Chapter 15 · International trade: Part II
Customs-union welfare effects
Compare trade creation and trade diversion in a customs union. Weigh consumption and production gains against lost revenue and diversion costs.
The economic idea
Economic integration is the progressive reduction of barriers and coordination of economic rules between economies. A preferential trade agreement grants selected partners better access; a free-trade area removes internal tariffs while members retain separate external tariffs; a customs union adds a common external tariff.
A common market extends a customs union by allowing freer movement of labour and capital. An economic union adds substantial coordination or harmonisation of economic policies, while a monetary union shares a currency or permanently fixed exchange rates and a common monetary policy.
Trade creation occurs when integration replaces higher-cost domestic production with lower-cost imports from a member. The domestic price falls, inefficient domestic production contracts and consumption expands, creating production and consumption efficiency gains.
Trade diversion occurs when preferential treatment causes imports to switch from a lower-cost non-member to a higher-cost member. The loss of tariff revenue is partly a transfer, but the difference between the member and non-member resource costs on diverted imports is a real national welfare loss.
The static welfare effect depends on the balance of trade-creation gains and trade-diversion losses. Dynamic effects may include stronger competition, scale economies, investment, technology transfer and bargaining power, but also adjustment unemployment, concentration, uneven regional gains and reduced policy autonomy.
Reading Ppartner against the protected and non-member prices
“Quantity” is measured horizontally and “Price / cost” vertically.
“D domestic” and “S domestic” describe the domestic market; the three horizontal lines represent perfectly elastic foreign supply at different landed prices.
“Price before union” is the pre-union domestic price with the common external tariff, “Ppartner” is the duty-free partner price, and “Pw · non-member” is the non-member’s underlying world price.
Before integration: A to B
At “Price before union”, domestic firms supply “Qs₁” and consumers demand “Qd₁”. Imports are the horizontal distance from “A” to “B”.
After integration: C to D
“Ppartner” lies below “Price before union”. Domestic production falls from “Qs₁” to “Qs₂”, domestic consumption rises from “Qd₁” to “Qd₂”, and imports increase after integration.
Identify the source of the imports
“Ppartner” remains above “Pw · non-member”. Preferential access therefore redirects at least some imports from the lower-cost non-member to the higher-cost partner: trade diversion accompanies the expansion of trade.
Complete the welfare judgment
The current trade-creation production and consumption gains exceed the resource-cost loss from diversion, so national total surplus rises in this partial-equilibrium calculation.
Build the net welfare comparison
The production gain comes from replacing domestic output between “Qs₂” and “Qs₁” with imports available at “Ppartner”. The consumption gain comes from extra purchases between “Qd₁” and “Qd₂” whose benefit exceeds that new price. These are the two gains to identify before reaching a national-welfare conclusion.
For the imports already purchased before entry, compare “Ppartner” with “Pw · non-member”. The higher resource cost on these diverted imports forms a rectangle: the partner–non-member price gap multiplied by imports before entry. It is not the whole rectangle of lost tariff revenue.
Add the production and consumption gains, then subtract any diversion loss. Only after this comparison should you discuss longer-run effects such as competition, investment and economies of scale.
How to reason from membership to a welfare conclusion
Establish the pre-union price “Price before union” and read “Qs₁” and “Qd₁”.
Remove the internal tariff and read the new domestic price from “Ppartner”; trace the movements along “S domestic” and “D domestic”.
Use “Pw · non-member” to test sourcing efficiency. If “Ppartner” is above it, some imports have been diverted from a lower-cost non-member.
Treat tariff revenue as a transfer first, then isolate the real production, consumption and diversion effects before calculating net welfare.
Add dynamic and distributional considerations only after the static mechanism is correct.
Common mistakes to avoid
- An increase in imports is not by itself proof of trade creation; identify what production those imports replace and compare source costs.
- Lost tariff revenue is not automatically a deadweight loss. The real diversion loss is the excess resource cost of partner imports over lower-cost non-member imports.
- A lower domestic price benefits consumers but does not by itself prove a positive national welfare result.
- A customs union has a common external tariff; a free-trade area does not.
- Static welfare triangles do not capture scale economies, investment, competition, bargaining power or transition costs.
How to draw this diagram
- Title the diagram “Customs-union welfare effects”. Label the horizontal axis “Quantity” and the vertical axis “Price / cost”.
- Draw D domestic first and label it clearly.
- Draw S domestic next and label it clearly.
- Draw Pw · non-member next and label it clearly.
- Draw Price before union next and label it clearly.
- Draw Ppartner next and label it clearly.
- Mark each relevant intersection and distinguish the initial, market and socially optimum or policy outcomes where applicable.
- Add the relevant guide lines and shade the economic area only after its boundaries are established.
- Check that every symbol used in the explanation appears on the diagram and add arrows showing the direction of change.
Assumptions and limitations
- The small-country model assumes perfectly elastic foreign supply and no influence on partner or non-member prices.
- Products are treated as homogeneous and transport, border, regulatory and rules-of-origin costs are omitted.
- Domestic demand and supply are unchanged even though integration may alter investment, productivity, market structure and expectations.
- Retaliation, bargaining, fiscal adjustment and the distribution of gains across workers, firms and regions are not plotted.
- The model is partial equilibrium: exchange rates, aggregate demand, public finances and other markets may change.