📚 International Trade and Access to Markets | 国际贸易与市场准入
International trade is a central topic in A-Level Economics. It examines why countries exchange goods and services, how they benefit from specialisation, and how barriers affect access to markets. This revision note covers trade theory, terms of trade, protectionism, market access, the WTO, exchange rates and development issues.
国际贸易是 A-Level 经济学的核心主题。它研究各国为何交换商品和服务、如何从专业化中获益，以及各种壁垒如何影响市场准入。本复习笔记涵盖贸易理论、贸易条件、保护主义、市场准入、WTO、汇率和发展问题。
1. The Gains from International Trade | 国际贸易的收益
International trade allows countries to obtain goods and services that domestic producers cannot supply at the same price or quality. It widens consumer choice and reduces costs through greater competition and economies of scale. For a small open economy, trade shifts resources into sectors where the country has a comparative advantage, raising allocative efficiency.
国际贸易使各国能够获得国内生产者无法以相同价格或质量提供的商品和服务。它扩大了消费者选择，并通过更激烈的竞争和规模经济降低成本。对于小型开放经济体，贸易将资源配置到具有比较优势的部门，从而提高配置效率。
Trade also encourages technology transfer and innovation because firms face foreign competitors. However, the gains are not automatic; they depend on how easily resources can move between industries and how well institutions support adjustment. In exam answers, link the gains from trade to lower prices, wider variety and increased output.
贸易还促进技术转让和创新，因为企业要面对外国竞争者。然而，收益并非自动产生；它取决于资源能否在行业间顺利流动，以及制度是否支持调整。在答题时，应将贸易收益与更低价格、更多种类和更高产出联系起来。
2. Absolute and Comparative Advantage | 绝对优势与比较优势
Adam Smith explained trade using absolute advantage: a country should export goods it can produce with fewer resources than another country. David Ricardo developed a more powerful theory of comparative advantage: even if a country is less efficient in producing all goods, it should specialise in the good where its opportunity cost is lowest.
亚当·斯密用绝对优势解释贸易：一国应出口它能以更少资源生产的商品。大卫·李嘉图提出了更有力的比较优势理论：即使一国在生产所有商品上效率都较低，它也应专门生产机会成本最低的商品。
For example, suppose Country A gives up 2 units of cloth to produce 1 unit of wine, while Country B gives up 3 units of cloth for 1 unit of wine. Country A has a comparative advantage in wine, and Country B has a comparative advantage in cloth. If both countries specialise and trade, total world output rises.
例如，假设 A 国生产 1 单位酒需要放弃 2 单位布，而 B 国生产 1 单位酒需要放弃 3 单位布。A 国在酒上具有比较优势，B 国在布上具有比较优势。如果两国实行专业化并开展贸易，世界总产出就会增加。
Remember that comparative advantage is about opportunity cost, not absolute productivity. A country always has a comparative advantage in some good, unless opportunity costs are identical. This is why trade can benefit both developed and developing economies.
请记住，比较优势关注的是机会成本，而不是绝对生产率。除非机会成本完全相同，否则一国总会在某种商品上具有比较优势。这就是为什么贸易可以使发达经济体和发展中经济体都获益。
3. Specialisation, Interdependence and Trade Costs | 专业化、相互依赖与贸易成本
Specialisation raises output per worker and allows economies of scale, but it also increases interdependence. A country that specialises in manufacturing may rely on imported food and energy, so a supply shock abroad can disrupt domestic production. This makes access to reliable trade routes and suppliers a strategic issue.
专业化提高人均产出并带来规模经济，但同时也增加了相互依赖。一个专门从事制造业的国家可能依赖进口食品和能源，因此国外供给冲击可能扰乱国内生产。这使得可靠贸易路线和供应渠道的准入成为战略问题。
Not all trade is frictionless. Transport, insurance, language differences, legal systems and border delays create transaction costs. These costs reduce the net gains from trade and can prevent small firms from entering foreign markets even when tariffs are low.
贸易并非没有摩擦。运输、保险、语言差异、法律制度和边境延误都会产生交易成本。这些成本减少了贸易净收益，即使关税较低，也可能阻碍小企业进入外国市场。
4. Terms of Trade | 贸易条件
The terms of trade measure the ratio of export prices to import prices. An improvement means that a country can buy more imports for the same volume of exports, which raises national income. The standard formula is export price index divided by import price index, multiplied by 100.
贸易条件衡量出口价格与进口价格之比。贸易条件改善意味着一国用同样数量的出口可以购买更多进口，从而提高国民收入。标准公式是出口价格指数除以进口价格指数，再乘以 100。
Terms of trade = (Export price index ÷ Import price index) × 100
Terms of trade can improve because of higher world commodity prices, a stronger currency or faster productivity growth in export industries. A deterioration means more exports are needed to pay for a given volume of imports, which may lower living standards.
贸易条件可能因世界大宗商品价格上升、本币走强或出口行业生产率增长更快而改善。贸易条件恶化意味着需要更多出口才能支付同样数量的进口，这可能降低生活水平。
5. Tariffs and Their Welfare Effects | 关税及其福利效应
A tariff is a tax on imported goods. It raises the domestic price from the world price Pw to Pw + t. Domestic producers expand output because they receive a higher price, while consumers buy less, and imports fall from Q1Q4 to Q2Q3 in a standard diagram.
关税是对进口商品征收的税。它使国内价格从世界价格 Pw 上升到 Pw + t。国内生产者因获得更高价格而扩大产量，消费者购买减少，进口量在标准图形中从 Q1Q4 降至 Q2Q3。
- Consumer surplus falls | 消费者剩余下降
- Producer surplus rises | 生产者剩余上升
- Government gains tariff revenue | 政府获得关税收入
- Society loses from deadweight welfare triangles | 社会因无谓损失三角而遭受损失
The two deadweight loss triangles show productive inefficiency and consumption inefficiency. The tariff therefore protects domestic producers and workers, but it does so at a net cost to society. Higher prices also hit lower-income households hardest because they spend a larger share of income on imported goods.
两个无谓损失三角形表示生产无效率和消费无效率。因此，关税保护了国内生产者和工人，但其代价是社会净损失。更高的价格对低收入家庭的打击也最大，因为他们将较大份额的收入用于购买进口商品。
6. Quotas and Non-Tariff Barriers | 配额与非关税壁垒
A quota is a physical limit on the quantity of a good that can be imported. It raises the domestic price and creates quota rents for those who hold import licences. Unlike a tariff, a quota does not automatically generate government revenue unless licences are auctioned.
配额是对某种商品进口数量的物理限制。它提高了国内价格，并为持有进口许可证的人创造了配额租金。与关税不同，除非许可证被拍卖，否则配额不会自动产生政府收入。
Non-tariff barriers include product standards, labelling rules, customs procedures, health and safety regulations, local content requirements and administrative delays. These barriers can be more restrictive than tariffs because they are less transparent and harder for exporters to challenge.
非关税壁垒包括产品标准、标签规则、海关程序、健康与安全法规、本地含量要求和行政拖延。这些壁垒可能比关税更具限制性，因为它们不太透明，出口商也更难提出质疑。
7. The Protectionism Debate | 保护主义之争
Supporters of protection argue that tariffs can protect infant industries until they become internationally competitive. Protection may also preserve jobs in import-competing sectors, prevent dumping by foreign firms, raise tax revenue and protect industries important for national security.
保护主义的支持者认为，关税可以保护幼稚产业，直到它们具备国际竞争力。保护还可以保留进口竞争部门的就业，防止外国企业倾销，增加税收，并保护对国家安全至关重要的行业。
Critics respond that protection raises prices for consumers and firms, reduces choice, invites retaliation from trading partners and shelters inefficient producers. In the long run, sheltered industries have little incentive to cut costs or innovate. Most economists favour gradual liberalisation combined with retraining and
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