📚 Deadweight Loss | 无谓损失
Deadweight loss is one of the most important welfare concepts in the Cambridge A-Level Economics syllabus. It measures the loss of total social surplus when a market is prevented from reaching its allocatively efficient equilibrium. Understanding deadweight loss helps you evaluate taxes, subsidies, price controls, monopoly power, and externalities.
无谓损失是剑桥 A-Level 经济学大纲中最重要的福利概念之一。它衡量当市场无法达到配置效率均衡时，社会总剩余的净损失。理解无谓损失有助于你评价税收、补贴、价格管制、垄断势力和外部性等问题。
1. Definition and Allocative Efficiency | 定义与配置效率
Deadweight loss is the net reduction in consumer surplus and producer surplus that occurs when output is not at the socially efficient level. Allocative efficiency is achieved where marginal social benefit equals marginal social cost, which in a competitive market without externalities is where demand equals supply. At this equilibrium quantity, total welfare is maximised.
无谓损失是当产量不处于社会有效水平时，消费者剩余和生产者剩余的净减少。配置效率在边际社会收益等于边际社会成本时实现，在没有外部性的竞争市场中，这就是需求等于供给的位置。在均衡数量上，总福利达到最大。
When a policy or market failure moves quantity away from equilibrium, the lost surplus is not transferred to anyone else; it simply disappears. That is why economists call it a deadweight loss or excess burden.
当政策或市场失灵使数量偏离均衡时，损失的剩余并没有转移给任何其他人，而是直接消失了。因此经济学家称之为无谓损失或超额负担。
2. Consumer and Producer Surplus Recap | 消费者剩余与生产者剩余回顾
Consumer surplus is the difference between the maximum price consumers are willing to pay and the market price they actually pay. Graphically, it is the area below the demand curve and above the price line. Producer surplus is the difference between the market price and the minimum price producers are willing to accept; it is the area above the supply curve and below the price line.
消费者剩余是消费者愿意支付的最高价格与他们实际支付的市场价格之间的差额。在图形中，它是需求曲线以下、价格线以上的面积。生产者剩余是市场价格与生产者愿意接受的最低价格之间的差额，它是供给曲线以上、价格线以下的面积。
Total social surplus is the sum of consumer surplus and producer surplus. Any reduction in total surplus that is not converted into some other benefit is a deadweight loss.
社会总剩余是消费者剩余与生产者剩余之和。总剩余的任何减少，如果没有转化为其他收益，就是无谓损失。
3. Deadweight Loss as a Welfare Loss | 无谓损失作为一种福利损失
It is important to distinguish between a transfer and a deadweight loss. For example, when a government imposes a tax, part of the consumer and producer surplus is transferred to the government as tax revenue. That transfer is not automatically a deadweight loss if the government spends it on public goods or transfers it back to households.
区分转移与无谓损失非常重要。例如，当政府征税时，部分消费者剩余和生产者剩余以税收收入的形式转移给政府。如果政府将其用于公共品或转移回家庭，这种转移并不自动构成无谓损失。
The deadweight loss arises because the tax raises the buyer price and lowers the seller price, causing the equilibrium quantity to fall. The trades that no longer occur would have generated positive net benefits to buyers and sellers. Those lost net benefits make up the deadweight loss triangle.
无谓损失之所以产生，是因为税收提高了买方价格并降低了卖方价格，导致均衡数量下降。不再发生的交易本来会给买方和卖方带来正的净收益。这些损失的净收益构成了无谓损失三角形。
4. Indirect Tax and Deadweight Loss | 间接税与无谓损失
An indirect tax, such as a specific tax on petrol, shifts the supply curve vertically upward by the amount of the tax. The price paid by consumers rises to Pb, the price received by producers falls to Ps, and the quantity traded falls from Qe to Qt. Tax revenue per unit is Pb – Ps, so total government revenue is (Pb – Ps) × Qt.
间接税，例如对汽油征收的从量税，使供给曲线垂直向上移动税额的幅度。消费者支付的价格上升到 Pb，生产者获得的价格下降到 Ps，交易数量从 Qe 下降到 Qt。每单位税收收入为 Pb – Ps，因此政府总收入为 (Pb – Ps) × Qt。
The welfare loss is the triangle between the demand and supply curves from Qt to Qe. Its area is:
福利损失是需求曲线与供给曲线之间从 Qt 到 Qe 的三角形面积。其面积为：
Deadweight loss = 0.5 × tax per unit × (Qe – Qt)
This triangle represents the value of mutually beneficial trades that are destroyed by the tax. The greater the reduction in quantity, the larger the deadweight loss.
该三角形代表被税收摧毁的互利交易的价值。数量减少越多，无谓损失越大。
5. Subsidy and Deadweight Loss | 补贴与无谓损失
A subsidy lowers the cost of production and shifts the supply curve downward or to the right. Consumers pay a lower price, producers receive a higher price, and the quantity traded rises above the free-market equilibrium. This causes overproduction: beyond Qe, the marginal social cost of production exceeds the marginal social benefit.
补贴降低了生产成本，使供给曲线向下或向右移动。消费者支付更低的价格，生产者获得更高的价格，交易数量上升到高于自由市场均衡的水平。这会导致过度生产：超过 Qe 后，生产的边际社会成本大于边际社会收益。
The deadweight loss is the triangular area of overproduction between Qe and Qsub, where the supply curve lies above the demand curve. Government spending is the subsidy per unit multiplied by Qsub, but much of that spending is a transfer from taxpayers to producers and consumers. Only the overproduction triangle is a net welfare loss.
无谓损失是 Qe 与 Qsub 之间过度生产的三角形区域，在该区域供给曲线位于需求曲线之上。政府支出为每单位补贴乘以 Qsub，但大部分支出只是从纳税人向生产者和消费者的转移。只有过度生产三角形构成净福利损失。
6. Price Ceilings and Price Floors | 价格上限与价格下限
A price ceiling set below the equilibrium price, such as rent control, creates a shortage. The quantity traded falls to the quantity supplied, which is lower than the efficient quantity. Consumers may gain lower prices for some units, but the lost trades create a deadweight loss. There can also be welfare losses from waiting, search costs, and black markets.
低于均衡价格的价格上限，例如租金管制，会造成短缺。交易数量下降到供给量，低于有效数量。消费者可能因部分单位价格更低而获益，但损失的交易造成了无谓损失。此外，等待、搜寻成本和黑市也会带来福利损失。
A price floor set above equilibrium, such as a minimum price for agricultural goods, creates a surplus. The quantity traded falls to the quantity demanded. The resulting underproduction creates a deadweight loss triangle. Some surplus goods may be bought and stored by the government, which adds a fiscal cost but does not eliminate the welfare loss.
高于均衡价格的价格下限，例如农产品最低价格，会造成过剩。交易数量下降到需求量。由此导致的产量不足产生了无谓损失三角形。部分过剩商品可能由政府购买和储存，这会增加财政成本，但并不能消除福利损失。
7. Monopoly and Deadweight Loss | 垄断与无谓损失
A profit-maximising monopolist produces where marginal revenue equals marginal cost and charges the highest price consumers are willing to pay for that quantity. Since price exceeds marginal cost at the monopoly output, output is lower than the competitive level. The monopolist gains producer surplus, but the reduction in quantity destroys some consumer surplus and some potential producer surplus.
利润最大化的垄断者在边际收益等于边际成本处生产，并对该数量收取消费者愿意支付的最高价格。由于在垄断产量上价格高于边际成本，产量低于竞争水平。垄断者获得了生产者剩余，但数量减少摧毁了部分消费者剩余和部分潜在生产者剩余。
The deadweight loss is the triangle between the demand curve and the marginal cost curve from the monopoly quantity to the competitive quantity. It shows the net value of output that society loses because monopoly restricts supply.
无谓损失是需求曲线与边际成本曲线之间从垄断产量到竞争产量的三角形区域。它显示了由于垄断限制供给，社会损失的产出净价值。
8. Externalities and Deadweight Loss | 外部性与无谓损失
When external costs or benefits exist, the free market equilibrium is allocatively inefficient. In a negative production externality, such as pollution, the marginal social cost is above the marginal private cost. The market overproduces relative to the social optimum, creating a deadweight loss of overproduction.
当存在外部成本或收益时，自由市场均衡是配置无效率的。在负生产外部性中，例如污染，边际社会成本高于边际私人成本。市场相对于社会最优水平过度生产，产生了过度生产的无谓损失。
In a positive consumption externality, such as education or vaccination, the marginal social benefit is above the marginal private benefit. The market underproduces relative to the social optimum, creating a deadweight loss of underproduction. Corrective policies such as Pigouvian taxes or subsidies can internalise the externality and reduce deadweight loss.
在正消费外部性中，例如教育或疫苗接种，边际社会收益高于边际私人收益。市场相对于社会最优水平生产不足，产生了生产不足的无谓损失。庇古税或补贴等矫正政策可以将外部性内部化，从而减少无谓损失。
9. Elasticity and the Size of Deadweight Loss | 弹性与无谓损失的大小
The size of the deadweight loss from a tax or subsidy depends on the price elasticities of demand and supply. When demand and supply are more elastic, a given tax causes a larger fall in quantity, so the deadweight loss is larger. When demand and supply are inelastic, quantity changes less, so the deadweight loss is smaller.
税收或补贴造成的无谓损失大小取决于需求和供给的价格弹性。当需求和供给更具弹性时，给定税收会导致数量下降更多，因此无谓损失更大。当需求和供给缺乏弹性时，数量变化较小，因此无谓损失较小。
| Market condition / 市场条件 | Effect on DWL / 对无谓损失的影响 |
|---|---|
| Elastic demand and elastic supply / 需求与供给均富有弹性 | Large deadweight loss because quantity falls sharply / 无谓损失大，因为数量大幅下降 |
| Inelastic demand and inelastic supply / 需求与供给均缺乏弹性 | Small deadweight loss because quantity changes little / 无谓损失小，因为数量变化很小 |
更多咨询请联系16621398022（同微信）
CommentsMore posts |
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导