international-trade

# International Trade

International trade examines why countries exchange goods, services, and capital, who benefits and who is harmed, and what policies governments adopt in response. The central insight -- comparative advantage -- is among the most counterintuitive results in all of economics: a country benefits from trade even if it is less productive than its trading partner at everything. This skill covers the classical and modern theories of trade, policy instruments, exchange rate determination, and the political economy of trade agreements.

**Agent affinity:** smith (comparative advantage, gains from trade), sen (development and trade, global inequality), hayek (free trade, spontaneous order in global markets)

**Concept IDs:** econ-comparative-advantage, econ-supply-demand, econ-market-structures, econ-gdp-growth

## The International Trade Toolbox at a Glance

| # | Topic | Core question | Key framework |
|---|---|---|---|
| 1 | Comparative advantage | Why do countries specialize and trade? | Opportunity cost comparison |
| 2 | Trade models | What determines trade patterns? | Ricardian, Heckscher-Ohlin, new trade theory |
| 3 | Gains and losses | Who wins and who loses from trade? | Stolper-Samuelson, specific factors |
| 4 | Trade policy | How do tariffs and quotas affect markets? | Partial equilibrium welfare analysis |
| 5 | Exchange rates | How are currencies valued? | PPP, interest parity, monetary models |
| 6 | Balance of payments | How do international flows balance? | Current account + capital account = 0 |
| 7 | Globalization | How has trade changed the world economy? | Trade/GDP ratios, GVC integration, inequality |

## Topic 1 -- Comparative Advantage

**The Ricardian model.** David Ricardo (1817) showed that trade is mutually beneficial whenever countries have different opportunity costs of production, regardless of absolute productivity differences. If England can produce cloth at a lower opportunity cost than wine, and Portugal can produce wine at a lower opportunity cost than cloth, both gain by specializing and trading -- even if Portugal is absolutely more productive at both.

**Worked example.** Suppose Country A can produce 10 units of wheat or 5 units of steel per labor hour. Country B can produce 4 units of wheat or 2 units of steel per labor hour. A has absolute advantage in both goods. But the opportunity costs are: A gives up 2 wheat per steel, B gives up 2 wheat per steel -- identical! No comparative advantage, no gains from trade. Now change B to 4 wheat or 1 steel. B's opportunity cost of steel is 4 wheat. A's is 2 wheat. A has comparative advantage in steel, B in wheat. Trade makes both better off at any exchange rate between 2 and 4 wheat per steel.

**Why it is counterintuitive.** People naturally think in absolute terms. "If we're worse at everything, how can trade help us?" The answer is opportunity cost: trade frees resources to do what you are relatively best at, even if you are absolutely worse at everything.

**Paul Samuelson's observation.** When asked by a mathematician to name one proposition in all of social science that is both true and non-trivial, Samuelson nominated comparative advantage. "That it is logically true need not be argued before a mathematician; that it is not trivial is attested by the thousands of important and intelligent men who have never been able to grasp the doctrine for themselves or to believe it after it was explained to them."

## Topic 2 -- Trade Models

**Heckscher-Ohlin.** Countries export goods that use their abundant factor intensively. A labor-abundant country exports labor-intensive goods; a capital-abundant country exports capital-intensive goods. This extends Ricardo by explaining comparative advantage in terms of factor endowments rather than technology.

**The Leontief paradox.** Leontief (1953) found that the US -- the most capital-abundant country -- exported labor-intensive goods, contradicting Heckscher-Ohlin. Resolutions include human capital (skilled labor is a form of capital), technology differences, and taste biases. The paradox forced trade theory to become more sophisticated.

**New trade theory.** Krugman (Nobel 2008) showed that economies of scale and product differentiation can generate trade between similar countries with similar factor endowments. Two-thirds of world trade is between rich countries trading similar products (intra-industry trade). This is unexplained by comparative advantage alone but falls naturally out of models with increasing returns and consumer preference for variety.

**Gravity model.** Trade between two countries is proportional to their economic size (GDP) and inversely proportional to the distance between them. This simple empirical regularity explains 60-80% of bilateral trade variation and is the workhorse of empirical trade analysis.

**Melitz model.** Melitz (2003) introduced firm heterogeneity into trade theory. Within any industry, firms differ in productivity. Trade liberalization forces the least productive firms to exit (they cannot compete with imports), allows the most productive firms to expand into export markets, and leaves medium firms serving only the domestic market. The result: trade raises average industry productivity even without any individual firm becoming more efficient -- a composition effect. This explains why empirical studies consistently find that exporters are more productive than non-exporters.

**Worked multi-model example.** Consider two countries, Home and Foreign, both producing wine and cheese. Ricardian model: if Home has lower opportunity cost for cheese, Home exports cheese and imports wine. Heckscher-Ohlin: if cheese is capital-intensive and Home is capital-abundant, same prediction. New trade theory: even if both countries are identical, firms in each country may produce differentiated varieties of cheese and wine, and consumers in both countries buy some of each -- intra-industry trade. Gravity model: the volume of