Microeconomics · Unit 2: Supply and Demand · 14 min read · Updated 2026-05-11
International Trade and Public Policy — AP Microeconomics
AP Microeconomics · Unit 2: Supply and Demand · 14 min read
1. Core Framework: Small Open Economy Assumption★★☆☆☆⏱ 3 min
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. This topic accounts for roughly 2-3% of the total AP Microeconomics exam score, appearing in both multiple-choice and free-response sections.
For all AP Microeconomics trade problems, world supply is drawn as a perfectly elastic horizontal line at the fixed world price $P_w$. AP Micro focuses exclusively on partial equilibrium analysis of a single market, aligned with the Unit 2 focus on supply and demand fundamentals.
2. Gains From Trade in an Open Economy★★☆☆☆⏱ 4 min
In autarky (a closed economy with no trade), the domestic market settles at its own equilibrium price $P^*$ and quantity $Q^*$. When the economy opens to trade, domestic price converges to the fixed world price $P_w$.
If $P_w < P^*$: Domestic price falls to $P_w$, domestic quantity supplied is less than domestic quantity demanded, and the gap is filled by imports.
If $P_w > P^*$: Domestic price rises to $P_w$, domestic quantity supplied is more than domestic quantity demanded, and the gap is exported.
Welfare is measured as total surplus (TS), equal to 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 loses surplus, the other group's gains exceed the losses, so overall social welfare increases.
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★★★☆☆⏱ 4 min
Higher domestic prices from a tariff have four key welfare effects: (1) domestic producers increase output, so producer surplus rises, (2) consumers reduce consumption, so consumer surplus falls, (3) the government collects tariff revenue equal to $t \times \text{imports after the tariff}$, (4) there is net deadweight loss (DWL) from two efficiency losses: inefficient domestic production and lost consumer surplus from reduced consumption.
Where $\Delta Q_s$ is the increase in domestic output, and $\Delta Q_d$ is the decrease in domestic consumption after the tariff.
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★★★☆☆⏱ 3 min
Binding quotas work almost exactly like tariffs: by restricting supply, they raise the domestic price above the world price. The only key difference is who receives the *quota rent* (the gap between 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 keeps the quota rent, and the outcome is identical to an equivalent tariff.
If the government gives quota licenses to foreign producers for free: the quota rent leaves the domestic economy, so total domestic loss equals DWL plus the entire quota rent.
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. AP-Style Concept Check★★★☆☆⏱ 2 min
Common Pitfalls
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.
Why: Students only track one group's outcome and ignore the larger gain to producers.
Why: Students confuse tariffs with a general per-unit tax on all units, but tariffs only apply to imported goods.
Why: Students assume all price gaps are deadweight loss, regardless of who gets the revenue.
Why: Students mix up the effect of higher prices on the two groups.
Why: Students confuse domestic supply with world supply.