars are not one good. A German consumer wanting a Fiat cannot be satisfied by a Volkswagen. Trade in differentiated varieties raises welfare through variety itself, independent of any cost difference. This is the demand-side explanation.

Economies of scale. If average cost falls with output, it is efficient for each variety to be produced in one location at large scale and shipped everywhere, rather than for every country to produce every variety at small scale. Which country makes which variety may be arbitrary — determined by history and accident, not by cost fundamentals. This is Paul Krugman's "new trade theory" (from 1979), and it explains why trade patterns can be path-dependent rather than endowment-determined.

Vertical differentiation within a category. "Cars" spans budget hatchbacks and luxury saloons. A country may have a comparative advantage in the high-quality segment and import the low-quality segment, so what appears as intra-industry trade is inter-industry trade at finer resolution.

How to use this in an exam. Intra-industry trade does not refute comparative advantage. It shows the theory is incomplete — it explains inter-industry trade (why Bangladesh sells garments to Germany) and not intra-industry trade (why Germany sells cars to France). Stating that boundary precisely is a Level 4 move; saying "the theory is unrealistic" is a Level 2 move.

#11.2 Global value chains: comparative advantage in tasks, not products

The deepest modern amendment. Ricardo assumed a good is made in one country. Most manufactured goods are now made in many, with each country performing the stage at which it has a comparative advantage. Comparative advantage now operates over tasks, not finished products.

Global value chains account for close to half of global trade flows (World Bank, World Development Report 2020: Trading for Development in the Age of Global Value Chains).

The measurement consequence is severe. Conventional trade statistics attribute the full value of an exported good to the last country in the chain. Consider the iPhone 7: total factory cost approximately US$237.45, of which value added captured in China — the assembling country and therefore the recorded exporter — was approximately US$8.46, about 3.6% (analysis based on IHS Markit teardown data, reported by Xing and Detert via The Conversation, 2018). The remaining 96.4% accrued to component suppliers in Japan, South Korea, Taiwan and the United States, and to Apple itself. Yet every iPhone shipped from China is recorded as a Chinese export at full value, inflating the measured bilateral deficit of importing countries with China.

Three implications worth carrying into evaluation.

• Bilateral trade balances are misleading. They measure gross flows, not value added. Trade-in-value-added statistics tell a materially different story about who gains.
• Development strategy changes. A country need not build an entire industry to enter world markets; it can enter at one task. This lowers the entry barrier dramatically — which is why GVC participation has spread industrialisation faster than the twentieth-century model of building complete national industries.
• But the value captured at the entry task may be very small, and moving up the chain — from assembly to component design to branding — is difficult. This is the modern, empirically grounded version of the dependency critique, and it is far stronger than the older version because it is measurable.

#11.3 Why the model still matters

Given all of the above, why teach a 1817 model at all?

It predicts the broad pattern of specialisation correctly. Labour-abundant economies do export labour-intensive goods. Resource-rich economies do export resources. Skill-abundant economies do export skill-intensive goods and services. The UK's pattern is textbook: in the three months to May 2026 it ran a goods deficit of £60.9 billion and a services surplus of £51.8 billion (ONS, UK trade, May 2026), exactly what a high-skill, high-wage, capital-abundant economy with a large financial and professional services sector should look like.

It identifies the source of the gain correctly. Whatever the complications, trade raises output by allowing production to be located where opportunity cost is lowest. Every subsequent theory — Heckscher–Ohlin, new trade theory, GVC analysis — is an addition to that insight, not a replacement for it.

It is the correct answer to the most common policy error. The belief that a country must be "competitive" — meaning absolutely more productive — to benefit from trade is false, and its falsity is a direct implication of section 4.4. Every generation rediscovers this error, and comparative advantage is the standing refutation.

It disciplines the debate. By locating the gains precisely, it also locates the objections precisely. If you accept the model, you cannot argue trade destroys aggregate output; you must argue about distribution, adjustment costs, terms of trade, or dynamic effects. That is a more productive argument.

#The scale of the phenomenon justifies the attention: world trade in goods and services was equivalent to 56.6% of world GDP in 2024 (World Bank, World Development Indicators, series NE.TRD.GNFS.ZS), and world merchandise trade volume grew 2.4% in 2025, with growth forecast to slow to 0.5% in 2026 (WTO, Global Trade Outlook and Statistics update, 7 October 2025).

#12. Exam application: converting theory into WEC14 marks

#12.1 Where this topic appears

Specification section 4.3.2.1 requires: the theory of comparative advantage, numerical and diagrammatic; the distinction between absolute and comparative advantage; assumptions and limitations of the theory (Pearson Edexcel International Advanced Level Economics specification, Unit 4).

Unit 4 (WEC14) is a two-hour, 80-mark paper. Assessment objective weightings for the unit are AO1 knowledge 20%, AO2 application 20%, AO3 analysis 30%, AO4 evaluation 30%. Evaluation and analysis together carry 60% of the marks. Knowledge alone carries a fifth.

This topic appears in three places.

• Section A — multiple choice with quantitative skills. Opportunity cost calculations, identification of comparative advantage from a table, terms of trade range.
• Section B — data response. Short calculations (2 marks), definitions with application (4 marks), analysis with evaluation (8 marks), and an extended 14-mark part.
• Section C — 20-mark essay. Marked as 12 marks for knowledge, application and analysis (four levels) plus 8 marks for evaluation (three levels).

#12.2 The country gate

This is the highest-value single piece of exam information in this chapter.

Every WEC14 Section C essay carries a note of the following form in the mark scheme:

N.B. Award maximum of Level 3 (9 marks) if a candidate does not refer to a developed country in their answer.

The variants observed across published mark schemes are "a developed country", "a developing country", and "a country of their choice", depending on the question wording. Examples from the Summer 2022 series alone: Question 8 required "a country of their choice", Question 9 required "a developed country", Question 10 required "a developing country" (Pearson Edexcel WEC14/01 mark scheme, Summer 2022). This note appears in 44 of the 48 Section C questions across 16 series.

The Level 4 descriptor for the 12-mark component reads: "Ability to link knowledge and understanding in context using appropriate examples which are fully integrated to address the broad elements of the question." The gate is the operational enforcement of "fully integrated".

THE COUNTRY GATE
Name a country. Name it in the first paragraph.
Match the type the question asks for.
Integrate it — do not bolt it on at the end.
Use at least one dated, sourced figure.
Without it you are capped at 9 of 12 marks.

What "integrated" means in practice. A named country mentioned once in the conclusion is a bolt-on. An integrated answer uses the country to drive the analysis: the country's factor endowment explains its comparative advantage, its export data quantifies the specialisation, its institutional context supplies the evaluation.

Compare two openings.

Not integrated: "Comparative advantage means a country should produce the good with the lower opportunity cost. This benefits countries such as Bangladesh."

Integrated: "Bangladesh, with a large low-skill labour force relative to its capital stock, has a comparative advantage in readymade garments: the opportunity cost of producing garments, measured in forgone alternative output, is lower there than in the EU. Specialisation on this basis is visible in the data — RMG accounted for US$39.35bn of Bangladesh's US$48.28bn of total exports in FY2024-25, or 81.49% (BGMEA, from Export Promotion Bureau data)."

The second contains the same theory and additionally satisfies AO2 and clears the gate.

#12.3 Chain construction: 4+ stages ending in a named outcome with a direction

Analysis marks are awarded for chains of reasoning. A chain has stages, each following from the last, ending in a specific outcome with a stated direction of change.

CHAIN RULES
Each stage must follow from the one before.
Minimum four stages. Five is safer.
End with a named variable and a direction.
"Living standards improve" is too vague.
"GNI per capita rises" names a variable and a direction.

Chain 1 — Benefit of specialisation for a developing country (Bangladesh).

1. Bangladesh's factor endowment is labour-abundant relative to capital, so the wage is low relative to the return on capital and the opportunity cost of producing labour-intensive readymade garments is lower than in capital-abundant economies.
2. Specialising in RMG allows Bangladesh to produce garments at lower real cost and exchange them for capital goods it would produce expensively — RMG was 81.49% of exports in FY2024-25 (BGMEA/EPB).
3. Rising export revenue increases the X component of aggregate demand (AD = C + I + G + X − M), shifting AD rightward.
4. Higher AD raises real GDP through the multiplier; because RMG is labour-intensive, employment rises more than proportionately, particularly formal female employment.
5. Higher household incomes raise consumption and income tax receipts, funding public health and education spending, so GNI per capita and the HDI rise.

Chain 2 — Cost of over-specialisation for a developing country (Zambia).

1. Zambia's comparative advantage lies in copper extraction, and traditional exports — overwhelmingly copper — were 74.3% of total export earnings in January 2026 (Zambia Statistics Agency, 25 February 2026).
2. Export revenue is therefore determined by a single world price Zambia does not set, and copper supply is price-inelastic in the short run because mine capacity cannot be adjusted quickly.
3. A fall in the world copper price cuts export revenue almost one-for-one, deteriorating the terms of trade index (export price index falls relative to import price index).
4. Lower foreign exchange earnings worsen the current account and cause the kwacha to depreciate.
5. Depreciation raises the domestic-currency price of imported fuel, machinery and food, generating cost-push inflation, so real household incomes fall and the poverty headcount rises.

Chain 3 — Cost for a developed country (structural adjustment).

1. As productivity rises in lower-cost economies, comparative advantage in labour-intensive manufacturing shifts away from developed economies.
2. Domestic firms in the affected sector lose market share to imports, contract output and shed labour.
3. Released workers are occupationally immobile (skills are industry-specific) and geographically immobile (housing costs impede relocation), so they are not absorbed by expanding sectors.
4. Structural unemployment rises and is concentrated in particular regions — US import competition from China is estimated to have reduced US employment by 2.4 million between 1999 and 2011 (Autor, Dorn and Hanson, 2016).
5. Prolonged unemployment causes skill atrophy and hysteresis, so regional GDP per head falls relative to the national average and government welfare spending rises.

Chain 4 — Terms of trade transmission.

1. Specialisation in primary commodities means export prices are set in competitive world markets while imported manufactures are supplied by firms with pricing power.
2. Productivity gains in commodity production are competed away into lower prices; productivity gains in manufacturing are retained as higher profits and wages.
3. The export price index therefore rises more slowly than the import price index, so the terms of trade index falls — for example, from 100 to 80 over a decade.
4. A ToT index of 80 means a given export volume buys 20% fewer imports, requiring 25% more export volume to import the same quantity.
5. To maintain import capacity the country must expand extraction, increasing resource depletion and environmental cost, so the volume of exports rises while real income per unit exported falls.

#12.4 Evaluation that reaches Level 3 (7–8 of 8 marks)

The Level 3 evaluation descriptor requires evaluation that "recognises different viewpoints and/or is critical of the evidence, leading to an informed judgement", with "appropriate reference to evidence/context", "supported by a logical chain of reasoning".

Three components: a viewpoint, evidence, and a judgement. Listing drawbacks satisfies none of them.

EVALUATION STRUCTURE
State the counter-position.
Give the mechanism, not the label.
Anchor it in the named country.
State what the answer depends on.
Give a judgement with a reason.

Six evaluation lines for this topic, each with its dependency.

Model judgement paragraph (note the structure: position, condition, magnitude, reason):

On balance, specialisation according to comparative advantage has been net beneficial for Bangladesh, because the scale of the gain is large and measurable — RMG exports of US$39.35bn in FY2024-25 support employment on a scale no alternative sector currently could — while the principal cost, exposure to a single sector, has so far been a risk rather than a realised loss. However, this judgement is conditional on the time horizon. Over the next decade the binding constraint is not whether to specialise but whether Bangladesh can move up the value chain from assembly, where captured value added is thin, toward design and branding. If it cannot, the static gains will persist while the country's relative position erodes — which is precisely the Prebisch–Singer concern applied to manufactures rather than commodities.

#12.5 Common errors and their mark cost

#12.6 Timing and structure for a 20-mark Section C essay

Twenty marks in roughly 24 minutes. Allocate: 3 minutes planning, 15 minutes writing analysis, 6 minutes writing evaluation.

SECTION C SKELETON
Paragraph 1: define, name the country, state the comparative advantage.
Paragraph 2: chain one, five stages, ending in a named variable.
Paragraph 3: chain two, five stages, ending in a named variable.
Paragraph 4: evaluation line one with its dependency.
Paragraph 5: evaluation line two with its dependency.
Paragraph 6: judgement with a condition and a reason.

Two chains and two evaluation lines, both developed, beat four of each stated briefly. Depth is what the level descriptors reward.

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#13. Self-test drills

Each drill has a binary pass criterion. You either meet it or you do not. Do not award yourself partial credit — the point is to find the gaps.

#Drill 1 — Opportunity cost from output data

Output per worker-year:

Calculate all four opportunity costs and identify each country's comparative advantage.

Pass criterion: all four values correct, both comparative advantages correctly assigned, and the reciprocal check stated.

<details> <summary>Model answer</summary>

Ardenia: OC of 1 tonne of steel = 6/12 = 0.5 hl of wine. OC of 1 hl of wine = 12/6 = 2 tonnes of steel. Check: 0.5 × 2 = 1 ✓

Borovia: OC of 1 tonne of steel = 3/3 = 1 hl of wine. OC of 1 hl of wine = 3/3 = 1 tonne of steel. Check: 1 × 1 = 1 ✓

Ardenia's steel is cheaper (0.5 < 1), so Ardenia has the comparative advantage in steel. Borovia's wine is cheaper (1 < 2), so Borovia has the comparative advantage in wine. Note Ardenia has the absolute advantage in both (12 > 3 and 6 > 3), which is irrelevant to the allocation.

</details>

#Drill 2 — Opportunity cost from input data

Worker-hours required per unit:

Identify each country's absolute and comparative advantage.

Pass criterion: you used h_A/h_B and not b/a, and reached Ravenna–cars, Solano–coffee.

<details> <summary>Model answer</summary>

Absolute advantage: Ravenna needs fewer hours for both (40 < 120 cars; 20 < 30 coffee). Ravenna has the absolute advantage in both.

Opportunity costs, using OC of A = h_A/h_B: Ravenna: 1 car = 40/20 = 2 tonnes of coffee; 1 tonne of coffee = 20/40 = 0.5 cars. Solano: 1 car = 120/30 = 4 tonnes of coffee; 1 tonne of coffee = 30/120 = 0.25 cars.

Ravenna has the comparative advantage in cars (2 < 4). Solano has the comparative advantage in coffee (0.25 < 0.5).

</details>

#Drill 3 — Feasible terms of trade range

Using Drill 1's data, state the feasible range for the terms of trade, in hectolitres of wine per tonne of steel, and explain both bounds.

Pass criterion: range stated as 0.5 < ToT < 1 with units, and both bounds justified by a participation constraint.

<details> <summary>Model answer</summary>

Ardenia exports steel. It can obtain 0.5 hl of wine per tonne of steel at home, so it will only export if the world rate exceeds 0.5.

Borovia imports steel. It can produce a tonne of steel at home for 1 hl of wine, so it will only import if the world rate is below 1.

Feasible range: 0.5 < terms of trade < 1 hectolitre of wine per tonne of steel. At exactly 0.5 Ardenia gains nothing; at exactly 1 Borovia gains nothing.

</details>

#Drill 4 — Full gains calculation

Ardenia has 200 worker-years, Borovia has 600. In autarky each splits labour equally between the two goods. Both then specialise completely. Ardenia exports 1,000 tonnes of steel at terms of trade of 0.75 hl of wine per tonne. Show that both countries gain in both goods.

Pass criterion: your post-trade world totals equal the specialised production totals exactly, and both countries show a positive gain in both goods.

<details> <summary>Model answer</summary>

Autarky: Ardenia (100 workers each way) = 1,200 steel, 600 wine. Borovia (300 each way) = 900 steel, 900 wine. World = 2,100 steel, 1,500 wine.

Specialisation: Ardenia = 200 × 12 = 2,400 steel. Borovia = 600 × 3 = 1,800 wine. World = 2,400 steel, 1,800 wine — a world gain of +300 steel and +300 wine.

Trade: Ardenia exports 1,000 steel, imports 1,000 × 0.75 = 750 wine.

Consistency check: 1,400 + 1,000 = 2,400 steel ✓; 750 + 1,050 = 1,800 wine ✓. The terms of trade 0.75 lies inside the range 0.5 to 1 ✓.

</details>

#Drill 5 — The no-gains case

Should these countries trade? Justify.

Pass criterion: you computed the ratios, found them identical, and concluded no gains — without being distracted by Nordia's absolute advantage.

<details> <summary>Model answer</summary>

Nordia: OC of 1 tonne of fish = 24/8 = 3 m³ of timber. Vestia: OC of 1 tonne of fish = 9/3 = 3 m³ of timber.

The ratios are identical, so neither country has a comparative advantage in either good. The feasible terms of trade range is 3 < ToT < 3, which is empty. There are no gains from trade, despite Nordia having a large absolute advantage in both goods (8 > 3 and 24 > 9). Gains require different relative costs, not superior absolute productivity.

</details>

#Drill 6 — Terms of trade index

A country's export price index rises from 100 to 126 and its import price index rises from 100 to 105 over the same period. Calculate the terms of trade index and interpret it.

Pass criterion: answer of 120, described as an improvement, with a statement of what it means for import purchasing power.

<details> <summary>Model answer</summary>

ToT index = (126 / 105) × 100 = 120.

This is a 20% improvement. A given volume of exports now buys 20% more imports than in the base year. Real national income rises through the terms of trade effect even if output volumes are unchanged.

Evaluation: this is not unambiguously good. If demand for the country's exports is price-elastic, the 26% rise in export prices could reduce export volumes by more than 26% in proportionate terms, cutting total export revenue and worsening the current account.

</details>

#Drill 7 — Increasing opportunity costs

A country's PPF is concave. The opportunity cost of the first 100 units of cloth is 0.5 wheat per unit; the second 100 costs 1.0; the third 100 costs 1.5; the fourth 100 costs 2.0. The world price is 1.2 wheat per unit of cloth. How much cloth does the country produce, and what does this show?

Pass criterion: answer of 200 units, with the stopping rule stated as "domestic opportunity cost equals the world price".

<details> <summary>Model answer</summary>

Produce cloth while the domestic opportunity cost is below the world price of 1.2. The first block costs 0.5 (< 1.2, produce). The second costs 1.0 (< 1.2, produce). The third costs 1.5 (> 1.2, stop and import instead).

The country produces 200 units of cloth and imports the rest. This demonstrates that with increasing opportunity costs, specialisation is partial rather than complete, because the rising domestic cost eventually exceeds the world price. Complete specialisation is an artefact of the constant-cost assumption, which is why no real economy specialises fully.

</details>

#Drill 8 — Assumption diagnosis

For each of these three assumptions, name the specific conclusion it supports and what replaces that conclusion when it is relaxed: (a) constant opportunity costs; (b) perfect factor mobility within countries; (c) fixed technology.

Pass criterion: each answer names a specific conclusion, not "the model breaks down".

<details> <summary>Model answer</summary>

(a) Constant opportunity costs support the conclusion of complete specialisation. Relaxed, specialisation becomes partial, halting where domestic opportunity cost equals the world price; gains shrink but persist.

(b) Perfect internal factor mobility supports the conclusion that reallocation is costless and causes no unemployment. Relaxed, adjustment produces structural unemployment concentrated in specific industries and regions, so the net aggregate gain conceals a concentrated gross loss.

(c) Fixed technology supports the conclusion that permanent specialisation in the current comparative advantage good is optimal. Relaxed, comparative advantage is mobile and partly created by investment, so specialisation risks lock-in to a low-value position — the static versus dynamic distinction.

</details>

#Drill 9 — Full essay plan

"Evaluate the extent to which specialisation based on comparative advantage benefits a developing country of your choice. (20 marks)"

Produce a plan in five minutes.

Pass criterion: a named developing country appears in the first line; two chains of at least four stages each; two evaluation lines each with a stated dependency; a judgement containing a condition.

<details> <summary>Model answer (plan)</summary>

Country: Bangladesh. Comparative advantage in readymade garments; RMG = 81.49% of US$48.28bn exports, FY2024-25 (BGMEA/EPB).

Define: comparative advantage = lower opportunity cost. Distinguish from absolute advantage in one sentence.

Chain A (benefit): labour abundance → low relative wage → low OC of labour-intensive RMG → specialisation raises export revenue → X rises, AD shifts right → multiplier raises real GDP and labour-intensive employment, especially female → GNI per capita and HDI rise.

Chain B (cost): 81.49% export concentration → revenue depends on a single sector's world demand → a demand shock in the EU/US cuts export revenue → current account worsens and the taka depreciates → imported input and food prices rise → real incomes fall and inflation rises.

Evaluation 1: distribution of gains. Buyer power in the garment global value chain pushes the terms of trade toward Bangladesh's own opportunity cost, so value added captured is thin — compare the iPhone case, where the assembling country captured 3.6% of factory cost. Depends on: number of competing supplier countries and the degree of buyer concentration.

Evaluation 2: static versus dynamic. Specialising in assembly builds assembly capability, not design capability. Depends on: time horizon, and state capacity to fund the skills and infrastructure needed to move up the chain.

Judgement: net beneficial to date, because the employment and income effects are large and measurable and the principal cost has been risk rather than realised loss; but the judgement is conditional on Bangladesh converting static gains into dynamic capability within the next decade, failing which the gains plateau while relative position erodes.

</details>

#Drill 10 — Error hunt

Find the three errors: "Because Northland can produce more of both machines and rice per worker, it has a comparative advantage in both. Southland should therefore specialise in neither. Trade will shift both countries' production possibility frontiers outward."

Pass criterion: all three identified and corrected.

<details> <summary>Model answer</summary>

1. "Comparative advantage in both" — impossible. Producing more of both is absolute advantage. Comparative advantages are reciprocals and must be split between the two countries.
2. "Southland should specialise in neither" — wrong. Southland has the comparative advantage in the good where it is proportionately least disadvantaged, and should specialise in that.
3. "Shift production possibility frontiers outward" — wrong in the static model. Trade shifts the consumption possibility frontier outward. The PPF is unchanged unless there are dynamic effects such as imported capital goods or technology transfer.

</details>

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#14. Reference card

COMPARATIVE ADVANTAGE — 100 WORDS

Absolute advantage: more output per resource. Levels.
Comparative advantage: lower opportunity cost. Ratios.

Output data: OC of A = b/a. Divide the other good by this one.
Input data:  OC of A = hA/hB. Divide this good by the other.
Check: the two opportunity costs multiply to 1.

Comparative advantage is always split between countries.
Identical ratios mean no gains, whatever the absolute advantage.

Terms of trade lie strictly between the two opportunity costs.
Specialisation creates the gain. Terms of trade divide it.

Trade shifts the consumption frontier outward, not the PPF.

Exam: name a country in line one.

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#15. Sources

• Autor, D., Dorn, D. and Hanson, G. (2016) "The China Shock: Learning from Labor-Market Adjustment to Large Changes in Trade", Annual Review of Economics.
• BGMEA, Export Performance (compiled from Export Promotion Bureau data), FY2024-25.
• Destatis (Federal Statistical Office of Germany), Foreign trade — trading goods, 2025 data.
• Harvey, D., Kellard, N., Madsen, J. and Wohar, M. (2010) "The Prebisch–Singer Hypothesis: Four Centuries of Evidence", Review of Economics and Statistics 92(2); extended in IMF Working Paper 13/180 (2013).
• Leontief, W. (1953) "Domestic Production and Foreign Trade: The American Capital Position Re-Examined", Proceedings of the American Philosophical Society.
• Office for National Statistics, UK trade, May 2026 bulletin.
• Pearson Edexcel, International Advanced Level Economics specification; WEC14/01 mark schemes, Summer 2022 and January 2024.
• Ricardo, D. (1817) On the Principles of Political Economy and Taxation, ch. 7.
• Vietnamese Department of Customs, Ministry of Finance, export data to mid-November 2025.
• World Bank, World Development Report 2020: Trading for Development in the Age of Global Value Chains; World Development Indicators series NE.TRD.GNFS.ZS (2024).
• WTO, Global Trade Outlook and Statistics update, 7 October 2025.
• Xing, Y. and Detert, N., iPhone value-added analysis based on IHS Markit teardown data, reported in The Conversation, 2018.
• Zambia Statistics Agency, Acting Statistician General's media briefing, 25 February 2026.

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