📚 Market Failure | 市场失灵
Market failure is a central concept in Cambridge A-Level Economics. It explains situations where the free market, left to itself, fails to allocate resources efficiently. Understanding the causes and consequences of market failure is essential for evaluating government policy and writing high-scoring exam answers.
市场失灵是剑桥 A-Level 经济学的核心概念。它解释了自由市场在不受干预时无法有效配置资源的情形。理解市场失灵的原因和后果，对于评估政府政策和写出高分考试答案至关重要。
1. What is Market Failure? | 什么是市场失灵？
Market failure occurs when the price mechanism in a free market fails to allocate scarce resources in a socially efficient way. In A-Level Economics, social efficiency is achieved when marginal social benefit (MSB) equals marginal social cost (MSC). If MSB ≠ MSC, there is a misallocation of resources, leading to a deadweight welfare loss.
当自由市场中的价格机制无法以社会有效率的方式配置稀缺资源时，就会发生市场失灵。在 A-Level 经济学中，当边际社会收益（MSB）等于边际社会成本（MSC）时，社会效率得以实现。如果 MSB ≠ MSC，资源配置就会发生扭曲，并产生无谓福利损失。
Students often confuse market failure with a market simply being unfair or unprofitable. In strict economic terms, market failure means allocative inefficiency: resources are not being used to produce the combination of goods and services that maximises social welfare.
学生常常将市场失灵与市场不公平或不盈利混为一谈。在严格的经济学意义上，市场失灵意味着配置无效率：资源没有被用于生产能够最大化社会福利的商品和服务组合。
2. Types of Market Failure | 市场失灵的类型
Market failure can arise from several sources: externalities, public goods, information asymmetry, market power, and common access resources. Each source prevents the market from achieving allocative efficiency, where price equals marginal cost and consumer surplus plus producer surplus is maximised.
市场失灵可能源于多种因素：外部性、公共品、信息不对称、市场势力以及公共资源。每一种因素都会阻碍市场实现配置效率，即价格等于边际成本、消费者剩余与生产者剩余之和达到最大。
| Source of market failure | 市场失灵来源 | Key feature | 关键特征 | Example | 例子 |
|---|---|---|
| Externalities | 外部性 | Private costs or benefits diverge from social costs or benefits | 私人成本或收益偏离社会成本或收益 | Pollution, education | 污染、教育 |
| Public goods | 公共品 | Non-excludable and non-rivalrous | 非排他性和非竞争性 | National defence, street lighting | 国防、路灯 |
| Information asymmetry | 信息不对称 | One party knows more than the other | 一方比另一方知道更多 | Used cars, insurance | 二手车、保险 |
| Market power | 市场势力 | A firm can influence price | 企业可以影响价格 | Monopoly, oligopoly | 垄断、寡头 |
| Common access resources | 公共资源 | Rivalrous but non-excludable | 竞争性但非排他性 | Fish stocks, forests | 鱼类资源、森林 |
3. Externalities: Negative Production | 外部性：负生产外部性
A negative production externality occurs when the production of a good imposes external costs on third parties, such as air pollution from a factory. The marginal social cost exceeds the marginal private cost because producers ignore the external cost. In a free market, output is determined where marginal private cost equals marginal private benefit, so overproduction occurs relative to the social optimum.
负生产外部性发生在一种商品的生产给第三方带来外部成本时，例如工厂排放的空气污染。由于生产者忽略外部成本，边际社会成本大于边际私人成本。在自由市场中，产量由边际私人成本等于边际私人收益决定，因此相对于社会最优产量会出现过度生产。
e MEC is marginal external cost. The welfare loss can be estimated as ½ × (Qₘ − Qₛ) × MEC.
无谓福利损失是 MSC 与 MPB 之间从 Qₛ 到 Qₘ 的三角形区域。代数上，MSC = MPC + MEC，其中 MEC 是边际外部成本。福利损失可以估算为 ½ ×（Qₘ − Qₛ）× MEC。
Common exam examples include steel production causing air pollution, chemicals dumped into rivers, and carbon emissions from fossil-fuel power stations. Labels in diagrams should clearly show MSC above MPC.
常见的考试例子包括钢铁生产造成空气污染、化学物质排入河流，以及化石燃料发电站的碳排放。图表中的标注应清楚显示 MSC 位于 MPC 之上。
4. Externalities: Positive Consumption | 外部性：正消费外部性
Positive consumption externalities arise when consumption of a good generates external benefits to third parties, such as education improving productivity and social cohesion. The marginal social benefit is greater than the marginal private benefit. In a free market, consumers only consider private benefits, so the good is under-consumed relative to the social optimum.
正消费外部性发生在消费某种商品给第三方带来外部收益时，例如教育提高生产率和社会凝聚力。边际社会收益大于边际私人收益。在自由市场中，消费者只考虑私人收益，因此相对于社会最优水平，该商品会被消费不足。
n MSB and MPB from Qₘ to Qₛ. Here MSB = MPB + MEB, where MEB is marginal external benefit.
市场均衡数量 Qₘ 低于 Qₛ，造成的福利损失等于从 Qₘ 到 Qₛ 之间 MSB 与 MPB 之间的面积。此时 MSB = MPB + MEB，其中 MEB 是边际外部收益。
Other examples include vaccinations, which protect others through herd immunity, and public transport, which reduces congestion and pollution for non-users. The diagram shows MPB below MSB with under-consumption.
其他例子包括疫苗接种（通过群体免疫保护他人）和公共交通（减少非使用者的拥堵和污染）。该图显示 MPB 低于 MSB，并存在消费不足。
5. Public Goods and Free-Rider Problem | 公共品与搭便车问题
Public goods are non-excludable and non-rivalrous. Non-excludability means it is impossible or very costly to prevent non-payers from consuming the good; non-rivalry means one person's consumption does not reduce availability for others. Examples include national defence, street lighting, and flood defences.
公共品具有非排他性和非竞争性。非排他性意味着无法或很难阻止未付费者消费该商品；非竞争性意味着一个人的消费不会减少其他人可获得的数量。例子包括国防、路灯和防洪设施。
The socially efficient quantity is where MSB = MSC, but the market may produce zero units.
由于私人企业无法排除搭便车者，它们无法收取足以弥补成本的价格，因此自由市场会供给不足或完全不提供公共品。这就是搭便车问题。社会有效数量位于 MSB = MSC 之处，但市场可能生产零单位。
In exam answers, avoid calling public goods 'free goods'. A public good is not necessarily provided by the government, although the government often steps in because private provision is inefficient.
在考试答案中，避免将公共品称为“免费商品”。公共品不一定由政府提供，尽管由于私人提供效率低下，政府常常介入。
6. Information Asymmetry | 信息不对称
Information asymmetry occurs when one party in a transaction has more or better information than the other. This can lead to adverse selection before a transaction, such as in the used-car market where sellers know more about quality, and moral hazard after a transaction, such as a person with full insurance taking more risks.
信息不对称发生在交易中的一方比另一方拥有更多或更好的信息时。这可能导致交易前的逆向选择，例如在二手车市场中卖家更了解质量，以及交易后的道德风险，例如拥有全额保险的人承担更多风险。
Asymmetric information can cause markets to thin or collapse because consumers cannot distinguish high-quality from low-quality products. Government responses include compulsory labelling, licensing, warranties, and regulations.
信息不对称可能导致市场萎缩或崩溃，因为消费者无法区分高质量和低质量产品。政府对策包括强制标签、许可证、保修和监管。
In the used-car market, adverse selection may drive good cars out of the market because buyers are unwilling to pay high prices for unknown quality. This is sometimes called the 'lemons problem'.
在二手车市场中，逆向
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