📖 国际贸易周测（10 题）

Economics

I. Lesson Focus

This lesson consolidates core international trade concepts including comparative advantage, gains from trade, the welfare effects of tariffs, quotas, export subsidies, and the relationships among trade balances, capital flows, and exchange rates. Candidates must be able to calculate opportunity costs, quantify changes in consumer surplus, producer surplus, government revenue, and deadweight loss, and apply the Marshall-Lerner condition and J-curve effect.

II. The Problem

Should a country aim for complete self-sufficiency, or does opening to trade create net benefits? Which domestic industries gain and which lose from trade? How do tariffs, import quotas, export subsidies, and voluntary export restraints quantitatively affect domestic prices, output, consumer surplus, producer surplus, government revenue, and total welfare? Is a trade deficit always harmful? How do exchange-rate changes influence the trade balance? This lesson uses 10 high-quality practice questions to test and strengthen understanding of these mechanisms, enabling candidates to answer exam questions rapidly and accurately.

III. Comparative Advantage and Gains from Trade

Comparative advantage, developed by David Ricardo, is the foundation of international trade theory. The key criterion is opportunity cost, not absolute productivity.

• Absolute advantage exists when a country can produce a good using fewer resources than another country.
• Comparative advantage exists when a country can produce a good at a lower opportunity cost than another country.

When countries specialize in the goods in which they have a comparative advantage and trade, both obtain gains from trade, shown as an outward shift in the consumption possibility frontier.

Formula:
Opportunity cost = Quantity of good forgone / Quantity of good gained
Gains from trade = Post-trade consumption − Autarky consumption

IV. Trade Policy Tools and Welfare Effects

Governments frequently use tariffs, import quotas, export subsidies, and voluntary export restraints (VERs).

1. Effects of a Tariff (Small-Country Case)

• Domestic price = World price + Tariff
• Consumer surplus falls
• Producer surplus rises
• Government collects tariff revenue
• Net welfare loss = Production deadweight loss + Consumption deadweight loss

2. Effects of an Import Quota

• Quota raises domestic price to the quota-constrained level
• Quota rents are captured by importers or foreign exporters
• Welfare loss is typically larger than that of an equivalent tariff (no government revenue)

3. Export Subsidies

• Raise the domestic price in the exporting country
• Consumers lose, producers gain
• Government pays the subsidy, creating a net welfare loss

V. Trade Balances, Balance of Payments, and Exchange Rates

• The current account mainly reflects the trade balance.
• The sum of the current account and the capital and financial account is theoretically zero (ignoring statistical discrepancy).
• A trade deficit is not inherently harmful; it may reflect capital inflows that finance productive domestic investment.
• Exchange-rate effects:
• Domestic currency appreciation → Exports fall, imports rise → Trade balance worsens
• Domestic currency depreciation → Exports rise, imports fall → Trade balance improves (subject to possible short-term J-curve reversal)

Marshall-Lerner Condition: Depreciation improves the trade balance if the sum of the absolute values of export demand elasticity and import demand elasticity exceeds 1.

Worked Cases

Case 1: Comparative Advantage and Gains from Trade

The United States can produce 20 units of wheat or 10 units of cloth per labor unit; China can produce 6 units of wheat or 8 units of cloth per labor unit.
Identify each country’s comparative advantage. If the terms of trade are 1 unit of cloth for 1.5 units of wheat, how does consumption per labor unit improve for each country?

Solution:
U.S. opportunity cost of wheat = 10/20 = 0.5 cloth; China’s = 8/6 ≈ 1.33 cloth → United States has comparative advantage in wheat.
China’s opportunity cost of cloth = 6/8 = 0.75 wheat; U.S. = 20/10 = 2 wheat → China has comparative advantage in cloth.

In autarky, assume each country splits labor equally. The United States consumes 10 wheat + 5 cloth. After specialization and trade, the United States produces 20 wheat, exports 7.5 wheat for 5 cloth, and consumes 12.5 wheat + 5 cloth—an improvement. China produces 8 cloth, exports 5 cloth for 7.5 wheat, and consumes 7.5 wheat + 3 cloth—also an improvement.

Case 2: Welfare Effects of a Tariff (Small Country)

A small country faces a world auto price of $20,000. Domestic demand is 800 units and supply is 200 units. A $4,000 specific tariff is imposed. Calculate the new domestic price, change in producer surplus, change in consumer surplus, government revenue, and net welfare loss (assume linear supply and demand).

Solution:
New price = $24,000.
Assume the tariff reduces quantity demanded by 150 units and increases quantity supplied by 100 units.
Consumer surplus loss = ($4,000 × (800 + 650)/2) = $2,900,000
Producer surplus gain = ($4,000 × (200 + 300)/2) = $1,000,000
Government tariff revenue = $4,000 × (650 − 300) = $1,400,000
Net welfare loss (deadweight loss) = $2,900,000 − $1,000,000 − $1,400,000 = $500,000

Case 3: Exchange Rates and Trade Balance

A country’s export demand elasticity is 0.8 and import demand elasticity is 1.4. The domestic currency depreciates 10 %. Will the trade balance improve? The initial trade deficit equals 4 % of GDP. Discuss short-run versus long-run effects.

Solution:
0.8 + 1.4 = 2.2 > 1, satisfying the Marshall-Lerner condition; the trade balance improves in the long run.
In the short run the J-curve effect may cause an initial worsening because quantities adjust more slowly than prices. After quantities adjust, exports rise approximately 8 % and imports fall approximately 14 %, narrowing or eliminating the trade deficit.

Traps

Key Formulas

• Opportunity cost = Good forgone / Good obtained
• Domestic price with tariff = World price + Tariff
• Net welfare loss (small-country tariff) = Production DWL + Consumption DWL
• Marshall-Lerner Condition: εₓ + εₘ > 1
• Trade balance ≈ National saving − Investment (S − I)
• Real appreciation of domestic currency → Net exports decline
• Current account + Capital and financial account ≈ 0

Practice Questions

Q1. Country A’s opportunity cost of producing one unit of Good X is 2 units of Good Y; Country B’s is 0.8 units of Y. Country A has a comparative advantage in:
A. Good X
B. Good Y
C. Both goods
D. Cannot be determined

Q2. After a small country imposes a tariff on an imported good, the most likely outcome is:
A. Both consumer and producer surplus increase
B. Government revenue rises and national welfare necessarily rises
C. Domestic price rises, creating production and consumption deadweight losses
D. Import volume increases

Q3. Which statement about an import quota is correct?
A. A quota always produces a smaller welfare loss than an equivalent tariff
B. Quota rents are always captured by the government
C. The quota raises domestic price and increases producer surplus
D. The quota has no effect on consumption

Q4. A country’s export demand elasticity is 0.6 and import demand elasticity is 0.5. After a 10 % depreciation of its currency, the trade balance is most likely to:
A. Improve significantly
B. Worsen
C. Remain unchanged
D. Cannot be determined

Q5. Trade causes each country’s consumption possibility frontier to:
A. Shift inward
B. Remain unchanged
C. Shift outward
D. Shift outward only for the exporting country

Q6. The main difference between a large country and a small country imposing a tariff is that the large country:
A. Incurs no deadweight loss
B. May experience a terms-of-trade gain that can produce net national benefit
C. Collects less tariff revenue
D. Leaves consumers unaffected

Q7. A persistent current-account deficit is typically matched by:
A. A capital and financial account surplus
B. A capital and financial account deficit
C. A decrease in official reserves
D. Domestic currency appreciation

Q8. The effect of an export subsidy on the exporting country is to:
A. Lower domestic price and benefit consumers
B. Raise domestic price and create a net welfare loss
C. Require no government expenditure
D. Benefit producers in the importing country

Answers

Takeaways

• Comparative advantage is determined by opportunity cost, not absolute productivity.
• Free trade shifts both countries’ consumption possibility frontiers outward, generating mutual gains.
• A small-country tariff always creates net welfare loss equal to production plus consumption deadweight loss.
• Import quotas generate rents typically captured by importers or foreigners, not the government.
• Marshall-Lerner condition: export demand elasticity plus import demand elasticity > 1 implies currency depreciation improves the trade balance.
• A trade deficit is not inherently bad; its implications depend on the corresponding capital flows and the relationship between domestic saving and investment.