International Trade and Public Policy — AP Microeconomics Study Guide

For: AP Microeconomics candidates sitting AP Microeconomics.

Covers: World price, comparative advantage, gains from trade, imports, exports, tariff and quota trade barriers, changes in consumer and producer surplus, deadweight loss from restrictions, and welfare analysis of trade policies.

You should already know: How to calculate consumer and producer surplus from supply and demand curves; how to solve for equilibrium in a closed domestic market; the definition of comparative advantage and opportunity cost.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Microeconomics style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is International Trade and Public Policy?

International Trade and Public Policy applies core supply-and-demand welfare analysis to study the effects of cross-border trade and government trade restrictions on domestic consumers, producers, and overall social welfare. Within the AP Microeconomics CED, this topic falls in Unit 2 (Supply and Demand) and accounts for roughly 2-3% of total exam score weight, appearing regularly in both multiple-choice (MCQ) and free-response (FRQ) sections.

We almost always use the small open economy assumption for AP Micro problems: the domestic country is too small to influence the global price of the good in question, so world price is treated as an exogenous, fixed value that forms a perfectly elastic horizontal supply curve on the graph. This topic builds directly on basic supply-demand equilibrium and surplus concepts to answer core policy questions: when should a country import or export a good, who gains and who loses from open trade, and what is the efficiency cost of policies like tariffs and quotas that block trade. AP Micro focuses exclusively on partial equilibrium analysis of a single market, aligning with the Unit 2 focus on supply and demand fundamentals.

2. Gains From Trade in a Small Open Economy

Before trade (called autarky), a domestic market settles at its own equilibrium price $P^{\ast }$ and quantity $Q^{\ast }$. When the economy opens to trade, the domestic price converges to the fixed world price $P_w$, because consumers can buy cheaper imports and producers can sell exports for the higher global price.

If $P_w<P^{\ast }$, the domestic price falls to $P_w$. Domestic producers supply less than domestic consumers demand, and the gap is filled by imports. If $P_w>P^{\ast }$, the domestic price rises to $P_w$. Domestic producers supply more than domestic consumers demand, and the gap is exported.

Welfare is measured as total surplus, the sum of consumer surplus (CS) and producer surplus (PS): $$TS=CS+PS$$ Opening to trade always increases total surplus, meaning there are net gains from trade. While one group (consumers for exports, producers for imports) loses surplus, the other group's gains exceed the losses, so the overall pie grows.

Worked Example

The domestic market for cotton t-shirts in Country A has demand $P=30-0.5Q$ and supply $P=2+0.5Q$. The world price of t-shirts is $\$8$. Calculate the change in total surplus from opening to trade and confirm net gains.

1. Find autarky equilibrium by setting demand equal to supply: $30-0.5Q=2+0.5Q⟹Q^{\ast }=28$, $P^{\ast }=2+0.5(28)=\$16$.
2. Compare prices: $P_w=8<16$, so Country A will import t-shirts.
3. Find domestic quantity supplied and demanded at $P_w=8$: $8=2+0.5Q_s⟹Q_s=12$; $8=30-0.5Q_d⟹Q_d=44$.
4. Calculate total surplus before and after trade:
   • Autarky: $CS=0.5(30-16)(28)=196$, $PS=0.5(16-2)(28)=196$, $TS=196+196=392$
   • Free trade: $CS=0.5(30-8)(44)=484$, $PS=0.5(8-2)(12)=36$, $TS=484+36=520$
5. Change in $TS=520-392=+128$, so there is a net gain of $\$128$ from opening trade.

Exam tip: Always label CS, PS, and any surplus changes clearly on your graph for FRQs; AP readers award points for correctly shaded regions even if your numerical calculation is slightly off.

3. Welfare Effects of Import Tariffs

An import tariff is a per-unit tax on goods imported into the country. For a small open economy, the tariff does not change the world price, it only raises the domestic price of the imported good to $P_t=P_w+t$, where $t$ is the per-unit tariff.

Higher domestic prices have four key effects: (1) domestic producers increase output, so their surplus rises, (2) consumers reduce consumption, so their surplus falls, (3) the government collects tariff revenue equal to $t\times \text{imports after tariff}$, (4) there is net deadweight loss (DWL) from two sources: inefficient domestic production of units that could have been bought for less $P_w$ from abroad, and lost consumption of units that consumers valued more than $P_w$ but less than the new tariff-inclusive price. DWL is calculated as the sum of the two triangular efficiency loss areas: $$DWL=\frac{1}{2}t\Delta Q_s+\frac{1}{2}t\Delta Q_d$$ where $\Delta Q_s$ is the increase in domestic output, and $\Delta Q_d$ is the decrease in domestic consumption after the tariff.

Worked Example

Using the t-shirt market from the previous example ($P=30-0.5Q$ demand, $P=2+0.5Q$ supply, $P_w=8$), a $\$3$ per unit import tariff is imposed. Calculate the deadweight loss from the tariff and identify who gains and loses.

1. New domestic price after tariff: $P_t=8+3=\$11$.
2. Calculate new $Q_s$ and $Q_d$ at $P_t=11$: $11=2+0.5Q_s⟹Q_{s,t}=18$; $11=30-0.5Q_d⟹Q_{d,t}=38$. Imports after tariff: $38-18=20$, down from 32 under free trade.
3. Calculate changes in quantity: $\Delta Q_s=18-12=6$, $\Delta Q_d=44-38=6$.
4. Calculate DWL: $DWL=0.5(3)(6)+0.5(3)(6)=9+9=18$.
5. Welfare outcomes: Domestic producers gain $\$45$ of surplus, the government gains $\$60$ of tariff revenue, consumers lose $\$123$ of surplus, and net deadweight loss to the economy is $\$18$.

Exam tip: Never forget that a tariff for a small open economy does NOT change the world price, only the domestic price; large country tariff effects are not tested on AP Micro.

4. Welfare Effects of Import Quotas

An import quota is a legal limit on the quantity of a good that can be imported into a country. A quota is binding if it is set below the level of imports that would occur under free trade; non-binding quotas have no effect on the market.

Binding quotas work almost exactly like tariffs: by restricting supply, they raise the domestic price above the world price to the level where the gap between domestic demand and domestic supply equals the quota limit. The only meaningful difference between a quota and a tariff is who gets the quota rent (the gap between the domestic price and world price multiplied by the quota quantity, equivalent to tariff revenue). If the government auctions quota licenses to domestic importers, the government gets the rent, and the outcome is identical to an equivalent tariff. If the government gives quota licenses to foreign producers for free, the rent leaves the domestic economy, so total domestic loss is equal to DWL plus the entire quota rent.

Worked Example

Using the same t-shirt market (demand $P=30-0.5Q$, supply $P=2+0.5Q$, $P_w=8$), a binding quota of 20 imports is imposed. Compare the outcome to the $\$3$ tariff that also resulted in 20 imports.

1. Free trade imports are 32, so 20 is a binding quota. We know that $Q_d-Q_s=20$ under the quota.
2. Rewrite demand and supply as functions of price: $Q_d=60-2P$, $Q_s=2P-4$. Imports = $(60-2P_q)-(2P_q-4)=64-4P_q=20$.
3. Solve for domestic price: $4P_q=44⟹P_q=\$11$, the same price as under the $\$3$ tariff.
4. Calculate DWL: the two efficiency triangles are identical to the tariff, so DWL = $\$18$, same as the tariff.
5. Comparison: The only difference is the quota rent of $\$3\times 20=\$60$. If the government auctions quota licenses, it keeps the $\$60$ rent and the outcome is identical to the tariff. If the government gives the licenses to foreign producers, the $\$60$ rent leaves the domestic economy, so total domestic loss is $\$18+\$60=\$78$, far higher than the tariff.

Exam tip: Always check if a quota is binding first: if the quota limit is higher than free trade imports, it has no effect on price, quantity, or welfare.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Calculating deadweight loss from a tariff by forgetting to add government revenue back into total surplus when comparing to free trade. Why: Students remember CS falls and PS rises, but overlook that tariff revenue is a benefit to the domestic government that counts toward total surplus. Correct move: When calculating total surplus after a tariff, always compute $TS=CS+PS+\text{tariff revenue}$ before comparing to free trade TS.
• Wrong move: Claiming total surplus decreases when a country opens to export a good because consumer surplus falls. Why: Students only track one group's outcome and ignore the larger gain to producers. Correct move: Always calculate total surplus as the sum of CS + PS; net gains from open trade are always positive for a small open economy.
• Wrong move: Calculating tariff revenue as $t\times$ total domestic consumption after the tariff, instead of $t\times$ imports after the tariff. Why: Students confuse tariffs with a general per-unit tax on all units, but tariffs only apply to imported goods. Correct move: Tariff revenue = per-unit tariff × (domestic quantity demanded - domestic quantity supplied after tariff).
• Wrong move: Treating quota rent as part of deadweight loss when the government auctions quota licenses. Why: Students assume all price gaps are deadweight loss, regardless of who gets the revenue. Correct move: Only the two triangular efficiency areas count as DWL for quotas; quota rent is a transfer, not an efficiency loss, unless it accrues to foreigners.
• Wrong move: Stating that a tariff increases both consumer and producer surplus. Why: Students mix up the effect of higher prices on the two groups. Correct move: Higher domestic prices from tariffs/quotas always increase producer surplus and decrease consumer surplus.
• Wrong move: Drawing a downward-sloping world supply curve for a small open economy problem. Why: Students confuse domestic supply with world supply. Correct move: For all AP Micro trade problems, draw world supply as a horizontal line at the fixed world price.

6. Practice Questions (AP Microeconomics Style)

Question 1 (Multiple Choice)

The autarky equilibrium price of wheat in Country B is $\$6$ per bushel, and the world price is $\$4$ per bushel. Country B is a small open economy. When opening to free trade, which of the following is true? A) Country B will export wheat, consumer surplus increases, producer surplus decreases, total surplus increases B) Country B will import wheat, consumer surplus increases, producer surplus decreases, total surplus increases C) Country B will import wheat, consumer surplus decreases, producer surplus increases, total surplus decreases D) Country B will export wheat, consumer surplus decreases, producer surplus increases, total surplus decreases

Worked Solution: First, compare autarky price ($\$6$) to world price ($\$4$): world price is lower than domestic autarky price, so Country B will import wheat, eliminating options A and D. When domestic price falls from $\$6$ to $\$4$, consumer surplus increases (lower price expands the area under demand above price) and producer surplus decreases (lower price shrinks the area above supply below price). Free trade always generates net gains for a small open economy, so total surplus increases. This matches option B. Correct answer: B.

Question 2 (Free Response)

The domestic market for sneakers in a small open economy has demand $P=100-Q$ and supply $P=20+Q$. The world price of sneakers is $\$50$. (a) Calculate the quantity of imports under free trade. Calculate total surplus under free trade. (b) The government imposes a $\$10$ per unit tariff on imported sneakers. Calculate the change in consumer surplus and the change in producer surplus relative to free trade. Calculate deadweight loss. (c) Explain one reason why governments impose import tariffs even though they create deadweight loss.

Worked Solution: (a) At $P_w=50$, $Q_d=100-50=50$, $Q_s=50-20=30$. Imports = $50-30=20$. $CS=0.5(100-50)(50)=1250$, $PS=0.5(50-20)(30)=450$. Total surplus $=1250+450=1700$. (b) After tariff, $P_t=50+10=60$. New $Q_d=100-60=40$, new $Q_s=60-20=40$. New $CS=0.5(100-60)(40)=800$, so change in $CS=800-1250=-450$ (CS falls by 450). New $PS=0.5(60-20)(40)=800$, so change in $PS=800-450=+350$ (PS rises by 350). Government tariff revenue = $10(40-40)=0$. $DWL=1700-(800+800+0)=100$. (c) Governments often impose tariffs to benefit politically connected domestic producer groups that lobby for protection. The concentrated gains to producers outweigh the diffuse losses to consumers in the political process, even though total domestic surplus falls.

Question 3 (Application / Real-World Style)

In 2022, a country imposed a 25% tariff on imported steel, with a world price of $\$800$ per ton. The domestic market (treat as small open) has annual demand $Q_d=200-0.1P$ (quantity in millions of tons) and supply $Q_s=0.1P-40$. Calculate the annual deadweight loss from this tariff if the government keeps all tariff revenue, and interpret the result.

Worked Solution: First, convert the ad valorem tariff to per-unit tariff: $t=0.25\times 800=\$200$ per ton. Domestic price after tariff: $P_t=800+200=1000$. At $P_w=800$, $Q_d=200-0.1(800)=120$ million tons, $Q_s=0.1(800)-40=40$ million tons. At $P_t=1000$, $Q_d=200-0.1(1000)=100$ million tons, $Q_s=0.1(1000)-40=60$ million tons. $DWL=0.5(200)(60-40)+0.5(200)(120-100)=2000+2000=4000$ million dollars = $\$4$ billion per year. This $\$4$ billion annual deadweight loss is the net efficiency loss to the economy, representing value lost forever due to inefficient domestic steel production and reduced consumption of steel.

7. Quick Reference Cheatsheet

8. What's Next

This topic is the first full application of supply and demand welfare analysis to real-world public policy, a framework that powers the rest of the AP Microeconomics course. Immediately next in Unit 2, you will apply the exact same consumer surplus, producer surplus, and deadweight loss framework to analyze domestic policies like price ceilings, price floors, and per-unit taxes. Without mastering how to track welfare changes for each group and calculate net efficiency loss here, you will struggle to correctly apply these tools to other policy problems later. This topic also builds on the comparative advantage concepts you learned earlier in the course, and feeds into later units where you will analyze how trade affects market power and industry concentration. The skills you practice here are tested repeatedly across both MCQ and FRQ sections, so mastering this content is critical for your overall exam score.

• Consumer and Producer Surplus
• Price Ceilings and Price Floors
• Taxes and Subsidies
• Comparative Advantage and Gains From Trade