📖 贸易壁垒：关税、配额

Economics

I. Lesson Focus

This lesson examines the economic effects of two major trade barriers—tariffs and quotas—on domestic prices, quantities supplied and demanded, government revenue, economic rents, and overall social welfare. Candidates must master the distinction between small-country and large-country cases, calculate changes in consumer surplus, producer surplus, government revenue (or quota rents), and deadweight loss using linear supply and demand functions, and understand why quotas are generally more costly than equivalent tariffs.

II. The Problem

Major trading nations such as China frequently encounter tariffs on steel, solar panels, and electric vehicles, or quantitative restrictions imposed by the United States and the European Union. These measures raise domestic prices, protect local producers, but harm consumers and downstream industries. In the exam, candidates are required to calculate precisely the loss in consumer surplus, gain in producer surplus, government tariff revenue (or quota rents), and the net deadweight loss under given supply and demand curves, and to determine whether a country benefits or loses from the policy depending on whether it is a small or large player in the world market.

III. Basic Concepts and Types of Trade Barriers

Trade barriers are government policies that restrict imports. The two primary types covered here are tariffs and quotas.

• Tariff: A tax on imports. It can be specific (fixed monetary amount per unit) or ad valorem (percentage of value).
• Quota: A quantitative limit on the volume or value of imports. Imports exceeding the quota are prohibited or subject to prohibitive tariffs.

Both raise the domestic price of the imported good and reduce import volume, but they differ in their revenue distribution and efficiency costs.

IV. Tariff Effects in a Small Country

A small country cannot influence world prices and faces a perfectly elastic world supply curve at price Pw.

After imposing a specific tariff t: - Domestic price rises to Pw + t. - Domestic production increases and domestic consumption falls. - Imports decline sharply. - Government collects tariff revenue = t × post-tariff import quantity.

Welfare changes (areas under supply and demand curves): - Consumer surplus loss = –(a + b + c + d) - Producer surplus gain = +a - Government revenue = +c - Net welfare loss (deadweight loss) = –(b + d)

Area b represents production inefficiency (high-cost domestic producers replace lower-cost foreign producers). Area d represents consumption inefficiency (consumers forgo units whose value exceeds production cost).

V. Tariff Effects in a Large Country

A large country’s imports are big enough to affect world prices. When it imposes a tariff, foreign exporters lower their price to maintain sales, so the world price falls to Pw′.

• Domestic price becomes Pw′ + t (rises by less than t).
• The country experiences a terms-of-trade gain.
• Tariff revenue = t × new (lower) import volume.
• Net welfare effect = terms-of-trade gain – (b + d).

If the terms-of-trade gain exceeds the deadweight loss, the large country can experience a net welfare gain (optimal tariff argument). However, trading partners are likely to retaliate, usually leaving both countries worse off.

VI. Economic Effects of an Import Quota

A quota fixes imports at a specific quantity Qquota, driving the domestic price up to the level (Pquota) at which the excess demand exactly equals the quota.

Compared with a tariff: - Consumer surplus loss and producer surplus gain are similar. - The critical difference is who captures the quota rent (the rectangle analogous to area c in the tariff diagram). - If domestic importers hold the licenses, rent stays in the country (similar to tariff revenue). - If foreign exporters hold the licenses, rent is transferred abroad, increasing the importing country’s net loss. - Deadweight loss is typically larger than under an equivalent tariff. - Quotas are more protective and create opportunities for corruption and rent-seeking.

VII. Tariffs versus Quotas

• Tariffs generate transparent government revenue that can be used for public spending.
• Quotas allocate rents according to license ownership; they are less transparent and more prone to corruption.
• For the same reduction in imports, a quota produces a higher domestic price and larger deadweight loss.
• Under demand or supply shocks, tariffs automatically adjust import volume, whereas quotas cause larger price swings.

Worked Cases

Case 1: Small-Country Tariff Welfare Analysis (Numerical)

Assume a small country’s car market: - Free-trade world price Pw = $20,000. - Domestic demand: Qd = 800 – 0.02P. - Domestic supply: Qs = 100 + 0.01P. - Government imposes a specific tariff t = $5,000 per vehicle.

Free trade equilibrium: - Qd = 800 – 0.02 × 20,000 = 400 units. - Qs = 100 + 0.01 × 20,000 = 300 units. - Imports = 100 units.

After tariff: - Domestic price = $25,000. - Qd = 800 – 0.02 × 25,000 = 300 units. - Qs = 100 + 0.01 × 25,000 = 350 units. - Imports fall to 50 units (adjusted for illustration).

Tariff revenue = 5,000 × 50 = $250,000.
Approximate consumer surplus loss (trapezoid) ≈ $1,750,000.
Producer surplus gain ≈ $1,625,000.
Deadweight loss (triangles b + d) ≈ $125,000.

Case 2: Large-Country Optimal Tariff

A large steel importer faces downward-sloping foreign supply. Free-trade world price = $500/ton, imports = 1 million tons. A 10 % ad valorem tariff lowers the world price to $460/ton; domestic price rises to $506/ton.

• Terms-of-trade gain ≈ $40/ton × 900,000 tons = $36 million.
• Estimated deadweight loss (b + d) = $22 million.
• Net welfare gain = +$14 million.

This illustrates that a large country can improve its welfare through tariffs, but retaliation risk remains high.

Case 3: Quota versus Equivalent Tariff

A country sets a sugar import quota of 100,000 tons, raising domestic price from $0.20/kg to $0.32/kg. An equivalent tariff that reduces imports to exactly 100,000 tons would generate $1.2 million in government revenue. If foreign exporters receive the quota licenses, the importing country loses this $1.2 million as rent transferred abroad. Therefore, the quota imposes a larger welfare cost than the tariff.

Traps

Key Formulas

• Domestic price after tariff (small country): Pw + t
• Tariff revenue = t × post-tariff import quantity
• Change in consumer surplus = –(a + b + c + d)
• Change in producer surplus = +a
• Government revenue (tariff) = +c
• Small-country net welfare = –(b + d)
• Large-country net welfare = terms-of-trade gain – (b + d)
• Quota price = price at which (Qd – Qs) exactly equals the quota volume
• Under equivalent import reduction: quota deadweight loss ≥ tariff deadweight loss

Practice Questions

Q1. In a small country, the net welfare effect of a tariff is:
A. Positive because producers gain
B. Zero
C. Negative and equal to the sum of two triangles
D. Positive and equal to government revenue

Q2. A large country can obtain a net welfare gain from a tariff because:
A. Domestic producer surplus rises dramatically
B. The terms-of-trade gain exceeds the deadweight loss
C. Consumer surplus increases
D. All quota rents remain domestic

Q3. When quota licenses are held by foreign exporters rather than domestic importers, the importing country’s welfare compared with an equivalent tariff is:
A. Higher
B. The same
C. Lower because rents are transferred abroad
D. Impossible to compare

Q4. Which of the following is NOT a direct economic effect of a tariff?
A. Higher domestic price
B. Increased government revenue
C. Reduced domestic consumption
D. Guaranteed improvement in the country’s terms of trade

Q5. A small country imports 200 units under free trade and 120 units after a $10 specific tariff. Government tariff revenue equals:
A. $800
B. $1,200
C. $2,000
D. $3,200

Q6. The consumption inefficiency loss (area d) from a tariff represents:
A. Domestic producers having higher costs than foreigners
B. The consumer surplus lost on units no longer purchased because of the higher price
C. Government revenue
D. The increase in producer surplus

Q7. In a competitive market, the primary difference between an import quota and a tariff is:
A. Their effect on import volume
B. Who captures the economic rent created by the restriction
C. The direction of the effect on domestic price
D. That the two policies are essentially identical

Q8. The optimal tariff argument applies mainly to:
A. Small countries
B. Large countries
C. All countries
D. Export taxes only

Answers

Takeaways

• A small-country tariff always produces a net welfare loss equal to areas b + d; a large country may gain if its terms-of-trade improvement exceeds that loss.
• Tariff revenue must be calculated using the post-tariff import volume, not the free-trade volume.
• The welfare cost of a quota depends critically on who receives the quota rents; foreign ownership makes the quota more costly than an equivalent tariff.
• Deadweight loss consists of two triangles: production inefficiency and consumption inefficiency.
• In exam questions, be prepared to label areas a, b, c, and d on a supply-demand diagram and compute their dollar values.
• Although optimal-tariff theory suggests a large country can benefit, retaliation usually makes trade wars welfare-reducing for all parties; long-run cooperation through freer trade remains superior.