📚 IB & OCR Economics: International Trade Key Concepts | IB OCR 经济：国际贸易 考点精讲
International trade is a cornerstone of modern economics, examining why countries exchange goods and services, the welfare effects of trade policies, and the role of international financial flows. This revision guide consolidates essential concepts for IB and OCR Economics students, covering comparative advantage, protectionism, exchange rates, and balance of payments.
国际贸易是现代经济学的基石，研究国家之间交换商品和服务的原因、贸易政策的福利效应以及国际资金流动的作用。这份考点精讲整合了IB和OCR经济学学生必备的核心概念，涵盖比较优势、保护主义、汇率与国际收支。
1. Absolute and Comparative Advantage | 绝对优势与比较优势
Absolute advantage refers to a country’s ability to produce more of a good using the same amount of resources (or the same output with fewer resources) than another country.
绝对优势是指一国使用相同资源能比另一国生产更多产品（或用更少资源生产相同产出）的能力。
Comparative advantage arises when a country can produce a good at a lower opportunity cost than its trading partner. Opportunity cost is the quantity of the next best alternative sacrificed per unit of the good produced.
比较优势发生在一国生产某种商品的机会成本低于其贸易伙伴时。机会成本是每生产一单位该商品所放弃的次优替代品的数量。
To illustrate, consider a simple two-country, two-good model. Assume that Country A can produce 12 units of cloth or 6 units of wine per worker, while Country B can produce 8 units of cloth or 2 units of wine. The opportunity cost of 1 unit of cloth in Country A is 0.5 wine (6/12), while in Country B it is 0.25 wine (2/8). Country B has a comparative advantage in cloth because its opportunity cost is lower. Country A has a comparative advantage in wine (OC of 1 wine is 2 cloth for A, but 4 cloth for B). Even if one country has absolute advantage in both goods, trade based on comparative advantage can be mutually beneficial.
举例说明，考虑简单的两国两产品模型。假设A国每个工人可生产12单位布或6单位酒，B国每个工人可生产8单位布或2单位酒。A国生产1单位布的机会成本是0.5单位酒（6/12），而B国是0.25单位酒（2/8）。B国在布料生产上具有比较优势，因为其机会成本更低。A国在酒类生产上具有比较优势（A国1单位酒的机会成本是2单位布，B国是4单位布）。即使一国在两种产品上都具有绝对优势，基于比较优势的贸易仍可互惠。
2. Gains from Trade and Specialisation | 贸易收益与专业化
When countries specialise according to their comparative advantage and trade, global output increases and consumption possibilities expand beyond each country’s domestic production possibilities frontier (PPF). The terms of trade (the ratio of export prices to import prices) determine how the gains from trade are distributed.
当各国根据比较优势进行专业化生产并开展贸易时，全球产出增加，消费可能性超出各国国内生产可能性边界（PPF）。贸易条件（出口价格与进口价格之比）决定了贸易收益的分配。
Graphically, trade allows a country to consume at a point outside its PPF. The new consumption bundle is determined by the international terms of trade line. Both trading partners can reach higher indifference curves. However, the assumption of free trade, zero transport costs, and perfect factor mobility within countries enables this result.
从图形上看，贸易使一国能够在其PPF之外的点进行消费。新的消费组合由国际贸易条件线决定。双方贸易伙伴都能达到更高的无差异曲线。然而，这一结果建立在自由贸易、零运输成本和国内要素完全流动的假设之上。
3. Terms of Trade | 贸易条件
The terms of trade (TOT) index measures the relative price of exports compared to imports. TOT = (Index of export prices ÷ Index of import prices) × 100. An improvement in the terms of trade means a country can buy more imports for a given quantity of exports, raising living standards. However, an improvement may reduce export competitiveness if it results from rising export prices, potentially worsening the trade balance.
贸易条件指数衡量出口价格相对于进口价格的水平。TOT = (出口价格指数 ÷ 进口价格指数) × 100。贸易条件改善意味着一国能用给定数量的出口换取更多进口，从而提高生活水平。但如果改善源于出口价格上涨，则可能削弱出口竞争力，进而恶化贸易收支。
Factors affecting terms of trade include changes in global demand and supply for a country’s exports and imports, exchange rates, and productivity. Developing countries often face declining terms of trade due to dependence on primary commodities with low income elasticity of demand.
影响贸易条件的因素包括全球对该国进出口的需求与供给变化、汇率以及生产率。发展中国家常因依赖需求收入弹性低的初级产品而面临贸易条件恶化。
4. Free Trade vs. Protectionism | 自由贸易与保护主义
Free trade promotes efficiency through specialisation, increases consumer choice, and lowers prices. It encourages competition and the diffusion of technology. However, it can also lead to structural unemployment, infant industry damage, and a loss of cultural identity.
自由贸易通过专业化提高效率，增加消费者选择，并降低价格。它促进竞争与技术传播。然而，自由贸易也可能导致结构性失业、损害幼稚产业，并造成文化认同缺失。
Protectionism involves imposing barriers to shield domestic industries from foreign competition. Arguments for protectionism include protecting infant industries, preserving national security, preventing dumping, conserving jobs, and generating government revenue. Counterarguments stress that protection leads to higher consumer prices, production inefficiency, retaliation, and a misallocation of global resources.
保护主义指设置壁垒以保护国内产业免受外国竞争。支持保护主义的理由包括保护幼稚产业、维护国家安全、防止倾销、保留就业和增加政府收入。反对观点强调保护会导致消费者价格上升、生产效率低下、引发报复以及全球资源配置不当。
5. Tariffs | 关税
A tariff is a tax imposed on imported goods. It raises the domestic price from the world price P_w to P_w + t, where t is the tariff. The higher price reduces import quantity, increases domestic production, and generates tariff revenue for the government.
关税是对进口商品征收的税。它将国内价格从世界价格P_w提高至P_w + t（t为关税额）。更高的价格降低进口量、增加国内生产，并为政府带来关税收入。
The welfare effects of a tariff include a loss in consumer surplus, a gain in producer surplus, government revenue equal to tariff × imports, and a deadweight loss (welfare loss) comprising two triangles: the production inefficiency loss and the consumption inefficiency loss. The net welfare effect is negative for a small country that cannot influence world prices. For a large country, an optimal tariff may exist if the terms-of-trade gain outweighs the deadweight loss.
关税的福利效应包括消费者剩余减少、生产者剩余增加、等于关税额乘以进口量的政府收入，以及由两个三角形组成的无谓损失（福利损失）：生产低效损失和消费低效损失。对于无法影响世界价格的小国，净福利效应为负。对大国而言，如果贸易条件改善收益超过无谓损失，可能存在最优关税。
The diagram analysis: Domestic supply and demand curves; world supply is perfectly elastic at P_w. With tariff, price rises to P_t, imports fall from Q₄ – Q₁ to Q₃ – Q₂. Consumer surplus falls by (a+b+c+d). Producer surplus rises by a. Government revenue is c. Deadweight loss is b+d.
图形分析：国内供给和需求曲线；世界供给在P_w处完全弹性。征收关税后，价格升至P_t，进口量从Q₄ – Q₁降至Q₃ – Q₂。消费者剩余减少 (a+b+c+d)。生产者剩余增加 a。政府收入为 c。无谓损失为 b+d。
6. Quotas and Subsidies | 配额与补贴
An import quota is a quantitative restriction on the volume of a good that can be imported. Like a tariff, a quota raises the domestic price and reduces import quantity. However, the government does not collect revenue; instead, the quota rents typically go to import licence holders, creating potential for corruption and administrative costs. An equivalent tariff would generate revenue for the government, making tariffs generally more efficient than quotas, unless voluntary export restraints (VERs) are used.
进口配额是对进口商品数量的数量限制。与关税一样，配额提高国内价格并减少进口量。然而，政府不获得收入；配额租金通常流向进口许可证持有者，可能滋生腐败和行政成本。等效的关税能为政府创收，因此通常关税比配额更有效率，除非采用自愿出口限制。
An export subsidy is a payment by the government to domestic firms for each unit exported. It raises the producer’s price above the world price, encouraging exports. In the domestic market, consumers face a higher price, and the subsidy causes a deadweight loss. The subsidy also worsens the government’s budget and can provoke retaliatory subsidies from trading partners.
出口补贴是政府对国内企业每出口一单位商品给予的支付。它使生产者价格高于世界价格，鼓励出口。在国内市场上，消费者面临更高价格，补贴造成无谓损失。补贴还会恶化政府预算，并可能引发贸易伙伴的报复性补贴。
7. Exchange Rates and International Trade | 汇率与国际贸易
An exchange rate is the price of one currency in terms of another. It can be expressed as a floating, fixed, or managed system. Appreciation of a currency (increase in value) makes exports more expensive and imports cheaper, potentially worsening the trade balance in the short run. Depreciation makes exports cheaper and imports more expensive, improving the trade balance if the Marshall-Lerner condition holds (the sum of price elasticities of demand for exports and imports > 1).
汇率是一种货币以另一种货币表示的价格。汇率制度可以是浮动、固定或有管理的。货币升值（价值上升）使出口更贵、进口更便宜，短期内可能恶化贸易收支。贬值使出口更便宜、进口更贵，如果满足马歇尔-勒纳条件（出口和进口需求价格弹性之和大于1），则可改善贸易收支。
The J‑curve effect describes the short-term worsening of the trade balance following a depreciation, because demand is inelastic in the immediate period, but improves later as quantities adjust.
J曲线效应描述了贬值后贸易收支在短期内恶化的情况，因为需求在即期缺乏弹性，但随着数量调整后改善。
8. Balance of Payments – Current Account | 国际收支 – 经常帐户
The current account records trade in goods (visible trade), trade in services (invisible trade), primary income (investment income and compensation of employees), and secondary income (current transfers). A current account deficit means a country spends more on imports, income and transfers than it earns, requiring financing via the financial account (capital inflows) or drawing down reserves.
经常账户记录商品贸易（有形贸易）、服务贸易（无形贸易）、初次收入（投资收入和雇员报酬）以及二次收入（经常转移）。经常账户赤字表示一国在进口、收入和转移上的支出超过其收入，需要通过金融账户（资本流入）或动用储备来
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