JL · AP MICROECONOMICS

Unit 6 · Market Failure and the Role of Government

市场失灵与政府的作用 — 社会效率、外部性、公共物品与公共资源、从量税与补贴、不同市场结构中的价格管制、反垄断、收入与财富不平等 · 对应 CED 6.1–6.5 · 选择题占比约 8–13%

这一单元在问什么前五个单元告诉我们：完全竞争市场在没有“副作用”时能把资源配置到最有效率的地方。这一单元问反过来的问题：什么时候市场做不到？政府能做什么？做了一定更好吗？
上一单元：→ 打开 Unit 5 Factor Markets（与本文件放在同一文件夹）

从前面带过来的工具

消费者剩余、生产者剩余、无谓损失，以及从量税、价格上限和下限（Unit 2 的 2.6、2.8）。复习：Unit 2 Supply and Demand。
边际分析：边际收益等于边际成本时最好（Unit 1）。这一单元把它升级为“社会的边际收益 = 社会的边际成本”。
垄断的 P > MC、无谓损失，以及自然垄断的管制（Unit 4 上册 4.2）：Unit 4（上）。
MRP（Unit 5）：每个要素得到它的边际产品价值——这也是收入差别的来源之一（6.5）。

本单元学习目标 LEARNING OBJECTIVES

说明社会最优数量在 MSB = MSC 处，并列出市场失灵的几种来源 · Define social efficiency and identify sources of market failure
画出正、负外部性图（生产或消费），找出市场数量、社会最优数量和无谓损失 · Graph externalities and deadweight loss
分析纠正外部性的政策：庇古税与补贴、管制、产权、排放许可交易 · Explain policies that correct externalities
按“竞争性”和“排他性”给物品分类，解释搭便车问题和公地悲剧 · Classify goods; explain free riders and the tragedy of the commons
计算从量税和补贴对价格、数量、剩余、税收和无谓损失的影响，并说明弹性决定税负归宿 · Analyze per-unit taxes and subsidies
比较价格管制在完全竞争、垄断和买方垄断中的不同效果；说明自然垄断的补贴与反垄断政策 · Analyze price controls across market structures
读洛伦兹曲线和基尼系数，区分累进、累退和比例税，列出收入和财富不平等的来源 · Describe measures and sources of inequality

Term 术语	Meaning 含义	中文
Social Efficiency	Quantity where marginal social benefit equals marginal social cost	社会效率
Market Failure	The market outcome is not socially efficient	市场失灵
Externality (positive / negative)	A benefit or cost that falls on people outside the transaction	外部性（正 / 负）
MSB / MPB / MEB	Marginal social benefit = marginal private benefit + marginal external benefit	边际社会 / 私人 / 外部收益
MSC / MPC / MEC	Marginal social cost = marginal private cost + marginal external cost	边际社会 / 私人 / 外部成本
Pigouvian Tax / Subsidy	Per-unit tax (subsidy) equal to the marginal external cost (benefit)	庇古税 / 庇古补贴
Rival / Excludable	One person's use reduces others' use / people who do not pay can be kept out	竞争性 / 排他性
Public Good / Common Resource	Non-rival and non-excludable / rival but non-excludable	公共物品 / 公共资源
Free Rider	Someone who benefits without paying	搭便车者
Tax Incidence	How the burden of a tax is split between buyers and sellers	税负归宿
Antitrust Policy	Laws that promote competition (block mergers, ban price fixing, break up monopolies)	反垄断政策
Lorenz Curve / Gini Coefficient	Graph / number (0 to 1) showing how unequally income or wealth is distributed	洛伦兹曲线 / 基尼系数
Progressive / Regressive / Proportional Tax	Tax rate rises / falls / stays the same as income rises	累进 / 累退 / 比例税

考试怎么考选择题：外部性图上读市场数量、社会最优数量和 DWL；该征税还是补贴、税额是多少；物品分类和搭便车；从量税后买方付的价格、卖方拿到的价格、税收和 DWL；价格上限对垄断的效果；洛伦兹曲线和累进税。 FRQ：几乎每年都有一道外部性题（画 MPB、MSB、MPC、MSC，标 Q_M、Q_S，涂 DWL，求矫正税或补贴），常和垄断、价格管制、要素市场组合在一起。

6.1Socially Efficient and Inefficient Market Outcomes 社会效率与市场失灵

The Optimal Quantity 最优数量：MSB = MSC

定义 DEFINITION（CED POL-2.A.1–A.2） Social efficiency 社会效率：生产的数量使最后一单位的边际社会收益（MSB）= 边际社会成本（MSC），此时总剩余最大。
只有当所有收益和成本都由市场里的买卖双方承担（“内部化”）时，市场均衡数量才等于社会最优数量。

点按钮比较三个数量：

① 刚好：Q = 9：需求反映买方的边际收益，供给反映卖方的边际成本。在 Q = 9 处 MB = MC = $11：再多一个，成本超过收益；少一个，就放弃了收益大于成本的交易。总剩余（CS + PS）= ½ × 18 × 9 = 81，最大。

② 太少：Q = 6：只生产 6 个：第 7–9 个本来 MB > MC（第 6 个处 MB = $14 > MC = $8），却没有生产。损失的剩余 = 红色三角形 = ½ × 6 × 3 = 9。

③ 太多：Q = 12：生产 12 个：第 10–12 个 MC > MB（第 12 个处 MC = $14 > MB = $8），每多一个都在“亏”。多出来的损失也是一个三角形 = ½ × 6 × 3 = 9。

一句话生产任何不是社会最优的数量——无论太多还是太少——都会产生无谓损失（CED POL-2.C.2）。

WORKED EXAMPLE 1要不要把河里的污染清到 100%？原创

A city is deciding how much of a polluted river to clean up. Cleanup is done in six equal stages. The marginal social benefit of each stage (less harm to residents, in ten-thousands of yuan) is 90, 70, 50, 30, 15, 5, and the marginal social cost is 10, 20, 35, 50, 70, 95. (a) How many stages should the city clean up? Explain using marginal analysis. (b) What is the net benefit at that level? (c) A resident says, "Any pollution is too much, so we should clean up all six stages." Evaluate this claim.

(a) 第 1–3 段 MSB ≥ MSC（90 > 10，70 > 20，50 > 35），值得清；第 4 段 MSB 30 < MSC 50，不值得 → 3 段（CED POL-2.B.3：决策者用成本收益分析评估政策）。

(b) 净收益 = (90 + 70 + 50) − (10 + 20 + 35) = 210 − 65 = 145（万元）。

(c) 不对。清第 4–6 段一共要多花 50 + 70 + 95 = 215，只带来 30 + 15 + 5 = 50 的好处，净损失 165。这些钱用在别处（学校、医院）能带来更多好处。社会最优不是零污染，而是 MSB = MSC 的那一点。

越往后，治理越难（成本越来越高），带来的好处越来越少。清到第 3 段时 MSB = 50 ≥ MSC = 35；第 4 段起 MSB < MSC——最优的污染并不是零。

Sources of Market Failure 市场失灵的来源（CED POL-2.B–C）

市场里的每个人都很理性：比较自己的边际收益和边际成本（POL-2.B.2），有市场势力就会用它（POL-2.B.1）。问题在于，个人的最优不一定是社会的最优。

👑

市场势力：垄断、寡头、垄断竞争都在 P > MC 处生产，产量太少（Unit 4）。

🏭

外部性：生产或消费影响了交易以外的人，而当事人不考虑这部分成本或收益（6.2）。

🎆

公共物品：无法排除不付钱的人，私人企业不愿提供，供给不足（6.3）。

🚗

信息不对称：一方比另一方知道得多。二手车卖家知道车的毛病，买家不知道，只愿意出“平均价”，好车车主不愿卖——好车的交易就没有发生。

WORKED EXAMPLE 2这里为什么“失灵”了？原创

Identify the source of market failure in each case and state whether the market produces too much or too little compared with the socially efficient quantity.

The only water company in a city charges a price far above its marginal cost.
A chemical plant dumps waste into a river, harming fishing villages downstream.
Many students would not get a flu vaccine on their own, although each vaccinated student protects classmates.
A town wants a fireworks show, but anyone nearby can watch it for free, so no private firm will put it on.
Sellers of used cars know much more about the cars' problems than buyers do, so many good used cars never get sold.

	市场失灵的来源	结果
1	垄断（市场势力，P > MC）	太少
2	负外部性（生产）	太多
3	正外部性（消费）	太少
4	公共物品（搭便车）	太少（甚至没有）
5	信息不对称	太少（好车卖不出去）

共同点：每个人都按自己的边际收益和边际成本做决定（CED POL-2.B.2），而社会关心的是所有人的边际收益和边际成本。两者不一致时，市场数量就偏离了社会最优数量，产生无谓损失。

政策的目标（CED POL-2.B.4）设计政策，让市场在 MSB = MSC 的数量上运行：对外部性用税或补贴把外部成本、收益“内部化”；对垄断用价格管制或反垄断；对公共物品由政府提供。6.2–6.4 一一展开。

练习 Practice 6.1 · 市场效率与市场失灵

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.1-12021 AP Micro 国际卷 Q55

A perfectly competitive market produces the socially efficient level of output when the marginal social benefit of the last unit of output produced is

equal to the marginal social cost
less than the marginal social cost
zero
greater than the total producer surplus
equal to the total producer surplus

完全竞争市场在最后一单位的 MSB = MSC 时生产社会有效的产量。

6.1-22024 AP Micro Q18

For the last unit produced, allocative efficiency is achieved when

price is equal to marginal cost
price is equal to average total cost
price is equal to marginal revenue
marginal cost is equal to average total cost
marginal cost is equal to average variable cost

配置效率：最后一单位的价格（买方的边际收益）等于边际成本，P = MC。(B) P = ATC 是零经济利润；(D) MC = ATC 是 ATC 最低点（生产效率）。

6.1-32023 AP Micro 国际卷 Q2

The allocatively efficient level of output is produced in any market structure when

firms are experiencing economies of scale
marginal revenue is equal to zero
long-run economic profit is equal to zero
price is equal to marginal cost
average variable cost is at a minimum

不管哪种市场结构，只要 P = MC 就达到配置效率。(C) 长期零利润的垄断竞争企业 P = ATC > MC，仍然不是配置效率。

6.1-42000 AP Micro Q12

Monopolies are inefficient compared to perfectly competitive firms because monopolies

produce output with average total cost exceeding average revenue
produce more output than is socially desirable
charge a price less than marginal revenue
charge a price greater than marginal cost
charge a price less than average total cost

垄断企业在 MR = MC 处生产，再从需求曲线读价格 → P > MC：最后一单位对买方的价值高于它的成本，产量不足，有 DWL。(A) ATC 高于平均收益是亏损，不是低效率的原因。

6.1-52005 AP Micro Q12

Generally, monopolies are considered inefficient because they

produce at a point where marginal cost is less than marginal revenue
produce at a point where marginal cost exceeds price
produce more output than does a competitive industry with similar cost conditions
lead to an overallocation of resources in the affected market
lead to an underallocation of resources in the affected market

P > MC → 产量少于有效数量 → 资源配置不足（underallocation）。(C)(D) 方向反了；(A) MC < MR 只说明还没到利润最大。

6.1-62012 AP Micro Q44

Most economists argue that a monopoly is inefficient because it

has no incentive to minimize its costs
produces too little output and sets a price above marginal cost
earns too much profit by charging consumers any price it wants
produces too much output and thus wastes scarce resources
usually produces unsafe products if not regulated by government

垄断产量太少、价格高于 MC，造成 DWL。经济学家批评的是配置效率，不是“利润太高”(C) 本身。

6.1-72023 AP Micro 国际卷 Q7

Apples are produced in a perfectly competitive market with no government intervention. Which of the following price changes will cause the total economic surplus to increase for the apple market depicted in the graph provided?

按原题重绘

A decrease in price from $8 to $6
A decrease in price from $6 to $4
A decrease in price from $6 to $0
An increase in price from $6 to $8
An increase in price from $8 to $12

均衡在 $6、数量 6，这时总剩余最大。价格 $8 时 Qd = 4 < Qs = 8，只能成交 4 个，有 DWL；降到 $6，成交增加到 6，DWL 消失 → 总剩余增加。(B)(C) 把价格压到 $6 以下，Qs 减少，成交量下降；(D)(E) 把价格抬到 $6 以上，Qd 减少。

6.2Externalities 外部性

定义 DEFINITION（CED POL-3.A） Externality 外部性：生产或消费给交易以外的第三方带来的成本（负外部性）或收益（正外部性）。
外部性的根源是产权不清楚（河水归谁？安静归谁？）和/或交易成本太高（成千上万的受影响者没法一一谈判）。理性的人只对私人成本和收益作出反应，不考虑外部成本和收益（POL-3.A.3）。 MSC = MPC + MEC　　MSB = MPB + MEB 社会最优数量在 MSB = MSC 处（POL-3.A.1）；市场数量在 MPB = MPC 处。两者不同，就有无谓损失。

Negative Externalities 负外部性

点按钮，一步一步把“社会成本”加到图上：

① 市场均衡：印染厂只看自己的成本（MPC，就是供给），买方只看自己的收益（MPB，就是需求）：市场在 Q_M = 9、P = $11 成交。

② 加上外部成本：每生产一单位，下游村民承担 $6 的损害（边际外部成本 MEC）。社会成本 MSC = MPC + MEC，比 MPC 高出 $6。

③ 社会最优：社会最优在 MSB = MSC：Q_S = 6、P = $14。市场生产得太多（9 > 6）、价格太低。

④ 无谓损失：第 7–9 单位的 MSC 高于 MSB，每多生产一个都让社会损失。DWL = ½ × (17 − 11) × (9 − 6) = 9，三角形的尖指向社会最优点。

⑤ 庇古税：对每单位征税 $6（= MEC）：企业的成本变成 MPC + 6 = MSC，市场自动在 Q = 6 成交。买方付 $14，卖方实得 $8，政府收税 6 × 6 = 36，DWL 消失——外部成本被“内部化”了。

WORKED EXAMPLE 3河上游的印染厂原创

In the perfectly competitive market for dyed fabric, demand is P = 20 − Q and supply (marginal private cost) is P = 2 + Q, where Q is thousands of meters. Dye factories dump waste into a river, causing $6 of damage to villages downstream for every meter produced. (a) Find the market equilibrium quantity and price. (b) Find the socially optimal quantity. (c) Calculate the deadweight loss at the market equilibrium. (d) The government imposes a per-unit tax equal to the marginal external cost. Find the tax, the price buyers pay, the price sellers keep, and the tax revenue. (e) Would a lump-sum tax on each factory achieve the same result? Explain.

(a) MPB = MPC：20 − Q = 2 + Q → Q_M = 9，P = $11。

(b) MSC = MPC + 6 = 8 + Q；MSB = MSC：20 − Q = 8 + Q → Q_S = 6。

(c) 第 6 到第 9 单位：MSC 从 $14 升到 $17，MSB 从 $14 降到 $11。DWL = ½ × (17 − 11) × (9 − 6) = 9（千元）。

(d) 税 = $6。新的供给 P = 8 + Q，与需求交于 Q = 6：买方付 $14，卖方实得 14 − 6 = $8，税收 = 6 × 6 = 36（千元）。

(e) 不能。定额税不随产量变化，不改变 MPC（边际成本），供给曲线不动，产量仍是 9（CED POL-4.A.2）。它只会减少工厂的利润，长期可能让一些工厂退出。

Positive Externalities 正外部性

正外部性（消费）：每个接种的人让身边的人少得病，边际外部收益 MEB = $4，MSB 比 MPB 高 $4。市场在 Q_M = 9，社会最优在 MSB = MSC 的 Q_S = 11——市场太少。DWL = ½ × 4 × 2 = 4，三角形同样指向社会最优点。

WORKED EXAMPLE 4校园流感疫苗：补贴多少才对？原创

In a city's market for flu shots, the marginal private benefit is P = 20 − Q and the marginal cost is P = 2 + Q (Q in thousands of shots; no external cost). Each shot also gives a $4 benefit to other people who are less likely to catch the flu. (a) Is the market quantity too high or too low? (b) Find the socially optimal quantity. (c) Calculate the deadweight loss at the market quantity. (d) The government gives buyers a $4 per-unit subsidy. Find the new quantity, the price sellers receive, the net price buyers pay, and the total cost of the subsidy.

(a) 正外部性（消费）：买的人只考虑自己的好处，太少。市场 Q_M = 9。

(b) MSB = 24 − Q；MSB = MSC：24 − Q = 2 + Q → Q_S = 11。

(c) DWL = ½ × (15 − 11) × (11 − 9) = 4（千元）。

(d) 补贴让需求上移 $4，变成 24 − Q：Q = 11，卖方拿到 $13，买方自己只付 13 − 4 = $9；补贴总额 = 4 × 11 = 44（千元）。

生产和消费都可能产生正、负外部性，一共四种。点按钮比较：

负外部性（生产）：印染厂排污：MSC 在 MPC 上方，市场生产太多（Q_M > Q_S）。

正外部性（生产）：养蜂人的蜜蜂顺便给周围果园授粉：MSC 在 MPC 下方（社会成本比私人成本低），市场生产太少。

负外部性（消费）：深夜在小区外放音乐：MSB 在 MPB 下方（别人受害），市场消费太多。

正外部性（消费）：接种疫苗，也保护了身边的人：MSB 在 MPB 上方，市场消费太少。

	谁的曲线“分家”	社会曲线在哪	市场数量 vs 社会最优	矫正
负外部性（生产）	成本：MSC ≠ MPC	MSC 在 MPC 上方	太多	每单位税 = MEC
正外部性（生产）	成本：MSC ≠ MPC	MSC 在 MPC 下方	太少	每单位补贴 = MEB
负外部性（消费）	收益：MSB ≠ MPB	MSB 在 MPB 下方	太多	每单位税 = MEC
正外部性（消费）	收益：MSB ≠ MPB	MSB 在 MPB 上方	太少	每单位补贴 = MEB

易错 TRAP

生产外部性移动的是成本那条线（MSC 与 MPC 分开），消费外部性移动的是收益那条线（MSB 与 MPB 分开）。
DWL 三角形在 Q_M 和 Q_S 之间、两条社会曲线（或社会曲线与私人曲线）之间，尖角指向社会最优点。
市场均衡价格读的是私人曲线（MPB = MPC）；社会最优读的是社会曲线（MSB = MSC）。

练习 Practice 6.2（一）· 认识外部性

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.2-12000 AP Micro Q59

Which of the following is the best example of a negative externality?

An increase in the price of oil due to the imposition of environmental regulations
An increase in the price of oil due to action taken by the Organization of Petroleum Exporting Countries (OPEC)
A decline in oil stock prices as a result of bad management
Oil leakages from drilling platforms in the Gulf of Mexico
Declining restrictions on the importation of foreign-made cars

漏油让渔民、海边居民承担了成本，他们不是交易的一方 → 负外部性。(A)(B)(C)(E) 都是通过市场价格传递的变化，不是外部性。

6.2-22010 AP Micro Q29

Which of the following would create a positive externality?

Congestion on city highways
Water pollution
A neighbor's loud music that wakes you up
Flu vaccination
The greenhouse effect and global warming

打疫苗的人也降低了别人被传染的机会 → 交易之外的人得到收益 → 正外部性。其他四个都是负外部性。

6.2-32013 AP Micro Q14

Which of the following best represents a positive externality?

Purchasing a pass for an amusement park
Dining at a favorite restaurant
Being disturbed by neighboring construction noise
Enjoying watching birds at a neighbor’s bird feeder
Dumping waste on someone else’s property

邻居的喂鸟器让你免费看鸟 → 交易之外的人得到收益 → 正外部性。(C)(E) 是负外部性；(A)(B) 是普通的私人交易。

6.2-42025 AP Micro 国际卷 Q29

Which of the following is an example of a negative externality in production?

An electronics store decreases the price of televisions during the holidays.
A car insurance company charges a higher price for teenage drivers.
A local paper mill emits an odor, imposing additional costs on nearby residents.
A pine tree in a public park provides pleasant shade for residents.
A photographer accepts a price for work below marginal cost.

造纸厂的气味让附近居民承担成本，造纸厂不付钱 → 生产的负外部性。(D) 是正外部性；(A)(B)(E) 是企业的定价行为。

6.2-52023 AP Micro 国际卷 Q5

Apple trees need bees to pollinate their flowers and produce apples. Apple farmers hire beekeepers to bring bees to their orchards to pollinate the flowers. This is an example of a

negative externality
tragedy of the commons
public good
market transaction
free-rider problem

果农付钱请养蜂人授粉，授粉这件事已经在市场上买卖了 → 是市场交易，不是外部性。如果是邻居的蜜蜂顺便给果园授粉、果农没付钱，那才是正外部性。

6.2-62000 AP Micro Q30

Whenever the production of a good creates negative externalities, an unregulated market will result in

too little of the good being produced
an optimal amount of the good being produced as long as the market is perfectly competitive
society's marginal cost being higher than the firm's marginal cost
the firm's marginal cost being higher than society's marginal cost
the firm's marginal cost being equal to society's marginal cost

负外部性：MSC = MPC + 外部成本 → 社会的边际成本高于企业的边际成本。市场会生产过多，不是过少 (A)；完全竞争也解决不了 (B)。

6.2-72013 AP Micro Q30

Which of the following is true when there are negative externalities associated with the production of a good?

The market will adjust automatically to equate marginal social costs and marginal social benefits.
Marginal social costs will exceed marginal private costs unless businesses are forced to internalize the external costs.
Marginal private costs will exceed marginal social costs, but the government can correct the problem.
Producers should be subsidized so that they will produce more of the good.
Consumers should be subsidized so that they will consume more of the good.

生产有负外部性时，除非企业被迫把外部成本内部化（比如征庇古税），否则 MSC > MPC。市场已经生产过多，再补贴 (D)(E) 会更多。

6.2-82022 AP Micro 国际卷 Q39

Which of the following is true if the production of a good creates negative externalities?

The market produces the socially efficient level of output.
Antitrust laws are violated.
The market price equals the marginal social cost.
The marginal social cost exceeds the marginal private cost.
The good is produced by the public sector.

生产有负外部性 → MSC > MPC。市场产量大于社会最优 (A 错)；市场价格等于 MPC，不等于 MSC (C 错)。

6.2-92019 AP Micro 练习卷 Q44

In a competitive market in which the production of a good causes pollution, the socially optimal output is different from the competitive market equilibrium output of that good because the

marginal social benefit is higher than the marginal social cost
marginal social benefit is lower than the marginal private cost
marginal social cost is higher than the marginal private cost
marginal social benefit is higher than the marginal private benefit
total social cost is less than the total social benefit

污染使 MSC > MPC：市场在 MPB = MPC 处生产，社会最优在 MSB = MSC 处，两个数量因此不同。

6.2-102005 AP Micro Q60

An industry will produce more than the socially efficient level of output under which of the following conditions?

The production or consumption of a good generates a positive externality.
The production or consumption of a good generates a negative externality.
The industry is a monopoly.
The industry produces a public good.
The industry produces a private good.

负外部性 → MSC > MPC（或 MSB < MPB）→ 市场产量大于有效产量。正外部性、垄断、公共物品都会导致产量不足。

6.2-112015 AP Micro 练习卷 Q29

If the production of a good generates a negative externality, which of the following is true at the private market equilibrium?

The private market equilibrium quantity is greater than the socially optimal quantity.
The private market equilibrium quantity is equal to the socially optimal quantity.
The marginal private cost is greater than the marginal social cost.
The marginal social benefit is greater than the marginal social cost.
The price of the product equals the marginal social cost.

负外部性：市场在 MPC = MB 处生产，超过了 MSC = MB 的数量 → 市场均衡数量大于社会最优数量。(C) 方向反了。

6.2-122016 AP Micro 练习卷 Q44

Assume that the firms in an industry pollute a river. If there is no government intervention, the firms will

produce more output than is socially efficient
pay production costs that are higher than actual social costs
charge a lower price than is necessary to maximize profit
charge a higher price to pay for the pollution
increase production to achieve allocative efficiency

企业不为河流污染付费，只看 MPC（< MSC）→ 产量大于社会有效产量。(B) 企业承担的成本低于社会成本，不是更高。

6.2-132014 AP Micro 练习卷 Q44

A perfectly competitive manufacturing industry pollutes public water in its production process, leaving the water unsuitable for use by the surrounding communities. At the market equilibrium output level, which of the following is true?

Marginal private cost exceeds marginal social cost.
Marginal private cost exceeds marginal private benefit.
Marginal social cost exceeds marginal social benefit.
The market equilibrium output is equal to the socially efficient output.
The market equilibrium output is less than the socially efficient output.

市场均衡处 MPC = MPB = MSB，而 MSC = MPC + 外部成本 > MPC → 在这个产量上 MSC > MSB（生产过多）。

6.2-142005 AP Micro Q45

The surgeon general has determined that smoking causes cancer and heart disease for both smokers and passive smokers, nonsmokers who breathe smoke-filled air. If cigarette prices are determined in a free market, which of the following will be true?

The price of cigarettes will be too low and the quantity sold will be too high.
The price of cigarettes will be efficient but the quantity sold will be too high.
The market will be efficient because markets always equate marginal benefits and marginal costs.
The price of cigarettes will reflect the marginal social benefit received by nonsmokers.
The price of cigarettes will overstate the true social cost imposed on nonsmokers.

二手烟是负外部性：市场价格只反映私人成本 → 价格太低、数量太多。(C) 市场只做到 MPB = MPC，做不到 MSB = MSC。

6.2-152021 AP Micro 国际卷 Q23

Which of the following is always true about a negative externality?

It can be corrected by the government subsidizing the good or activity that creates the externality.
It results in a market output that is smaller than the socially optimal output.
It results in a market output that is greater than the socially optimal output.
It results in a larger deadweight loss than a positive externality does.
It results in a smaller deadweight loss than a positive externality does.

负外部性一定使市场产量大于社会最优产量。DWL 有多大取决于具体曲线，不能说一定比正外部性大或小 (D)(E)；补贴 (A) 会让情况更糟。

6.2-162016 AP Micro 练习卷 Q14

If the production of good X creates a positive externality, then from society’s perspective, the industry that produces good X tends to produce

less than the socially optimal amount of good X
greater than the socially optimal amount of good X
the socially optimal amount of good X
a surplus of good X, unless subsidized
a shortage of good X, unless taxed

生产有正外部性：生产者拿不到外部收益 → 市场产量少于社会最优产量。

6.2-172017 AP Micro 练习卷 Q46

Good X is produced in a perfectly competitive industry in long-run equilibrium. If the consumption of good X generates positive externalities, the private market will produce

less than the socially optimum amount of good X
more than the socially optimum amount of good X
the socially optimum amount of good X
good X only if the government grants a subsidy for the production of good X
good X only if the government levies a corrective tax on the production of good X

消费有正外部性：MSB > MPB，市场在 MPB = MPC 处生产，少于 MSB = MSC 的数量 → 生产不足。(D) 没有补贴市场也会生产，只是生产得不够。

6.2-182015 AP Micro 练习卷 Q14

When consumption of a good generates a positive externality, which of the following must be true at the market equilibrium?

Marginal social benefit is less than marginal private cost.
Marginal social benefit is greater than marginal private benefit.
Marginal social cost is greater than marginal social benefit.
Marginal social cost is less than marginal private benefit.
Marginal social cost is equal to marginal external benefit.

消费的正外部性：MSB = MPB + 外部收益，在任何数量上（包括市场均衡）都有 MSB > MPB。

LAB 1外部性与矫正 · Externalities and the Fix

市场：需求（MPB）P = 20 − Q，供给（MPC）P = 2 + Q。选一种外部性，拖动每单位的外部成本或外部收益，再试试用税或补贴把它“内部化”。

每单位外部成本 / 收益 = $6

Fixing Externalities 纠正外部性的办法（CED POL-3.B.1）

🧾

税与补贴（庇古税、庇古补贴）：每单位税 = 边际外部成本，每单位补贴 = 边际外部收益，把外部影响算进私人决策。

📏

环境管制：直接规定排放上限、技术标准，或者禁止某种做法。简单直接，但不一定用最低成本达到目标。

🏫

公共提供：政府直接提供有正外部性的东西，比如免费疫苗、公立教育。

📜

界定产权：把“河水归谁”“安静归谁”说清楚，受害者就能索赔或谈判。

🤝

私人交易重新分配产权：当事人少、交易成本低时，双方可以自己谈；可交易的排放许可也是一种——减排便宜的企业多减排，把多余的许可卖给减排贵的企业。

易错 TRAP · 哪些办法“不管用” 定额税（lump-sum tax）不改变边际成本，产量不变，纠正不了外部性（2021、2023 年 FRQ 都考过）。价格上限、价格下限一般也纠正不了外部性。

WORKED EXAMPLE 5楼上邻居半夜练鼓：谁有权利，结果一样吗？原创

Lin practices the drums at night in her apartment. Drumming is worth $50 a night to her, and the noise costs her downstairs neighbor Chen $80 a night. They can bargain at no cost. (a) If the building rules give Chen the right to quiet, will Lin drum? (b) If the rules instead give Lin the right to drum, what will happen? (c) Is the outcome efficient in both cases? (d) Why does this kind of private solution rarely work for air pollution from a factory?

(a) Lin 最多愿意付 $50 换取练鼓，Chen 至少要 $80 才肯忍受 → 谈不拢，不练鼓。

(b) Chen 愿意付 $50 到 $80 之间的钱请 Lin 停练，Lin 收到 $50 以上就愿意停 → 谈成，也不练鼓（只是钱从 Chen 流向 Lin）。

(c) 两种情况都不练鼓：练鼓的收益 $50 < 损害 $80，不练才有效率。只要产权明确、交易成本低，私人谈判就能解决外部性，产权给谁只影响钱怎么分（这就是“科斯定理”的思路，CED POL-3.A.2、POL-3.B.1）。

(d) 受害的是成千上万的居民，人人谈判的成本极高，而且谁该负多少责任难以界定——交易成本太高，所以需要政府用税、管制或排放许可。

练习 Practice 6.2（二）· 外部性的图与纠正办法

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.2-192000 AP Micro Q15

A chemical plant pollutes a river that serves as the water supply for a nearby town. From an economist's point of view, pollution from the plant should be reduced until the

marginal benefit from cleaner water is equal to the marginal cost of making the water cleaner
marginal benefit from cleaner water is maximized
marginal benefit from cleaner water is zero
total benefit from cleaner water is equal to the total cost of making the water cleaner
total benefit from cleaner water is maximized

减少污染要减到边际收益 = 边际成本为止：再多减一点，花的成本就高于带来的好处。比的是边际，不是总量 (D)，也不是让收益最大 (B)(E) 或为零 (C)。

6.2-202010 AP Micro Q44

Under an efficient policy to address the problem of pollution created from manufacturing good X, which of the following is true?

Pollution is completely eliminated.
The marginal private cost of producing good X exceeds the marginal revenue from its sale.
The marginal private cost of producing good X equals the marginal revenue from its sale.
The marginal social benefit of pollution reduction exceeds the marginal social cost of pollution reduction.
The marginal social benefit of pollution reduction equals the marginal social cost of pollution reduction.

有效的污染政策让减污的边际社会收益 = 减污的边际社会成本。通常不会把污染完全消除 (A)——最后那一点污染清除起来太贵。

6.2-212012 AP Micro Q14

A per-unit tax on pollution produced by a firm will affect the firm’s output and pollution levels in which of the following ways?

Output: Increase；Pollution: Increase
Output: Increase；Pollution: Decrease
Output: Decrease；Pollution: Increase
Output: Decrease；Pollution: Decrease
Output: No change；Pollution: No change

按污染量征的每单位税提高了企业的边际成本 → 企业减少产量，污染也减少。

6.2-222014 AP Micro 练习卷 Q45

Assume that under new environmental regulations, tire companies have to pay taxes based on how much pollution is created when producing each tire. The price paid by consumers and the quantity of tires sold will most likely change in which of the following ways?

Price: Increase；Quantity: Decrease
Price: Increase；Quantity: Increase
Price: Decrease；Quantity: Increase
Price: Decrease；Quantity: Decrease
Price: No change；Quantity: No change

按每个轮胎的污染征税 = 每单位税 → 供给左移 → 消费者付的价格上升，数量减少。

6.2-232013 AP Micro Q45

Suppose that at the current output level the marginal social cost of producing a good is greater than its marginal private cost, and that the marginal social benefit associated with production of the good is equal to its marginal private benefit. Which of the following government actions would most likely increase efficiency?

Taxing the production of the good
Taxing the production of substitute goods
Giving tax rebates to people who purchase the good
Subsidizing production of complementary goods
Subsidizing consumption of the good

MSC > MPC 而 MSB = MPB：生产的负外部性，市场生产过多 → 对生产征税。(B)(D) 会增加这种商品的需求；(C)(E) 鼓励消费，更糟。

6.2-242010 AP Micro Q59

Which of the following is a government policy that is intended to address a market failure?

Taxing the use of toxic pesticides by farmers
Fining firms that outsource jobs to lower labor costs
Subsidizing corn farmers who face increasing production costs
Taxing short-run profits in competitive markets
Subsidizing entry of firms into a market with an existing natural monopoly

有毒农药伤害第三方（负外部性），对它征税是针对市场失灵的政策。(B)(C)(D) 都不是在纠正市场失灵；(E) 自然垄断由一家生产成本最低，补贴新企业进入反而提高成本。

6.2-252000 AP Micro Q57

In which of the following cases is the government's action appropriate for reducing inefficiency?

Taxation of the output of a chocolate factory that emits an aroma that residents of the city enjoy
Regulation that reduces the output of a monopoly
A lump-sum tax on a monopolist
A subsidy for the consumption of a good produced by a plant that emits a pollutant as a by-product
A toll on a congested bridge

拥堵的桥上每多一辆车都会拖慢其他人（负外部性），收过路费让司机承担这部分成本，车流量降到有效水平 → (E)。(A) 香味是正外部性，征税更糟；(B) 垄断已经产量不足，再限产更糟；(C) 定额税不改变边际决策；(D) 补贴有污染的商品更糟。

6.2-262010 AP Micro Q15

To correct for positive externalities, the government should

do nothing, since no harm is done by positive externalities
levy a tax on the output of the good or service
pay a subsidy equal to the marginal external benefit
impose a price ceiling on the good to discourage its production
impose a price floor on the good at which the marginal private benefit equals the marginal social cost

正外部性 → 市场生产不足 → 给每单位补贴，金额 = 边际外部收益，把 MPB 抬到 MSB。

6.2-272012 AP Micro Q29

If the production of a good generates a positive externality, the government can increase allocative efficiency by

taxing the producer of the good
subsidizing the producer of the good
prosecuting firms that produce the good without proper permits
setting a price ceiling to encourage production of the good
setting a price floor to discourage production of the good

生产有正外部性 → 生产不足 → 补贴生产者，让产量增加到有效水平。(D) 价格上限会减少供给量，不会鼓励生产。

6.2-282014 AP Micro 练习卷 Q59

Assume that the marginal social benefit of the last unit of vaccination provided is greater than the marginal social cost. Which of the following can be used to achieve efficiency in the market for vaccination?

A per-unit subsidy for vaccinations
A lump-sum tax for vaccinations
A limit on the quantity of vaccinations
A price floor for vaccinations
A price ceiling for vaccinations

最后一剂疫苗 MSB > MSC → 数量不足 → 用每单位补贴增加数量。(B) 定额税不改变边际决策；(C) 限量只会更少；(D)(E) 价格管制纠正不了外部性。

6.2-292023 AP Micro 国际卷 Q14

The use of environmentally friendly drinking straws has benefits to society beyond those enjoyed by their direct users. As a result, a government with the goal of maximizing total economic surplus in society would have an incentive to

allow the free market to determine the equilibrium price and quantity of environmentally friendly drinking straws
implement a binding price ceiling on environmentally friendly drinking straws
implement a binding price floor on environmentally friendly drinking straws
implement a per-unit tax on consumers of environmentally friendly drinking straws
implement a per-unit subsidy to consumers of environmentally friendly drinking straws

环保吸管的使用有正外部性（MSB > MPB），市场消费不足 → 给消费者每单位补贴，把 MPB 抬到 MSB。

6.2-302023 AP Micro 国际卷 Q50

Which of the following is most likely true if textile production generates pollution during the production process?

The market will underproduce textiles relative to the socially optimal quantity, and the government would need to provide a lump-sum subsidy to firms to correct the externality.
The market will underproduce textiles relative to the socially optimal quantity, and the government would need to provide a per-unit subsidy to firms to correct the externality.
The market will overproduce textiles relative to the socially optimal quantity, and the government would need to impose a lump-sum tax on firms to correct the externality.
The market will overproduce textiles relative to the socially optimal quantity, and the government would need to impose a per-unit tax on firms to correct the externality.
The market will produce the socially optimal quantity of textiles, and the government would not need to intervene in the market to influence the production decisions of firms.

污染 → 负外部性 → 市场生产过多；纠正要用每单位税（= 边际外部成本）。定额税 (C) 不改变边际成本，产量不变。

6.2-312018 AP Micro 练习卷 Q44

In a perfectly competitive market for a good with a downward sloping demand curve and an upward sloping supply curve, the marginal social benefit is greater than the marginal social cost at the market equilibrium quantity. The government imposition of a new per-unit tax on the production of the good would

have no effect on the price of the good
increase consumer surplus
increase the deadweight loss
increase producer surplus
increase the quantity sold of the good

在市场数量上 MSB > MSC，说明市场已经生产不足（正外部性）；再征每单位税，产量更少，离最优更远 → DWL 增加。

6.2-322016 AP Micro 练习卷 Q60

按原题重绘

Which of the following is true based on the graph above?

Quantity of education in the private market is more than the socially optimal quantity.
Consumers of education could be subsidized in order to achieve the socially optimal quantity.
Producers of education could be taxed in order to achieve the socially optimal quantity.
Consumers of education could be taxed in order to achieve the socially optimal quantity.
Consumption of education has a negative externality.

MSB 在 MPB 上方 → 消费教育有正外部性 → 市场数量（MPB 与成本线的交点）少于社会最优（MSB 与成本线的交点）。给消费者每单位补贴，把 MPB 抬到 MSB → (B)。征税只会更少。

6.2-332021 AP Micro 国际卷 Q30

The diagram provided illustrates the marginal private benefit (MPB), marginal social benefit (MSB), marginal private cost (MPC), and marginal social cost (MSC) measured in dollars for a competitive market.

按原题重绘

Assume that the goal of the government is to increase efficiency in the market. The cost of a government intervention that increases market output from Q₁ to Q₂ is $1.2 million. The intervention will definitely improve market efficiency if the area of

A is greater than $1.2 million
B is less than $1.2 million
C is less than $1.2 million
A + B is greater than or equal to $1.2 million
B + C is greater than or equal to $1.2 million

图中 MSB 在 MPB 上方：消费的正外部性。产量从 Q₁ 增加到 Q₂，社会多得到的收益 = MSB 下面的面积 A + B + C，多付的社会成本 = MSC 下面的面积 B + C，净增加 = A（正好是原来的 DWL）。政策要花 $1.2 million，只有 A > $1.2 million 才一定提高效率。

6.2-342012 AP Micro Q59

Questions 59-60 refer to the diagram below, which shows a market’s cost and revenue curves.

按原题重绘

The socially optimal quantity and the per-unit tax that will achieve the socially optimal quantity are which of the following?

Socially Optimal Quantity: Q₁；Per-Unit Tax: P₄ − P₂
Socially Optimal Quantity: Q₂；Per-Unit Tax: P₃ − P₂
Socially Optimal Quantity: Q₃；Per-Unit Tax: P₄ − P₂
Socially Optimal Quantity: Q₂；Per-Unit Tax: P₃ − P₁
Socially Optimal Quantity: Q₃；Per-Unit Tax: P₃ − P₂

社会最优：MSC = MSB 在 Q₂（价格 P₃）。纠正税 = 这个产量上的边际外部成本 = MSC − MPC = P₃ − P₁。(E) Q₃ 是市场产量（MPC = MSB）；(A)(C) 的 P₄ − P₂ 也等于两条成本线的距离，但数量不对。

6.2-352012 AP Micro Q60

Questions 59-60 refer to the diagram below, which shows a market’s cost and revenue curves.

按原题重绘

Given the position of the marginal social cost curve, one can conclude that the

private cost of producing good X exceeds the social cost of production at all levels of output
production of good X creates a negative externality
production of good X occurs in an imperfectly competitive market
market quantity, Q₃, is the socially optimal quantity
free market will produce too little of good X

MSC 在每个产量上都高于 MPC → 生产给别人带来了外部成本 → 负外部性。市场产量 Q₃ 大于最优 Q₂（生产过多），所以 (D)(E) 错；(A) 方向反了。

6.2-362018 AP Micro 练习卷 Q30

Questions 30-31 refer to the following diagram, which shows the marginal social cost (MSC), the marginal private cost (MPC), and the marginal social benefit (MSB) associated with the production of a good in a market.

按原题重绘

Which of the following describes the type of externality generated by the unregulated private market and the resulting deadweight loss?

Type of Externality: Positive；Deadweight Loss: egh
Type of Externality: Positive；Deadweight Loss: ehf
Type of Externality: Negative；Deadweight Loss: ehf
Type of Externality: Negative；Deadweight Loss: egh
Type of Externality: Negative；Deadweight Loss: P4ghP2

MSC 在 MPC 上方 → 负外部性。市场在 MPC = MSB 的 h 点生产 Q₂，社会最优在 MSC = MSB 的 e 点 Q₁。Q₁ 到 Q₂ 之间每单位 MSC > MSB，DWL = 三角形 egh。(C) ehf 夹在 MPC 和 MSB 之间，用错了曲线。

6.2-372018 AP Micro 练习卷 Q31

按原题重绘

The government can attempt to correct the externality by setting a

per-unit tax equal to (P₄ − P₁)
per-unit tax equal to (P₄ − P₂)
per-unit subsidy equal to (P₄ − P₂)
per-unit subsidy equal to (P₃ − P₂)
price ceiling at P₂

庇古税 = 边际外部成本 = MSC 与 MPC 的竖直距离，在 Q₂ 处是 g − h = P₄ − P₂（等于 Q₁ 处的 P₃ − P₁）。(A) P₄ − P₁ 不是同一产量上的距离；负外部性要征税，不是补贴 (C)(D)。

6.2-382022 AP Micro 国际卷 Q28

Questions 28-29 are based on the graph provided, which shows the marginal private cost (MPC), marginal social cost (MSC), marginal private benefit (MPB), and marginal social benefit (MSB) in a market for a good that has an external cost.

按原题重绘

What are the socially efficient and market equilibrium quantities?

Socially efficient quantity is 6, and market equilibrium quantity is 12.
Socially efficient quantity is 9, and market equilibrium quantity is 9.
Socially efficient quantity is 9, and market equilibrium quantity is 12.
Socially efficient quantity is 12, and market equilibrium quantity is 12.
Socially efficient quantity is 12, and market equilibrium quantity is 9.

社会有效：MSB = MSC 在数量 9（$5.25）；市场均衡：MPB = MPC 在数量 12（$5.00）→ 市场生产过多。

6.2-392022 AP Micro 国际卷 Q29

按原题重绘

What is the dollar value of the marginal external cost?

$0.25
$0.50
$1.00
$5.00
$5.50

边际外部成本 = MSC − MPC，在任何数量上都一样：数量 9 时 $5.25 − $4.75 = $0.50（数量 6 时 $5.00 − $4.50）。

6.2-402024 AP Micro Q60

Which of the following policies would address the externality problem in the market depicted in the graph provided?

按原题重绘

Granting a per-unit subsidy of P₄ − P₂ to consumers
Imposing a per-unit tax of P₃ − P₁ on producers
Setting a price floor at P₂
Setting a price ceiling at P₃
Limiting production to Q₂

图中 MSC 在 MPC 上方：生产的负外部性。市场在 MPC = MPB 的 Q₂ 生产，社会最优在 MSC = MSB 的 Q₁（价格 P₃）。对生产者征每单位税 = 边际外部成本 = P₃ − P₁，产量降到 Q₁。(E) Q₂ 就是现在的市场产量，限产到 Q₂ 什么也没改变。

6.3Public and Private Goods 公共物品与私人物品

Four Kinds of Goods 四种物品（CED POL-3.C.1）

两个问题 Rival 竞争性：一个人使用会减少别人能用的量。
Excludable 排他性：可以阻止不付钱的人使用。
私人物品既有竞争性又有排他性；公共物品既没有竞争性也没有排他性。

竞争性：我用了，你就少了（一杯奶茶）。排他性：不付钱就能把你挡在外面（视频网站要登录会员）。两个问题各有“是/否”，一共四种物品。CED 重点考左上（私人物品）、右下（公共物品）和左下（公共资源）。

易错 TRAP

“政府提供的”不等于“公共物品”。公立学校的教育有竞争性（班级容量有限）、有排他性（可以限制入学），本质上是私人物品，只是政府选择免费提供（CED POL-3.C.3），因为教育有正外部性，也关乎公平。
公共物品和公共资源都没有排他性；区别在竞争性：公共资源会被用完，公共物品不会。

LAB 2它是哪一种物品？ · Classify the Goods

先问“我用了你就少了吗？”（竞争性），再问“不付钱能挡住你吗？”（排他性）。（得分：0 / ）

Free Riders and Common Resources 搭便车与公共资源

搭便车 FREE RIDER（CED POL-3.A.4、POL-3.C.2）东西没有排他性时，人们有动机不付钱、坐享其成。私人企业卖不出去（收不到钱），所以不愿提供公共物品——市场提供得太少甚至没有，通常只能由政府提供，用税收支付。

WORKED EXAMPLE 6巷子里的路灯：人人都想别人出钱原创

Three families live on a dark lane. A streetlight costs $120 per month. The first light is worth $60, $50, and $40 per month to the three families; a second light would add only $30, $25, and $20. (a) Is a streetlight a public good? Explain. (b) Should the lane have one light? Two? Explain. (c) Why might the light never be installed if each family decides on its own whether to pay? (d) Suggest a way to get the efficient number of lights.

(a) 是。灯亮着，一家看得见不影响另一家（非竞争性）；也没法只照亮付了钱的人家门口（非排他性）。

(b) 公共物品的社会边际收益 = 所有人的边际收益加起来。第一盏：60 + 50 + 40 = $150 > $120，值得装；第二盏：30 + 25 + 20 = $75 < $120，不值得 → 一盏。

(c) 没有哪一家单独觉得值 $120（最高的才 $60），所以谁也不会自己装；如果大家凑钱，每家都想：“反正装了我也能照到，不如少出点。”——这就是搭便车（CED POL-3.A.4、POL-3.C.2）。结果可能谁都不出钱。

(d) 由居委会或政府出面，向每家收一笔费用（例如每家 $40）统一安装。这就是公共物品通常由政府提供、用税收付费的原因。

公共资源与公地悲剧（CED POL-3.C.4）有竞争性、没有排他性的自然资源（公海的鱼、地下水、公共草地）谁都可以用，每个人只算自己的收益和成本，不考虑自己用了以后别人少了——结果过度使用。解决办法：配额、许可证、收费，或者把产权明确给某个所有者。

WORKED EXAMPLE 7一个人人都能来打鱼的湖原创

Anyone can fish in a lake. Fish sell for $10 per ton, and running a boat costs $30 per day. The table shows the total daily catch with different numbers of boats.

Boats	1	2	3	4	5	6	7	8
Total catch (tons/day)	10	18	24	28	30	30	28	24

(a) Is the fish in the lake a public good? (b) How many boats maximize the total profit from the lake? (c) With open access, how many boats will fish? Explain. (d) Suggest two policies that would fix the problem.

(a) 不是。鱼有竞争性（我捞走的鱼你就捞不到），但没有排他性（谁都能来）——是公共资源（CED POL-3.C.4）。

(b) 多一艘船给全湖增加的收入：100、80、60、40、20、0、−20、−40。只要 ≥ $30 就值得 → 4 艘，总利润 = 280 − 120 = $160，最大。

(c) 每个船主只比较自己的平均收入和成本：第 8 艘船来了以后平均收入 = 240 ÷ 8 = $30，正好等于成本 → 8 艘。总利润 = 240 − 240 = $0，鱼也被过度捕捞。每艘新船都让其他船少捕鱼，但它不用为此付钱——这是公共资源的负外部性，叫公地悲剧。

(d) 发放 4 张捕鱼许可证或规定捕捞配额；把湖的产权交给一个所有者（他会只放 4 艘船）；对每艘船每天收费（例如收 $20，船主的平均收入减去费用后，只有 4 艘船还值得来）。

红线是每艘船每天的成本 $30。船主只要自己的平均收入（蓝）≥ $30 就会来——一直来到第 8 艘。但第 5 艘以后，多一艘船给全湖带来的收入（黄）不到 $30，甚至是负的：它抢走的是别的船的鱼。

练习 Practice 6.3 · 公共物品与搭便车

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.3-12010 AP Micro Q11

Which of the following is the best example of the free-rider problem?

A company gives away free samples of a new product.
A single firm is the sole supplier of corn oil.
There are economies of scale in the production of passenger elevators.
It is difficult to exclude those who do not pay for a good from enjoying the benefits of the good.
There is free entry and exit into production of toothpaste, but products are not homogeneous.

搭便车问题来自非排他性：很难把不付钱的人排除在受益之外，于是人人都不想付钱。

6.3-22014 AP Micro 练习卷 Q15

A free-rider problem arises when a good is

nonrival
nondepletable
nonexcludable
produced in a competitive market
produced in a monopolistic market

搭便车来自非排他性：没法把不付钱的人排除在外。

6.3-32005 AP Micro Q29

Characteristics of a pure public good include which of the following?

I. Nonpaying consumers cannot be excluded from consuming the good.
II. Consumption by any one person does not reduce the availability of the good to others.
III. The amount consumed depends on the price paid.

I only
II only
III only
I and II only
II and III only

纯公共物品：不能排除不付钱的人（I，非排他），一个人消费不减少别人能用的量（II，非竞争）。III 错：不付钱也能消费，消费量和付的价格无关 → I and II only。

6.3-42013 AP Micro Q15

Which of the following is a characteristic of a public good?

It is excludable.
It is nonrival in consumption.
It yields substantial spillover costs.
The private sector would produce a larger than socially optimal quantity.
Private-sector producers would realize too great a profit.

公共物品的特点之一：非竞争性。(A) 正好相反；私人部门会生产不足，所以 (D)(E) 错。

6.3-52017 AP Micro 练习卷 Q56

Which of the following best demonstrates nonrivalry in consumption?

Occupying a seat in a sold-out movie theatre
Sharing rent equally among roommates
Catching fish in a local pond
Playing golf on a crowded public golf course
Listening to a radio program

多一个人听广播，不减少别人能收听的内容 → 非竞争。(A) 满座的座位、(C) 池塘里的鱼、(D) 拥挤的球场都有竞争性。

6.3-62024 AP Micro Q55

For a good to be considered a pure public good, it must be

produced at zero marginal cost
rival and nonexcludable
rival and excludable
nonrival and excludable
nonrival and nonexcludable

纯公共物品：非竞争且非排他。(B) 是公共资源；(C) 是私人物品；(D) 是俱乐部物品。

6.3-72014 AP Micro 练习卷 Q43

Which of the following best defines a pure public good?

A good that is provided free of charge to the public
A good that is provided by the government, whether or not a fee is charged
A good that is efficiently produced only by a monopoly
A good whose benefits can be enjoyed by many consumers at the same time and from which consumers cannot be excluded
A good that by law must be available to all consumers without discrimination on the basis of sex, race, or religion

纯公共物品：很多人可以同时享受（非竞争），而且不能排除任何人（非排他）。是不是政府提供 (B)、是不是免费 (A) 都不是定义。

6.3-82012 AP Micro Q30

Which of the following is the best example of a pure public good?

Electricity from a public utility
Mail delivery service by the post office
Social Security payments
National defense
Imported oil

国防非竞争、非排他。电力和邮政按单位收费，可以排他；社会保障是转移支付；进口石油是私人物品。

6.3-92015 AP Micro 练习卷 Q15

National defense is an example of a public good because

it requires tax revenues to fund any production
one person’s use of it will decrease another person’s ability to use it
it is nonexcludable and nonrival
the private market typically produces the socially efficient level of output
the public is protected from invasion

国防是公共物品，因为它非排他、非竞争。需要税收支持 (A) 是结果，不是原因；(B) 描述的是竞争性。

6.3-102021 AP Micro 国际卷 Q37

National defense is generally provided by the public sector for which of the following reasons?

The opportunity cost of privately providing national defense is too high.
The government always provides the socially optimal quantity.
National defense is a natural monopoly.
National defense is subject to negative externalities.
The benefits of national defense are mostly nonrival and nonexcludable.

国防的收益基本上非竞争、非排他，私人市场会因为搭便车而提供不足，所以由公共部门提供。(B) 政府不一定总能提供最优数量。

6.3-112000 AP Micro Q60

Private supply of public goods is most likely to result in

less than the efficient level of output, due to the free-rider problem
less than the efficient level of output, due to the problem of insufficient competition
more than the efficient level of output, due to lower costs of private firms
more than the efficient level of output, due to the existence of externalities
an overuse of these goods

公共物品不能排他，人们可以“搭便车”不付钱 → 私人企业收不到足够的收入 → 产量低于有效水平，原因是搭便车问题。(B) 原因不是竞争不足。

6.3-122010 AP Micro Q60

Public goods are typically provided by the government, because private markets tend to

produce them at higher cost
produce less than the socially optimum quantity
produce more than the socially optimum quantity
charge a monopoly price that is greater than the marginal social cost
charge a monopoly price that is less than the marginal social cost

公共物品不能排他 → 搭便车 → 私人市场生产少于社会最优数量（常常一点也不生产），所以由政府提供。

6.3-132017 AP Micro 练习卷 Q5

The government must provide public goods such as national defense because

the production of public goods requires economies of scale that the private sector cannot achieve
it is generally impossible to exclude individuals who value public goods but do not pay for them
public goods cannot be produced in private competitive markets, since they have highly inelastic demand
private producers charge a price that is substantially greater than marginal cost
no single individual should have to pay for public goods, since they benefit society as a whole

公共物品无法排除不付钱的人 → 私人企业收不到钱 → 由政府出面提供。(C) 原因不是需求缺乏弹性；(E) 是价值判断，不是经济学解释。

6.3-142023 AP Micro 国际卷 Q57

Which of the following best explains why the government provides public goods?

Nonexcludable goods have a perfectly inelastic demand.
Nonrival goods benefit only private consumers.
Surpluses result from the private production of nonrival goods.
Production of socially optimal quantities results in positive economic profits.
Nonexcludable goods are underproduced by private firms.

非排他的物品人人可以搭便车，私人企业收不回成本 → 生产不足 → 由政府提供。

6.3-152018 AP Micro 练习卷 Q60

One major difference between public goods and private goods is that

public goods exclude nonpaying consumers, while private goods are available to all
public goods are inferior goods, while private goods are normal goods
public goods generate negative externalities, while private goods generate positive externalities
it is difficult to charge customers directly for the provision of public goods
it is impossible for private firms to produce public goods

公共物品很难直接向使用者收费（非排他、搭便车）。(A) 正好反了；(E) 太绝对——私人企业可以生产公共物品（比如受政府委托）。

6.3-162019 AP Micro 练习卷 Q45

Which of the following explains why free-riding can result in a market failure?

More than the socially optimal quantity is produced and consumed.
The socially optimal quantity is produced but less than the socially optimal quantity is consumed.
Private producers of nonexcludable goods are unable to charge everyone who consumes the good.
There is a market surplus of the good.
There is no consumer surplus derived from the good.

搭便车造成市场失灵：非排他物品的私人生产者没法向每个使用者收费，收入不够，于是生产不足。

6.3-172021 AP Micro 国际卷 Q51

Public goods are under produced in a competitive market because the free-rider problem causes

supply to be perfectly elastic
a price ceiling to become binding
consumers to not reveal their true demand
positive externalities to increase
demand to be perfectly inelastic

不付钱的人也不能被排除，消费者就不说出自己真实的需求（假装不需要，等着搭便车）→ 市场需求低估了真实收益 → 生产不足。

6.3-182022 AP Micro 国际卷 Q51

If a pure public good is provided only by a private market, then generally the good will

be overproduced relative to the socially optimal quantity
be underproduced relative to the socially optimal quantity
be excludable
be rivalrous
reduce deadweight loss

纯公共物品只靠私人市场：搭便车 → 相对社会最优生产不足。(C)(D) 公共物品本来就非排他、非竞争。

6.3-192025 AP Micro 国际卷 Q45

Good X, a popular item with consumers, is nonrival and non-excludable in consumption. What is the most likely outcome in the market for Good X if there is no government intervention?

Firms will provide the socially optimal quantity of Good X because consumers will have a demand for it.
Firms will provide the socially optimal quantity of Good X because they will be profitable in the short run.
Firms will provide the socially optimal quantity of Good X despite the fact that marginal social cost is greater than the marginal private cost at the equilibrium market quantity.
Firms will not provide the socially optimal quantity of Good X because people are able to consume it without having to pay for it.
Firms will not provide the socially optimal quantity of Good X because the marginal social cost is greater than the marginal private cost at the equilibrium market quantity.

非竞争、非排他 = 公共物品：人们不付钱也能消费 → 企业收不到钱 → 达不到社会最优数量。(E) 说的是负外部性，不是这里的原因。

6.3-202015 AP Micro 练习卷 Q44

Assuming that all residents of the city of Smallville prefer to have streetlights installed citywide, which of the following best explains why individual residents do not install the optimal number of streetlights?

Streetlights generate negative externalities.
Streetlights are nonexcludable.
Installing streetlights results in diseconomies of scale.
The marginal cost of installing streetlights is zero.
The marginal benefit of installing streetlights is zero.

路灯亮起来谁都能用，不能排他 → 每个居民都想等别人出钱装好再“搭便车” → 私人装的路灯少于最优数量。

6.3-212025 AP Micro 国际卷 Q32

Tornado-warning sirens are usually paid for by local governments rather than private individuals because

each individual does not find the sirens to be of value
sirens are rival and excludable in consumption
private individuals lack the incentive to cover the cost of providing the sirens
sirens will be overproduced in the market equilibrium
it is cheaper to produce sirens for local governments than to produce them for private individuals

警报声谁都能听到（非排他、非竞争），每个人都可以搭便车 → 个人没有动力出钱 → 市场提供不足，所以由地方政府出资。

6.3-222018 AP Micro 练习卷 Q15

The socially optimal quantity of a public good is provided when

marginal private cost equals marginal private benefit
marginal social cost equals marginal social benefit
total private cost equals total social benefit
total social cost equals total social benefit
marginal social benefit minus marginal social cost is at a maximum

任何物品（包括公共物品）的最优数量都在 MSB = MSC 处。(D) 比的是总量；(E) “MSB − MSC 最大”不是边际条件。

6.4The Effects of Government Intervention in Different Market Structures 政府干预的效果

政府的工具箱：从量税和补贴、定额税和补贴、价格上限和下限、管制价格、反垄断。同一件工具，用在不同的市场结构里，效果可能完全相反。

Per-Unit Taxes and Subsidies 从量税与补贴（CED POL-4.A.1–A.2）

对卖方征每单位 $6 的税：供给上移 $6。成交量从 9 降到 6；买方付的价格 P_B = $14（涨了 $3），卖方实得 P_S = $8（降了 $3）。CS 从 40.5 降到 18，PS 从 40.5 降到 18，政府税收 = 6 × 6 = 36，剩下的 9 是 DWL（CED POL-4.A.1）。

	征税前	征税 $6 后	变化
成交量	9	6	−3
买方付的价格	$11	$14	+$3（买方负担一半）
卖方实得的价格	$11	$8	−$3（卖方负担一半）
消费者剩余	40.5	18	−22.5
生产者剩余	40.5	18	−22.5
政府税收	0	36	+36
总剩余（含税收）	81	72	−9 = DWL

税负归宿：谁付得多？不管税向买方收还是向卖方收，结果一样：买方付的价格上升、卖方实得的价格下降，二者之差 = 税额。哪一方越缺乏弹性，负担的税越多——跑不掉的一方吃亏。

点按钮比较两种需求：

需求缺乏弹性：买方很难不买（比如香烟、汽油）：征税 $6，买方付的价格从 $11 涨到 $15.5（蓝色，负担 $4.5 = 75%），卖方只少拿 $1.5（绿色）。

需求富有弹性：买方很容易换别的（比如某个牌子的奶茶）：同样征税 $6，买方只多付 $1.5（25%），卖方少拿 $4.5。

定额税与定额补贴 LUMP-SUM（CED POL-4.A.2）定额税（补贴）的金额与产量无关，不改变边际成本和边际收益，只改变固定成本。所以企业的利润最大化产量和价格短期不变，只是利润变少（变多）。

WORKED EXAMPLE 8征税还是补贴：谁受益、谁吃亏？原创

In a perfectly competitive market with no externalities, demand is P = 20 − Q and supply is P = 2 + Q. (a) The government gives sellers a subsidy of $6 per unit. Find the new quantity, the price buyers pay, the price sellers receive (including the subsidy), the cost of the subsidy, and the deadweight loss. (b) Instead, the government gives each firm a lump-sum subsidy. What happens to the equilibrium price and quantity in the short run? (c) Explain why a per-unit tax creates deadweight loss in this market but removes deadweight loss in the dye market of 6.2.

(a) 补贴让供给下移 $6：P = −4 + Q。与需求 20 − Q 相交：Q = 12，买方付 $8，卖方连补贴共得 8 + 6 = $14。补贴总额 = 6 × 12 = 72。第 10–12 单位的成本（MC 从 $11 升到 $14）高于买方的评价（从 $11 降到 $8），DWL = ½ × 6 × 3 = 9——生产过多也是无效率。

(b) 定额补贴不改变每多生产一单位的成本（MC），供给曲线不动 → 价格和数量不变，只是企业利润增加（CED POL-4.A.2）。

(c) 这里原本没有外部性，市场数量已经是社会最优，税把数量压到最优以下 → 产生 DWL。印染市场有负外部性，市场数量高于最优，税（= MEC）正好把它压回最优 → 消除 DWL。同一个政策，效果取决于它是否对准了市场失灵的原因（CED POL-4.A.4）。

LAB 3税由谁来付？ · Taxes, Subsidies, and Incidence

供给 P = 2 + Q。选需求的弹性，再拖动每单位税（向左拖到负数就是补贴），看价格、数量、剩余、税收和 DWL。

每单位税 = $6

练习 Practice 6.4（一）· 税与补贴

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.4-12005 AP Micro Q18

Assume that both the supply of and the demand for a good are relatively price elastic. The imposition of a per-unit excise tax on the sale of the good would cause the equilibrium price and quantity to change in which of the following ways?

Price: Increase；Quantity: Increase
Price: Increase；Quantity: No change
Price: Increase；Quantity: Decrease
Price: Decrease；Quantity: No change
Price: Decrease；Quantity: Decrease

每单位消费税让供给向上移动（移动幅度 = 税额）→ 买方付的价格上升，均衡数量减少（两边都有弹性，数量减少得较多）。

6.4-22013 AP Micro Q24

Assuming a downward-sloping demand curve and an upward-sloping supply curve, which of the following will occur as a result of an increase in the sales tax on a good?

Supply will increase.
Demand will increase.
Quantity supplied will increase.
Quantity demanded will decrease.
The price paid by consumers will decrease.

销售税提高 → 供给向上（向左）移动 → 买方付的价格上升 → 沿着需求曲线需求量减少。需求曲线本身不动 (B)；供给是减少，不是增加 (A)。

6.4-32018 AP Micro 练习卷 Q33

If the government imposes a per-unit tax on personal computers, consumer surplus in the personal computer market will

decrease, because the price paid by consumers for personal computers will have increased
decrease, because there will be more personal computers on the market
increase, because personal computer manufacturers will pass the tax on to consumers
increase, because consumers will buy more imported personal computers
not change because the demand is unit elastic

每单位税提高了消费者付的价格、减少了成交量 → 消费者剩余减少。

6.4-42017 AP Micro 练习卷 Q52

Suppose that the government imposes a per-unit tax on the producers of a good that has a perfectly inelastic demand. After the tax, the price and quantity of the good sold would change in which of the following ways?

Price: Increase；Quantity: Increase
Price: Increase；Quantity: Decrease
Price: Increase；Quantity: Not change
Price: Not change；Quantity: Increase
Price: Decrease；Quantity: Not change

需求完全无弹性（竖直）：供给上移，价格上涨整个税额，数量不变 → 税全部由买方承担。

6.4-52024 AP Micro Q47

Mike's Bikes has a government license to produce bicycles. The government has increased the annual license fee on bicycle producers. Which of the following will occur in the short run for Mike's Bikes as a result of the increase in the annual license fee?

Total revenue will decrease.
Marginal revenue will decrease.
Marginal cost will increase.
Average total cost will increase.
Average variable cost will increase.

年度执照费与产量无关，是固定成本（相当于定额税）：AFC 上升 → ATC 上升；MC、AVC、MR 都不变，短期产量和总收益也不变。（2024 年非官方打字答案键给 (C)，有误：执照费不改变 MC。）

6.4-62024 AP Micro Q59

Which of the following will happen if a government increases a per-unit subsidy to consumers of a good in a market that has no externalities?

Producers will receive a lower price.
Government revenue will increase.
Producer surplus will increase.
Consumer surplus will decrease.
Deadweight loss will decrease.

给消费者的每单位补贴增加 → 需求右移 → 成交量和卖方收到的价格都上升 → 生产者剩余增加。(A) 卖方价格上升，不是下降；(B) 补贴是支出；(D) 消费者付的净价格下降，CS 增加；(E) 没有外部性的市场，补贴越大 DWL 越大。

6.4-72012 AP Micro Q18

Questions 18-19 are based on the diagram below, which shows the effect of a unit tax placed on a good.

按原题重绘

What is the price paid by consumers and the net price received by producers after the tax is paid?

Paid by Consumers: $11.00；Received by Producers: $10.45
Paid by Consumers: $11.00；Received by Producers: $10.00
Paid by Consumers: $10.45；Received by Producers: $10.00
Paid by Consumers: $10.45；Received by Producers: $9.45
Paid by Consumers: $10.00；Received by Producers: $9.45

税后买方付的价格读税后供给与需求的交点：$10.45（Q′）。卖方实际收到 = 原供给在 Q′ 的高度 = $9.45。

6.4-82012 AP Micro Q19

Questions 18-19 are based on the diagram below, which shows the effect of a unit tax placed on a good.

按原题重绘

According to the diagram, what is the dollar amount of the unit tax?

$0.00
$0.45
$0.55
$1.00
$1.45

税额 = 两条供给线的竖直距离：$10.45 − $9.45 = $1.00（在 Q 处也是 $11.00 − $10.00）。

6.4-92016 AP Micro 练习卷 Q18

按原题重绘

In the graph above, S is the original supply curve, and S₁ is the supply curve including an excise tax. The area representing the tax revenue to the government is

P₁P₃VT
P₂P₄RZ
P₂P₄XU
P₂UQ₃0
P₃VQ₂0

征税后成交点在 V：数量 Q₂，买方付 P₃，卖方收到 P₁（原供给 S 上的 T 点）。每单位税 = P₃ − P₁，税收 = (P₃ − P₁) × Q₂ = 矩形 P₁P₃VT。

6.4-102024 AP Micro Q24

The provided graph shows the imposition of a per-unit tax on the market for a good. After the tax is imposed, what is the change in producer surplus?

按原题重绘

It will decrease by $25.
It will decrease by $40.
It will increase by $90.
It will increase by $40.
It will increase by $25.

每单位税 $2。征税前：Q = 30，P = $3，PS = ½ × 30 × 3 = $45。征税后：Q = 20，买方付 $4，卖方得 $2，PS = ½ × 20 × 2 = $20 → PS 减少 $25。

6.4-112025 AP Micro 国际卷 Q40

The graph shows a per-unit subsidy being granted to producers of Good X. The letters on the graph represent enclosed areas.

按原题重绘

What is the total cost of the per-unit subsidy to the government?

Areas B, C, and J
Areas B, C, and D
Areas A, F, and G
Areas A, B, C, F, G, H, and J
Areas A, B, C, D, E, J, and K

补贴使供给下移，新的成交点在 (Q₂, P₃)：消费者付 P₃，生产者实得 P₁（原供给在 Q₂ 的高度）。每单位补贴 = P₁ − P₃，政府总支出 = (P₁ − P₃) × Q₂ = 0 到 Q₂、P₃ 到 P₁ 的矩形 = A + B + C + F + G + H + J。

6.4-122025 AP Micro 国际卷 Q41

The graph shows a per-unit subsidy being granted to producers of Good X. The letters on the graph represent enclosed areas.

按原题重绘

The change in consumer surplus as a result of the per-unit subsidy granted to producers of Good X is equal to

zero
Area A
Area J
Areas J and K
Areas B, C, and J

补贴前：价格 P₂、数量 Q₁，CS = A。补贴后：价格 P₃、数量 Q₂，CS = A + B + C + J。CS 增加了 B + C + J。

Price Controls in Different Market Structures 不同市场结构中的价格管制（CED POL-4.A.3–A.5）

在完全竞争市场，有效的价格上限造成短缺、减少成交量（Unit 2）。但对一家垄断企业定价格上限，结果可能正好相反——产量增加。原因：上限以内，多卖一个不必降价，MR 变成一条水平线。

上限 $14：价格不能超过 $14：在 5 个以内，多卖一个不用再降价，MR 变成 $14 的水平线。MC = $14 在 Q = 7，但 $14 时只有 5 个人买 → 卖 5 个，比不管制的 4 个多，DWL 变小。

上限 $12：上限正好定在 D = MC 的 $12：MR = $12 的水平线与 MC 交于 Q = 6，需求量也是 6 → 卖 6 个，达到配置有效的数量，DWL 消失（CED POL-4.A.5）。

上限 $10：上限定得太低（$10 < 竞争价格）：企业只愿意生产到 MC = $10 的 5 个，而 $10 时有 7 个人想买 → 短缺 2 个，DWL 又出现了。

有效的价格管制	完全竞争	垄断（垄断竞争短期类似）	买方垄断（劳动市场）
价格上限 / 最低工资定在竞争水平	—	上限 = 竞争价格：产量增加到有效数量	最低工资 = 竞争工资：就业增加到有效数量
价格上限低于竞争价格	短缺，成交量减少，DWL	短缺，产量可能仍比不管制多，也可能更少	—
价格下限 / 最低工资高于竞争水平	过剩，成交量减少，DWL	—	就业减少（与竞争市场的结论相同）

WORKED EXAMPLE 9小岛唯一的轮渡：给票价定上限原创

A ferry company is the only way to reach a small island. Demand for crossings is P = 30 − Q (Q in hundreds of trips per week), and the marginal cost is constant at $6. (a) Find the unregulated monopoly quantity and price, and the deadweight loss. (b) The island government sets a price ceiling of $12. Find the new quantity and deadweight loss. (c) What ceiling would eliminate the deadweight loss? (d) If the ferry also has a large fixed cost, what problem might that ceiling create?

(a) MR = 30 − 2Q = 6 → Q = 12，P = 30 − 12 = $18。有效数量在 P = MC：30 − Q = 6 → Q = 24。DWL = ½ × (18 − 6) × (24 − 12) = 72。

(b) 票价不能超过 $12：在需求量 18 以内，每多卖一张都能收 $12 > MC $6 → 一直卖到 $12 时的需求量 Q = 18。DWL = ½ × (12 − 6) × (24 − 18) = 18，比不管制时小得多。

(c) 上限 = MC = $6：Q = 24，DWL = 0。

(d) 价格 $6 只够付边际成本，付不了固定成本，公司会亏损、长期可能停运——和自然垄断一样，需要定额补贴，或者改用 P = ATC 的“公平回报”价格（Unit 4 上册）。

LAB 4给垄断企业定价格上限 · Price Ceiling on a Monopoly

垄断企业：需求 P = 24 − 2Q，MC = 2Q（不管制时卖 4 个、定价 $16）。拖动价格上限，看它卖多少、有没有短缺、DWL 多大。

价格上限 = $12

练习 Practice 6.4（二）· 价格管制

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.4-132000 AP Micro Q49

An increase in which of the following will most likely result in a long-run surplus of a product?

The number of suppliers of the product
A price that is set by law above the equilibrium price
The demand for the product
The costs of resources used to produce the product
The future expected price of the product

法律规定的价格高于均衡价格（有效价格下限），价格降不下来 → Qs > Qd 的过剩一直存在。供求曲线的移动只会让市场到达新的均衡，不会长期过剩。

6.4-142012 AP Micro Q17

Following a prolonged power outage, the price of flashlights normally increases significantly. If cities had passed laws prohibiting price increases for flashlights, during power outages such laws would most likely

make flashlights more available
create a shortage of flashlights
shift the demand curve for flashlights to the right
shift the supply curve for flashlights to the left
have no effect on the availability of flashlights

禁止涨价 = 在原价设价格上限；停电后需求增加，上限变成有效的 → Qd > Qs → 短缺。法律不会移动需求或供给曲线 (C)(D)。

6.4-152023 AP Micro 国际卷 Q41

Which of the following will occur in the market for bicycles if the government establishes a binding price floor?

A market shortage
A decrease in the demand for bicycles
A decrease in the quantity of bicycles purchased
An increase in consumer surplus
An increase in the equilibrium quantity of bicycles

有效价格下限高于均衡价 → 消费者买得少：购买量减少，同时出现过剩（不是短缺）；需求曲线不动，CS 减少。

6.4-162022 AP Micro 国际卷 Q48

An effective price floor placed on a market for a good will have which of the following effects on the quantity purchased by consumers and the consumer surplus?

Quantity will decrease, and consumer surplus will increase
Quantity will decrease, and consumer surplus will decrease
Quantity will increase, and consumer surplus will not change
Quantity will increase, and consumer surplus will increase
Quantity will not change, and consumer surplus will decrease

有效价格下限 → 价格上升 → 消费者买得更少，而且买的每一单位都付得更多 → CS 减少。

6.4-172014 AP Micro 练习卷 Q6

If a government eliminated an effective price floor in a market, all of the following would occur EXCEPT:

The surplus would be eliminated.
The price would decrease.
The quantity supplied would decrease.
The quantity demanded would increase.
The supply of the good would increase.

取消有效价格下限 → 价格降到均衡 → 过剩消失、Qs 减少、Qd 增加。这些都是沿着曲线的移动，供给曲线本身不变 → 例外是 (E)。

6.4-182022 AP Micro 国际卷 Q53

Removing a binding price floor on milk will affect the quantity of milk demanded and the quantity of milk supplied in the market in which of the following ways?

There will be no change to quantity demanded, and quantity supplied will decrease.
Quantity demanded will decrease, and quantity supplied will decrease.
Quantity demanded will decrease, and quantity supplied will increase.
Quantity demanded will increase, and there will be no change to quantity supplied.
Quantity demanded will increase, and quantity supplied will decrease.

取消有效价格下限，价格从下限降到均衡价 → 沿需求曲线需求量增加，沿供给曲线供给量减少，直到两者相等。

6.4-192000 AP Micro Q35

按原题重绘

The graph above shows the supply and demand curves for gasoline. Which of the following will occur if the government establishes a price ceiling of $1.20 per gallon?

A shortage of 900 million gallons
A shortage of 200 million gallons
A shortage of 100 million gallons
A surplus of 100 million gallons
Neither a surplus nor a shortage

均衡价格 $1.00、数量 1,000。上限 $1.20 高于均衡价格，不起作用 → 市场仍在 $1.00、1,000，没有短缺也没有过剩。（上限定在 $0.80 才会有 200 的短缺。）

6.4-202013 AP Micro Q47

按原题重绘

If the market depicted in the diagram above is initially in equilibrium, which of the following will result from the government’s setting a price ceiling at P₃?

Quantity demanded will exceed quantity supplied.
Quantity supplied will exceed quantity demanded.
Market price will increase.
Market price will be unaffected.
Market price will decrease.

均衡在 P₂、Q₃。价格上限 P₃ 高于均衡价格，不起作用 → 市场价格不变。

6.4-212015 AP Micro 练习卷 Q3

Price per Gallon	Quantity Supplied	Quantity Demanded
$2.50	11,000	17,000
$2.75	12,000	16,000
$3.00	13,000	15,000
$3.25	14,000	14,000
$3.50	15,000	13,000
$3.75	16,000	12,000

The table above shows the quantity of gasoline supplied and demanded at various prices in a country. If the government sets a price floor of $2.75 on a gallon of gasoline, what is the price per gallon?

$2.75
$3.00
$3.25
$3.50
$3.75

表中 Qs = Qd = 14,000 在 $3.25（均衡）。价格下限 $2.75 低于均衡价，不起作用 → 价格仍是 $3.25。

6.4-222022 AP Micro 国际卷 Q40

The graph shows the market supply (S) and demand (D) curves for a particular product. If the government imposes a price ceiling at P₂, which of the following statements is true?

按原题重绘

The quantity sold in the market falls from Q₂ to Q₁.
The quantity sold in the market remains at Q₂ because the price ceiling is not binding.
The price ceiling results in a shortage of (Q₃−Q₁).
The price ceiling results in a surplus of (Q₃−Q₁).
The price ceiling will benefit sellers at the expense of buyers.

均衡在 P₁、Q₂。上限 P₂ 高于均衡价格，不起作用 → 成交量仍是 Q₂。(C)(D) 的 Q₃ − Q₁ 是价格在 P₂ 时的差距，但市场价格不会停在 P₂。

6.4-232012 AP Micro Q4

按原题重绘

In the market depicted in the diagram above, if the government imposes a price ceiling of $1.00 per gallon on gasoline, which of the following will result?

A surplus of 6 billion gallons
A shortage of 6 billion gallons
A surplus of 12 billion gallons
A shortage of 12 billion gallons
Neither a surplus nor a shortage, because the price ceiling would not be effective

上限 $1.00 低于均衡价 $1.50，是有效的。在 $1.00：Qs = 6，Qd = 12（十亿加仑）→ 短缺 6。

6.4-242014 AP Micro 练习卷 Q33

按原题重绘

The diagram above depicts demand and supply curves in a city’s rental housing market. If a price ceiling of $1,000 is imposed on the market, which of the following will occur?

There will be a surplus of rental housing in the city.
The demand curve for housing will shift to the right.
The supply curve for housing will shift to the right.
The quantity of rental housing supplied will decrease.
The quantity of rental housing demanded will increase.

均衡租金 $2,000，上限 $1,000 是有效的。供给是竖直线，供给量不变；沿需求曲线需求量增加 → 出现短缺。两条曲线都不移动。

6.4-252000 AP Micro Q19

Questions 18-19 refer to the graph below. The market is currently in equilibrium.

按原题重绘

If a price floor is set at X, the quantity demanded will

increase from 0R to 0S
increase from 0R to 0T
decrease from 0S to 0R
decrease from 0T to 0R
not change

下限 X 高于均衡价 Y，是有效的。需求量沿需求曲线从 Z 点（0S）减少到 V 点（0R）；同时供给量增加到 0T，过剩 = RT。

6.4-262018 AP Micro 练习卷 Q5

按原题重绘

The diagram above depicts the demand and supply curves for good Y. The perfectly competitive market is initially in equilibrium, but the government decides to institute a price floor program by setting the price of good Y at P_F and agrees to purchase all unsold units of good Y from the producers. The cost of this program to the government is represented by which of the following areas?

0P_FJQ₂
0P_FKQ₃
GIKH
Q₁IJQ₂
Q₁IKQ₃

价格定在 P_F：Qd = Q₁（I 点），Qs = Q₃（K 点）。政府按 P_F 买下过剩的 Q₃ − Q₁，花费 = P_F × (Q₃ − Q₁) = 矩形 Q₁IKQ3。

6.4-272010 AP Micro Q4

Questions 3-4 refer to the diagram below. The letters on the graph represent enclosed areas.

按原题重绘

If a price ceiling is set at P₁, which of the following areas represent the resulting consumer surplus, producer surplus, and deadweight loss?

Consumer Surplus: f；Producer Surplus: k；Deadweight Loss: l + m
Consumer Surplus: f + g + h；Producer Surplus: k + l；Deadweight Loss: h + j
Consumer Surplus: f + g + h；Producer Surplus: i + j + k；Deadweight Loss: l + m
Consumer Surplus: f + g + i；Producer Surplus: k；Deadweight Loss: h + j
Consumer Surplus: f + g + i；Producer Surplus: h + j；Deadweight Loss: l + m

上限 P₁ 低于均衡 P₂，是有效的：成交量 = 供给量（左边那条竖虚线）。CS = 需求以下、P₁ 以上、到这个数量为止 = f + g + i；PS = P₁ 以下、供给以上 = k；DWL = 两个数量之间、需求与供给之间 = h + j。

6.4-282023 AP Micro 国际卷 Q9

Consumer surplus could be the shaded area in which of the following situations?

按原题重绘

A price floor set at P₁ in the market
A price floor set at P₂ in the market
A price ceiling set at P₁ in the market
A per-unit subsidy equal to P₂−P₁ granted to producers
A per-unit tax equal to P₂−P₁ imposed on producers

阴影 = 需求以下、P₁ 以上、从 0 到 Q₁ 的面积。价格上限定在 P₁ 时，买方付 P₁，但只能买到供给量 Q₁ → CS 正好是这块面积。(B) 下限 P₂ 时 CS 只剩 P₂ 以上的三角形；(E) 每单位税 P₂ − P₁ 让成交点移到 (Q₁, P₂)，CS 也只剩那个三角形；(D) 补贴让成交量增加到 Q₂，CS 比阴影大。

Natural Monopoly and Antitrust 自然垄断与反垄断（CED POL-4.A.4–A.7）

自然垄断按 P = MC = $4 定价：产量 16 达到配置效率，但 ATC = $7 > P，每单位亏 $3，共亏 48——需要一笔定额补贴 48 才能让它留在市场（CED POL-4.A.6）。定额补贴不改变 MC，所以不会改变它的产量。数字与 Unit 4（上）的地铁例子相同。

反垄断政策 ANTITRUST POLICY（让市场更有竞争性）

🚫

阻止合并：两家大企业合并后会形成垄断或很高的集中度，政府可以不批准。

✂️

拆分垄断企业：1911 年美国最高法院判决把标准石油公司拆成几十家公司；1984 年 AT&T 被拆分。

🤝

禁止合谋与价格操纵：卡特尔、串通定价是违法的（4.5 的卡特尔在美国被反垄断法禁止）。

易错 TRAP · 政府干预不一定提高效率只有对准了造成市场失灵的那个动机，干预才能提高效率（CED POL-4.A.4）：没有外部性的市场征税会造成 DWL；价格上限定得太低会造成短缺；对一家已经产量不足的垄断企业征“负外部性税”，可能让它的产量更少（2017 年 FRQ 就考这个，见本单元 FRQ 3）。

练习 Practice 6.4（三）· 反垄断与自然垄断管制

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.4-292015 AP Micro 练习卷 Q45

Antitrust policy is designed to

encourage businesses to produce public goods
establish trust between the government and the business community
prevent businesses from earning profits
maintain a competitive business environment
ensure that businesses earn only zero economic profits in the long run

反垄断政策的目的：维持竞争的市场环境（阻止垄断、合谋和反竞争的合并）。(C)(E) 企业赚利润本身不违法。

6.4-302016 AP Micro 练习卷 Q29

Antitrust policy seeks to prevent or eliminate which of the following practices?

Firing of workers
Collusion and price fixing
Pollution of rivers
Monopolistic competition
Discrimination in the labor market

反垄断政策禁止合谋与价格操纵。(D) 垄断竞争是一种正常的市场结构，不违法。

6.4-312022 AP Micro 国际卷 Q52

Which of the following is an example of a violation of antitrust laws?

Advertising to increase a firm’s market share in an industry
Making positive economic profits by charging a price above marginal cost
Making free-trade agreements with other countries
Engaging in collusive behavior to restrain competition
Charging a price below the price ceiling

合谋限制竞争（串通定价、划分市场）违反反垄断法。做广告 (A)、P > MC 赚利润 (B) 本身不违法。

6.4-322018 AP Micro 练习卷 Q58

Antitrust policies are put in place to limit which of the following?

Income inequality
Monopoly power
Double taxation of corporations
Price ceilings
The provision of public goods

反垄断政策限制的是垄断势力（市场势力）。

6.4-332019 AP Micro 练习卷 Q30

Antitrust laws are designed to maintain a competitive market environment by

eliminating monopolies wherever they exist
preventing monopolies from generating negative externalities
limiting practices that increase a firm's market power
imposing price ceilings on products produced by monopolies
making charging a price above marginal cost illegal

反垄断法通过限制增加市场势力的做法（反竞争的合并、合谋、掠夺性定价）来维持竞争。(A) 太绝对——自然垄断一般不拆；(E) P > MC 本身不违法。

6.4-342024 AP Micro Q36

Which of the following describes the purpose of government antitrust legislation?

To increase positive externalities in consumption in order to increase consumers' standard of living
To decrease negative externalities in production in order to achieve a socially optimal quantity of output
To increase competition in markets in order to reduce inefficiencies caused by monopoly power
To encourage firms to pay their workers a wage equal to their marginal revenue product in order to decrease income inequality
To limit imports in order to protect domestic producers

反垄断立法的目的：增加竞争，减少垄断势力造成的低效率（DWL）。(A)(B) 是外部性政策。

6.4-352000 AP Micro Q27

Antitrust legislation is designed to make it illegal for a firm to monopolize an industry. Which of the following best states the economic rationale for this legislation?

A monopolist produces too little of the good, producing an output that minimizes the average cost of production.
A monopolist produces too little of the good, charging consumers a price that exceeds the marginal cost of production.
A monopolist is more likely to pollute the environment than are firms in a competitive industry.
A monopolist engages in price discrimination, charging low-income people with elastic demand curves a higher price than that charged to high-income people with inelastic demand curves.
A monopolist produces too much of a good, attracting scarce factors of production that might be better utilized in other industries.

反垄断法的经济理由：垄断产量太少，价格高于边际成本（P > MC，有 DWL）。(A) 垄断一般不在 ATC 最低点生产；(D) 说反了——价格歧视是对需求缺乏弹性的人收高价。

6.4-362005 AP Micro Q56

A merger of two firms may increase economic efficiency by

decreasing average total cost through an increase in economies of scale
decreasing output to reduce marginal cost and equalize price
increasing economic profits but decreasing consumer surplus
increasing consumer surplus by decreasing economic profits
increasing consumer surplus by shifting the demand curve for the product to the right

合并可能提高效率的途径：利用规模经济降低 ATC。(B)(C) 说的是市场势力带来的低效率。

6.4-372025 AP Micro 国际卷 Q35

Initially, Markus Corporation is the only firm that produces aluminum. The government breaks up Markus’ monopoly, creating two firms in its place. The long-run average total cost for each firm is higher than if Markus operated alone in the industry. This would suggest that before the breakup, Markus Corporation was operating under conditions of

monopolistic competition
Nash equilibrium
decreasing returns to scale
economies of scale
diseconomies of scale

拆成两家后，每家的长期 ATC 反而更高 → 原来一家生产时 LRATC 随产量下降 → 规模经济（自然垄断）。拆分自然垄断会提高成本。

6.4-382014 AP Micro 练习卷 Q60

Which of the following government actions can correct the inefficiency caused by the existence of a monopoly?

Granting a per-unit subsidy on the monopolist’s output
Granting an annual lump-sum subsidy to the monopolist
Imposing a per-unit tax on the monopolist’s output
Imposing a price floor on the monopolist
Limiting the monopolist's production through regulation

垄断产量太少。每单位补贴降低 MC，MR = MC 的产量增加，向有效产量靠近。(B) 定额补贴不改变 MC；(C)(D)(E) 都会让产量更少。

6.4-392018 AP Micro 练习卷 Q59

If the government regulates a natural monopolist to produce the allocatively efficient level of output, it will require the monopolist to set a price that is

equal to its marginal cost and grant a subsidy to cover the loss
equal to its average total cost and levy a tax on the excess profit
greater than its marginal cost and levy a tax on the excess profit
greater than its marginal cost but that minimizes the deadweight loss
greater than its average total cost but that minimizes the deadweight loss

配置效率要求 P = MC。自然垄断的 ATC 一直在下降，MC < ATC，P = MC 时每单位都亏损 → 需要政府补贴弥补亏损。

6.4-402023 AP Micro 国际卷 Q3

The government of Annapolita has determined that the country’s power company, an unregulated natural monopolist, should be required to produce the allocatively efficient quantity of electricity. Which of the following actions will achieve the government’s goal?

The government imposes a per-unit tax on each unit of output the monopoly produces and imposes a lump-sum tax on the monopolist to eliminate any remaining profit.
The government sets the price at the point where demand and average total cost curves intersect and provides a lump-sum subsidy to the monopolist equal to the monopolist’s normal profit.
The government sets the price at the point where demand and average total cost curves intersect and imposes a lump-sum tax on the monopolist to eliminate the monopolist’s normal profit.
The government sets the price at the point where demand and marginal cost curves intersect and imposes a lump-sum tax on the monopolist to eliminate the monopolist’s normal profit.
The government sets the price at the point where demand and marginal cost curves intersect and provides a lump-sum subsidy to the monopolist to earn normal profit.

配置效率要 P = MC：把价格定在需求与 MC 的交点。自然垄断在这里 P < ATC，会亏损 → 政府给定额补贴，让它赚到正常利润、留在市场。定额补贴不改变 MC，所以不改变产量 → (E)。(B)(C) 是零经济利润定价，产量仍然不足。

6.4-412019 AP Micro 练习卷 Q59

As an unregulated monopolist, City Cable is earning positive economic profits. If the government regulated the firm by requiring it to produce the level of output that allowed the firm to earn zero economic profit, City Cable would set a price that is equal to its

marginal cost
marginal revenue
average total cost
average variable cost
total cost

零经济利润 → P = ATC（“公平回报”定价，fair-return pricing）。

6.5Inequality 收入与财富不平等

市场有效率，不等于结果公平。即使每个市场都完全竞争、没有外部性，人们的收入和财富也可能相差很大。6.5 讨论怎样衡量不平等，以及不平等从哪里来。

收入与财富（CED POL-5.A.1） 收入（income）是一段时间内的流量：工资、利息、租金、利润、转移支付。财富（wealth）是某一时点的存量：资产减去负债（房子、存款、股票减去贷款）。财富通常比收入更不平等，因为它会代代积累。
收入水平和贫困率在不同群体（年龄、性别、种族）之间、不同国家之间、同一国家内部都相差很大。

Lorenz Curve and Gini Coefficient 洛伦兹曲线与基尼系数（CED POL-5.A.2）

家庭（从穷到富）	最穷 20%	第二个 20%	中间 20%	第四个 20%	最富 20%
Alpha 的收入份额	10%	15%	20%	25%	30%
Beta 的收入份额	4%	8%	13%	22%	53%

两个虚构国家的洛伦兹曲线：横轴是从最穷到最富累计的家庭比例，纵轴是他们累计拿到的收入比例。离对角线（完全平等）越远，越不平等——Beta 比 Alpha 不平等。基尼系数 = A ÷ (A + B)：A 是对角线与洛伦兹曲线之间的面积，在 0（完全平等）到 1（一个人拿走全部收入）之间。

读图规则

洛伦兹曲线越靠近对角线，分配越平等；越弯（越远），越不平等。
基尼系数在 0 到 1 之间：0 = 完全平等，1 = 完全不平等；数字越大越不平等。
可以用来比较不同国家、不同政策（税前与税后）、不同年份的分配。
AP 不考画洛伦兹曲线或计算基尼系数（CED 排除项）。

WORKED EXAMPLE 10读洛伦兹曲线原创

Use the Lorenz curves and the table for the fictional countries Alpha and Beta above. (a) In Beta, what percentage of total income goes to the poorest 40% of households? To the richest 20%? (b) Which country has the larger Gini coefficient? (c) Beta adopts a more progressive income tax and uses the revenue for cash transfers to low-income families. How would Beta's Lorenz curve and Gini coefficient change? (d) What would the Lorenz curve look like if every household had exactly the same income?

(a) 最穷 40%：4% + 8% = 12%（读 Beta 曲线在横轴 40% 处的高度）。最富 20%：100% − 47% = 53%。

(b) Beta 的曲线离对角线更远，A 面积更大 → Beta 的基尼系数更大，更不平等。

(c) 税后、转移支付后的收入分配更平均 → 洛伦兹曲线向对角线靠近，基尼系数下降。

(d) 就是那条 45° 的完全平等线，基尼系数 = 0。（AP 不要求画洛伦兹曲线或计算基尼系数，只要求会读、会比较——CED 排除项。）

Tax Structures and Redistribution 税制与再分配

三种税制（看平均税率怎么随收入变化） 累进税 Progressive：收入越高，平均税率越高（例：分档的个人所得税）→ 缩小不平等。
累退税 Regressive：收入越高，平均税率越低（例：对生活必需品的销售税，相对于收入而言）→ 扩大不平等。
比例税 Proportional：所有人平均税率相同 → 不改变相对差距。
平均税率 = 总税额 ÷ 收入；边际税率 = 多挣 $1 要多交的税。

WORKED EXAMPLE 11三种税：谁的负担更重？原创

The table shows the tax paid under three tax plans by families with different incomes.

Income	Plan A	Plan B	Plan C
$20,000	$1,000	$1,000	$1,600
$50,000	$2,500	$4,000	$2,500
$100,000	$5,000	$12,000	$3,000

(a) Classify each plan as progressive, regressive, or proportional. (b) Which plan would most reduce income inequality? (c) A sales tax on groceries is often described as regressive even though everyone pays the same rate on each purchase. Explain. (d) Under another plan, income up to $10,000 is taxed at 10% and income above $10,000 at 20%. For a family earning $30,000, calculate the total tax, the average tax rate, and the marginal tax rate.

收入	A 的平均税率	B 的平均税率	C 的平均税率
$20,000	5%	5%	8%
$50,000	5%	8%	5%
$100,000	5%	12%	3%

(a) 看平均税率（税 ÷ 收入）怎么随收入变化：A 不变 → 比例税；B 随收入上升 → 累进税；C 随收入下降 → 累退税。注意不是看税额：C 的富人交的钱更多，但占收入的比例更低。

(b) B（累进税）：高收入者交的比例更高，税后收入差距缩小；如果再把税收用于低收入家庭的转移支付，效果更明显。

(c) 低收入家庭把收入的大部分花在食品上，高收入家庭只花一小部分。同样的税率乘以食品支出，占低收入家庭收入的比例更高——所以对收入来说是累退的。

(d) 税 = 10% × $10,000 + 20% × $20,000 = $5,000；平均税率 = 5,000 ÷ 30,000 ≈ 16.7%；边际税率（多挣 $1 要交的税）= 20%。边际税率高于平均税率，是累进税的特征。

再分配的工具（了解即可）累进税 + 转移支付（失业救济、食品补助、低收入补贴、公共医疗）可以缩小税后收入差距。代价是：税会降低工作和投资的激励，政府需要在“公平”和“效率”之间权衡。

Sources of Inequality 不平等的来源（CED POL-5.B）

要素按边际产品价值得到报酬（CED POL-5.B.1）在竞争的要素市场里，每个要素的报酬等于它的边际产品价值（Unit 5：W = MRP）。技能稀缺、生产率高、产品值钱的工人收入高；拥有更多土地和资本的人还会得到租金和利息——这些都会扩大差距。

CED 列出的来源（POL-5.B.2）

🎓

人力资本：教育、技能、健康。受教育多的人生产率高、工资高。

🧑‍🤝‍🧑

社会资本：人脉和社会关系网，带来工作机会和信息。

🏠

继承：财富和机会在代际之间传递。

⚖️

歧视：同样生产率的人因为性别、种族等拿到更低的工资或更少的机会。

🏦

进入金融市场的机会：借不到钱就没法上学、创业、买房。

🚌

流动性：能否搬到机会更多的地方、换到收入更高的职业。

🤝

议价能力：在企业、工会、家庭内部谈判的能力不同，分到的份额也不同。

🧾

税制结构：累进税缩小差距，累退税扩大差距。

练习 Practice 6.5 · 收入与财富不平等

以下为历年 AP 真题（带图和表格的题按原题重绘），点选项即时判分并显示解析。

6.5-12005 AP Micro Q44

A Lorenz curve can be used by economists to do which of the following?

Show the distribution of personal income
Show the amount of wealth in a nation
Show the amount of consumption in a nation
Explain why different households have different incomes
Explain why income is equally distributed in socialist countries

洛伦兹曲线显示个人收入的分配。它不显示收入或财富的总量 (B)(C)，也不解释原因 (D)。

6.5-22017 AP Micro 练习卷 Q58

The Lorenz curve is used to study the

relationship between output and unemployment
relationship between income and consumption
extent of income inequality within a society
incidence of an excise tax on buyers and sellers in a market
impact of foreign trade on domestic production of goods

洛伦兹曲线用来研究一个社会收入不平等的程度。(D) 是税收归宿的问题，用供求图研究。

6.5-32019 AP Micro 练习卷 Q29

The Lorenz curve represents the relationship between

the cumulative percentage of households and the cumulative percentage of income
income tax rates and income tax revenues
child labor rates and the poverty levels
income inequality and education level
market structure and the number of firms in the market

洛伦兹曲线：横轴是家庭的累计百分比，纵轴是收入的累计百分比。

6.5-42023 AP Micro 国际卷 Q52

Income inequality is measured by what economic concept?

Market failure
Unemployment
Production function
Gini coefficient
Deadweight loss

收入不平等用基尼系数衡量（0 = 完全平等，1 = 完全不平等），它由洛伦兹曲线算出。

6.5-52013 AP Micro Q57

Country A has a Gini coefficient of 0.5, and Country B has a Gini coefficient of 0.3. Which of the following statements is definitely true?

Country A has higher average income than Country B.
Country B has higher average income than Country A.
Income is more evenly distributed in Country A than in Country B.
Income is more evenly distributed in Country B than in Country A.
Country A has more deadweight loss than Country B.

基尼系数越小越平等：0.3 < 0.5 → B 国的收入更平均。基尼系数不反映平均收入 (A)(B)。

6.5-62010 AP Micro Q45

按原题重绘

Based on the information in the Lorenz curve above, which of the following is true?

People in Country X make more money than people in Country Z.
Income is more equally distributed in Country X than in Country Z.
The top 20 percent of households in Country X received 60 percent of the total income.
The top 20 percent of households in Country Z received 40 percent of the total income.
The average income in Country X is higher than the average income in Country Z.

X 的曲线离对角线（完全平等线）更近 → X 的收入分配更平等。(C) X 在 80% 处是 60%，最富的 20% 拿 100 − 60 = 40%，不是 60%；(D) Z 最富的 20% 拿 60%，不是 40%。洛伦兹曲线不显示收入高低 (A)(E)。

6.5-72018 AP Micro 练习卷 Q45

按原题重绘

The Lorenz curves in the graph above show the effects of a tax system on income inequality. Which of the following statements must be true?

Before the tax, 20% of families lived below the poverty level, and after the tax, none did.
The tax redistributed income only from the top 20% of families to the lowest 20%.
The tax system is proportional.
The tax system is regressive.
The tax system is progressive.

税后曲线比税前曲线更靠近完全平等线 → 税制缩小了不平等，说明高收入者交的比例更大 → 累进。(B) 曲线在每一段都变了，不只是最高 20% 转给最低 20%。

6.5-82019 AP Micro 练习卷 Q60

A progressive income tax is characterized by

a higher average tax rate at low income levels than at high income levels
tax rates that increase total tax revenues
marginal tax rates that do not change as income changes
marginal tax rates that increase as income increases
marginal tax rates that decrease as income increases

累进所得税：边际税率随收入增加而上升。(A) 是累退税；(C) 是比例税；(E) 也是累退。

6.5-92000 AP Micro Q45

In the United States, the federal government redistributes income primarily by

setting up planning commissions to set wage rates
taxing different income levels at different rates
guaranteeing every person a minimum income through minimum-wage laws
providing the same goods and services to all citizens
relocating and retraining structurally unemployed people

美国联邦政府再分配收入主要靠对不同收入水平征不同税率（累进所得税），再加上转移支付。最低工资法 (C) 不保证每个人都有收入。

6.5-102005 AP Micro Q15

A society that wishes to achieve greater income equality is most likely to have which of the following?

A regressive income tax system and high estate and gift taxes
A regressive income tax system and low estate and gift taxes
A progressive income tax system and high estate and gift taxes
A progressive income tax system and low estate and gift taxes
A proportional income tax system and high estate and gift taxes

想要更平等：累进所得税（收入越高税率越高）+ 高遗产税、赠与税（限制财富代代集中）。

6.5-112012 AP Micro Q15

Which of the following will most likely lead to a more equal distribution of income?

More regressive national sales tax
More progressive income taxes
An increase in the high school dropout rate
An increase in structural unemployment
An increase in earnings for owners of capital

更累进的所得税从高收入者拿走更大的比例，缩小税后收入差距。(A) 累退的销售税、(C) 辍学率上升、(D) 结构性失业、(E) 资本所有者收入增加，都会扩大差距。

6.5-122014 AP Micro 练习卷 Q14

Which of the following taxes contributes most to decreasing inequality in the distribution of income?

Progressive income taxes
Sales taxes
Proportional income taxes
Excise taxes
Import tariffs on necessities

累进所得税最能缩小不平等。销售税、消费税、对必需品的关税是累退的；比例所得税对分配基本是中性的。

6.5-132015 AP Micro 练习卷 Q30

Which of the following is most likely to make a country’s income distribution more equal?

Establishing a national sales tax on food and clothing
Establishing a progressive income tax system
Eliminating excise taxes on luxury goods
Proportionally decreasing the tax rate for all income groups
Reducing the subsidy to education

累进所得税让税后收入更平等。(A) 对食品服装征销售税是累退的；(C)(E) 偏向高收入者；(D) 所有人同比例降税，分配基本不变。

6.5-142024 AP Micro Q50

Which of the following is most likely to cause an increase in income inequality in a country?

Increasing access to financial markets for people in low-income groups
Providing subsidized job training to people in low-income groups
Providing free higher education to people in low-income groups
Implementing a progressive income tax structure
Implementing a regressive income tax structure

累退所得税从低收入者拿走更大的比例 → 税后不平等扩大。(A)–(D) 都会缩小不平等。

6.5-152022 AP Micro 国际卷 Q19

One reason many governments provide free access to mandatory primary education is because primary education

generates negative externalities
helps to reduce future income inequality
is only provided by a natural monopoly
is nonrival and therefore subject to overcrowding
is nonexcludable and therefore subject to the free rider problem

免费普及的小学教育让所有孩子（不论家庭收入）都能积累人力资本 → 减少未来的收入不平等。教育有正外部性，不是负外部性 (A)；教育可以排他，不是公共物品 (D)(E)。

6.5-162025 AP Micro 国际卷 Q7

Discrimination in the workplace is one source of which of the following?

Income and wealth inequality
Productive efficiency
Regressive income tax systems
Formation of monopolies
Scarcity of resources

工作中的歧视让一些人的收入低于他们生产率对应的水平 → 是收入和财富不平等的来源之一（CED POL-5.B.2）。

ΣSummary 本单元总结与易错点

6.1 Socially Efficient and Inefficient Market Outcomes

没有外部性的完全竞争市场在 MSB = MSC（P = MC）处生产，总剩余最大，是社会有效的。数量少于或多于这个数量都有 DWL。市场失灵 = 市场达不到社会有效结果：外部性、公共物品、市场势力、信息不对称……

6.2 Externalities

MSC = MPC + MEC，MSB = MPB + MEB。负外部性 → 生产过多；正外部性 → 生产不足。DWL 在 Q_M 与 Q_S 之间，尖角指向社会最优点。纠正：庇古税/补贴（= 边际外部成本/收益）、管制、界定产权、私人谈判、可交易许可；定额税不管用。

6.3 Public and Private Goods

按竞争性和排他性分四类：私人物品、公共物品（非竞争、非排他）、公共资源（竞争、非排他）、俱乐部物品（非竞争、排他）。公共物品有搭便车问题 → 私人市场提供不足，常由政府提供；公共资源会被过度使用（公地悲剧）。

6.4 The Effects of Government Intervention

税和补贴在供求之间打进一个“楔子”，税负由弹性决定（更缺乏弹性的一方负担更多）。没有外部性时税、补贴、价格管制都造成 DWL。对垄断：价格上限可能增加产量；自然垄断按 P = MC 定价要补贴，按 P = ATC 定价是零经济利润；反垄断法维护竞争。

6.5 Inequality

洛伦兹曲线：家庭累计 % 对收入累计 %；越靠近对角线越平等。基尼系数 0（完全平等）到 1（完全不平等）。累进税：边际税率随收入上升；累退税：低收入者交的比例更高；比例税：比例相同。不平等的来源：人力资本、继承、歧视、市场势力、税制等。

Government Policy Toolbox 政府干预工具对照

政策工具	它改变了什么	什么时候能提高效率	什么时候反而降低效率
每单位税	供给（或需求）竖直移动 = 税额；买方付的价格上升、卖方实得下降、数量减少	税额 = 边际外部成本（负外部性，庇古税）	没有外部性的市场：造成 DWL
每单位补贴	供给向下（或需求向上）移动 = 补贴额；数量增加	补贴 = 边际外部收益（正外部性）；对垄断的每单位补贴也能增加产量	没有外部性的市场：生产过多，造成 DWL
定额税 / 定额补贴	只改变固定成本和利润，MC 不变，短期产量和价格不变	自然垄断按 P = MC 定价后，用定额补贴弥补亏损	纠正不了外部性（不改变边际决策）
价格上限	竞争市场：数量 = 供给量，出现短缺	对垄断定在 P = MC 价与垄断价之间：产量增加、DWL 减少	竞争市场里有效的上限：成交量减少、DWL
价格下限	数量 = 需求量，出现过剩	（一般不能）	成交量减少、DWL；政府收购过剩还要花钱
管制、界定产权、可交易许可	直接限制数量，或让外部成本有人“认领”	把产量拉到社会最优；当事人少时私人谈判也行	限得过多或过少
反垄断	阻止合并、拆分垄断、禁止合谋	增加竞争，减少 P > MC 的 DWL	拆分自然垄断会提高平均成本
公共提供	政府直接提供（用税收支付）	公共物品：私人市场因搭便车提供不足	提供得太多或太少（政府也不一定知道 MSB）
累进税与转移支付	高收入者交更大比例，低收入者得到补助	目标是公平（缩小不平等），不是效率	可能削弱工作、储蓄的激励——效率与公平的权衡

易错点 TOP 10

市场均衡在 MSB = MSC 处 ✗ → 市场看私人曲线（MPB = MPC），社会最优才看社会曲线（MSB = MSC）。
负外部性使市场生产不足 ✗ → 负外部性生产过多，正外部性才生产不足。
生产的外部性让 MSB 和 MPB 分开 ✗ → 生产外部性分开的是成本线（MSC ≠ MPC），消费外部性分开的是收益线。
庇古税 = MSC 的高度 ✗ → 庇古税 = MSC 与 MPC 的竖直距离（边际外部成本），在社会最优产量处量。
定额税也能纠正外部性 ✗ → 定额税不改变 MC，产量不变。
政府提供的就是公共物品 ✗ → 公共物品看非竞争 + 非排他；邮政、电力可以排他，不是公共物品。
公共资源被过度使用是因为非竞争 ✗ → 公共资源是竞争但非排他：谁都能用，用了别人就少了。
法律规定谁交税，税就由谁负担 ✗ → 税负由弹性决定，和名义上谁交无关。
给垄断企业定价格上限一定减少产量 ✗ → 上限在 P = MC 价和垄断价之间时，产量增加、DWL 减少。
基尼系数越大越平等 ✗ → 越大越不平等；洛伦兹曲线离对角线越远越不平等。累进税看边际（和平均）税率随收入上升。

✎Unit 6 Test 单元测验

20 道选择题，全部选自历年 AP 真题（与各节练习不重复，带图和表格的题按原题重绘），建议用时 24 分钟。先全部选完，再点底部“交卷”查看得分和解析。

T12022 AP Micro 国际卷 Q58

The efficient market outcome shown in the graph provided is

按原题重绘

at price P₁, quantity Q₁, and total economic surplus is J+K
at price P₂, quantity Q₂, and total economic surplus is B+C+D+E+F+G+H+I
at price P0, quantity Q₀, and total economic surplus is B+C+E+F
at price P0, quantity Q₀, and total economic surplus is A+B+C+E+F
at price P0, quantity Q₀, and total economic surplus is A+B+C+E+F+J

有效结果在供求交点：价格 P₀、数量 Q₀。CS = 需求以下、P₀ 以上 = A + B + C；PS = 供给以上、P₀ 以下 = E + F + J；总剩余 = A + B + C + E + F + J。(D) 漏了 J。

T22005 AP Micro Q10

At the current production level of good X, price is greater than marginal cost. Which of the following actions would lead to greater efficiency?

Decreasing the production of good X
Increasing the production of good X
Maintaining the current level of production of good X
Imposing a tax on the production of good X
Imposing an effective price floor on good X

P > MC：最后一单位对买方的价值高于它的成本 → 多生产能增加总剩余 → 增加产量，直到 P = MC。征税、有效价格下限都会让产量更少。

T32024 AP Micro Q30

Quantity	Marginal Private Cost ($)	Marginal Private Benefit ($)
21	25	65
22	35	55
23	45	45
24	55	35
25	65	25

The table provided shows the marginal private cost and marginal private benefit for Good X. The market for Good X is in equilibrium. If the production of Good X generates a constant marginal external cost of $20 per unit, which of the following actions will improve efficiency in this market?

Imposing a $20 per-unit tax on producers, which will decrease output to 22 units
Imposing a $20 per-unit tax on consumers, which will decrease output to 21 units
Granting a $20 per-unit subsidy to consumers, which will increase output to 25 units
Granting a $20 per-unit subsidy to producers, which will decrease output to 21 units
Implementing a price floor at $20, which will increase output to 24 units

市场均衡：MPC = MPB = $45，数量 23。MSC = MPC + $20 = 45、55、65……，MSB = MPB = 65、55、45…… → MSC = MSB = $55 在数量 22。对生产者征 $20 每单位税，把 MPC 抬到 MSC，产量降到 22 → (A)。(B) 对消费者征 $20 税，产量也是降到 22，不是 21；(C)(D)(E) 都增加产量。

T42023 AP Micro 国际卷 Q44

Home remodeling services are provided in a perfectly competitive market but generate noise that angers neighbors. What is the socially optimal quantity of home remodeling services and the area that represents deadweight loss associated with the market equilibrium?

按原题重绘

Q₁ and area ACD
Q₁ and area ABC
Q₂ and area ABC
Q₂ and area ACD
Q₁ and area CEF

负外部性：市场在 MPC = MPB 的 C 点（Q₂）生产，社会最优在 MSC = MSB 的 A 点（Q₁）。Q₁ 到 Q₂ 之间每单位都 MSC > MSB，DWL = 三角形 ABC。(A) ACD 夹在 MPC 和 MSB 之间，用错了曲线。

T52021 AP Micro 国际卷 Q27

Which of the following is NOT an effective way to manage the inefficiency resulting from a negative externality?

Government intervention in the form of obligatory controls
Per-unit taxes
Assignment of property rights
Lump-sum taxes
Private negotiations among affected parties

定额税不改变多生产一单位的边际成本，企业仍在 MPB = MPC 处生产，过度生产没有被纠正。管制、每单位税、界定产权、私人谈判都能起作用。

T62019 AP Micro 练习卷 Q15

Assume the demand curve for a good is perfectly inelastic and the production of each unit of this good generates external costs. A profit-maximizing firm producing the good in an unregulated free market will

generate deadweight loss because marginal social cost is greater than marginal private cost.
generate deadweight loss only if marginal costs are constant
not generate deadweight loss because the equilibrium quantity is socially optimal
not generate deadweight loss unless marginal costs are constant
not generate deadweight loss unless fixed costs are zero

需求完全无弹性（竖直）：MSB 就是那条竖线，不管成本线在哪里，MPC = MSB 与 MSC = MSB 都在同一个数量上 → 市场数量正好是社会最优，没有 DWL。外部成本仍然存在，只是不造成数量上的扭曲。

T72025 AP Micro 国际卷 Q59

The graph shows the market equilibrium for a good.

按原题重绘

What is the socially optimal quantity, and what is the marginal external benefit at that quantity?

Q₁, and the marginal external benefit is P₃ − P₂.
Q₁, and the marginal external benefit is P₂ − P₁.
Q₂, and the marginal external benefit is P₃.
Q₂, and the marginal external benefit is P₁.
Q₂, and the marginal external benefit is P₂ − P₁.

MSB 在 MPB 上方：消费的正外部性。社会最优在 MSB = MSC 的 Q₂；在 Q₂ 处 MSB = P₂、MPB = P₁，边际外部收益 = P₂ − P₁。(A) 的 P₃ − P₂ 是 Q₁ 处的距离，但最优数量不是 Q₁。

T82023 AP Micro 国际卷 Q34

Which of the following is an example of a good that is nonexcludable and rival in consumption?

Fish sandwich
Public highway
Private dance lesson
Online music
National defense

非排他但有竞争性 = 公共资源。免费的公共道路谁都能上（非排他），但多一辆车就让别人更堵（竞争）→ (B)。(A)(C) 是私人物品；(D) 网络音乐非竞争但可以排他（俱乐部物品）；(E) 国防是公共物品。

T92013 AP Micro Q60

Public goods are underproduced in a competitive market because the free-rider problem causes which of the following?

Fixed supply
Effective price ceilings
Reduced market demand
Negative externalities
Increased imports

搭便车的人不付钱也能享受，他们的支付意愿没有表现在市场上 → 市场（付费的）需求被低估 → 生产不足。

T102005 AP Micro Q59

An increase in the marginal social benefit of consuming a public good should result in

a decrease in public production of the good
an increase in the optimal quantity of the good
a decrease in the optimal quantity of the good
a decrease in the social costs associated with producing the good
a decrease in the social costs associated with consuming the good

最优数量在 MSB = MSC 处；MSB 上移而 MSC 不变 → 交点右移 → 最优数量增加。

T112024 AP Micro Q48

The market in the graph provided is currently in equilibrium. The government wishes to increase the equilibrium quantity of this good to 300 units. Which of the following interventions will achieve the government's goal?

按原题重绘

Implementing a price floor at $4
Imposing a $2 per-unit tax on producers
Granting a $2 per-unit subsidy to consumers
Granting a $2 per-unit subsidy to consumers and imposing a $2 per-unit tax on producers
Granting a $2 per-unit subsidy to consumers and granting a $2 per-unit subsidy to producers

均衡 Q = 200、P = $6。在 Q = 300：需求价格 $4，供给价格 $8，需要每单位共 $4 的“推力”。给消费者 $2 补贴（需求上移 $2）+ 给生产者 $2 补贴（供给下移 $2）→ 新交点正好在 300 → (E)。(C) 只给消费者 $2，数量只到 250；(D) 补贴和税互相抵消，仍是 200；(A) $4 的下限低于均衡价，不起作用。（2024 年非官方打字答案键给 (C)，有误。）

T122016 AP Micro 练习卷 Q48

按原题重绘

In the diagram above, if there is a price ceiling set at P₁, consumer surplus will be represented by the area

ABC
P₂BP₀
P₃BP₂
P₃ACP₁
P₃BCP₁

上限 P₁ 低于均衡 P₂，是有效的：只能买到 C 点的供给量。CS = 需求以下、P₁ 以上、到这个数量为止 = 梯形 P₃ACP₁。(E) P₃BCP₁ 把 C 点以后的数量也算进去了，但成交量没有那么多。

T132021 AP Micro 国际卷 Q5

Assume that the government imposes a binding price ceiling on a market for an inferior good. Which of the following is most likely to occur in the market if consumers’ incomes increase?

The market demand will shift to the right.
The market supply will shift to the right.
The shortage in the market will decrease.
The surplus in the market will increase.
The market price will increase.

低档品：收入增加 → 需求左移。价格被上限压着不动，需求量减少、供给量不变 → 短缺缩小。(A) 低档品的需求是左移，不是右移。

T142016 AP Micro 练习卷 Q55

Suppose that the demand for soft drinks is price elastic and the supply is price inelastic. If the government imposes a sales tax on soft drinks, which of the following will occur in the short run?

The tax burden will fall equally on both consumers and producers.
The tax burden will fall more on producers.
The tax burden will fall more on consumers.
The percentage increase in the price of soft drinks will be greater than the percentage increase in the quantity demanded.
The percentage decrease in total revenue will be greater than the percentage decrease in the quantity demanded.

税负更多落在更缺乏弹性的一方。需求富有弹性、供给缺乏弹性 → 生产者负担更多。

T152021 AP Micro 国际卷 Q38

The graph provided shows the demand (D), marginal revenue (MR), marginal cost (MC), and average total cost (ATC) curves for a monopolist. If a regulatory agency requires that the monopolist set a price that earns the monopolist a normal profit, what price will the monopolist set?

按原题重绘

P₁
P₂
P₃
P₄
P₅

“正常利润” = 零经济利润 → P = ATC：需求与 ATC 交于 Q₃ → 价格 P₃。P₅ 是不管制的垄断价格（MR = MC 在 Q₁）；P₁ 是 P = MC 的社会最优价格（Q₄）。

T162025 AP Micro 国际卷 Q51

Two monopolies with identical cost structures are compared. One is an unregulated profit-maximizing monopoly, and the other is regulated to produce the quantity at which it earns zero economic profit. Which of the following accurately describes the difference between the two monopolies?

The Unregulated Monopoly: Charges a price equal to average total cost；The Regulated Monopoly: Charges a price equal to marginal cost
The Unregulated Monopoly: Creates deadweight loss in the market；The Regulated Monopoly: Eliminates deadweight loss from the market
The Unregulated Monopoly: Experiences economies of scale；The Regulated Monopoly: Experiences diseconomies of scale
The Unregulated Monopoly: Collects all consumer surplus from the consumers；The Regulated Monopoly: Transfers all consumer surplus to the government
The Unregulated Monopoly: Produces where marginal revenue equals marginal cost；The Regulated Monopoly: Produces where price equals average total cost

不受管制的垄断企业在 MR = MC 处生产；被管制到零经济利润的企业在 P = ATC 处生产。(B) 只有 P = MC 才消除 DWL，P = ATC 只是减少 DWL；(A) 两边都说错了。

T172016 AP Micro 练习卷 Q45

The following table shows Kent’s annual income over two years and the income taxes paid in each year.

	Annual Income	Income Taxes
Year 1	$100,000	$20,000
Year 2	$120,000	$30,000

With no changes in the marginal tax-rate schedule, which of the following is true about the tax rates?

The marginal tax rate was equal to the average tax rate for both incomes.
The marginal tax rate on the last $20,000 earned was greater than the marginal tax rate at $100,000 income, but the average tax rate decreased with the increase in income.
The average tax rate when Kent earned $120,000 was less than the marginal tax rate on the last $20,000 he earned.
The amount paid in income tax increased as income increased, so the tax is proportional.
The amount paid in income tax increased as income increased, so the tax is regressive.

平均税率：$20,000/$100,000 = 20%，$30,000/$120,000 = 25%。最后 $20,000 的边际税率 = $10,000/$20,000 = 50%。所以收入 $120,000 时平均税率 25% < 边际税率 50% → (C)。平均税率随收入上升，是累进税，(D)(E) 错。原考试只有 33% 的人答对。

T182017 AP Micro 练习卷 Q26

In a given year, Jennifer earns $50,000 and spends $40,000. During the same period, Steve earns $30,000 and spends $27,000. If Jennifer and Steve both must pay a 10 percent sales tax on goods purchased, the sales tax is

a higher percentage of income for Jennifer than for Steve
a higher percentage of spending for Steve than for Jennifer
progressive with respect to income
regressive with respect to income
proportional with respect to income

Jennifer 交税 $4,000 = 收入的 8%；Steve 交税 $2,700 = 收入的 9%。收入低的人交的比例更高 → 销售税相对收入是累退的。

T192012 AP Micro Q45

Quintile	Before Taxes and Transfers	After Taxes and Transfers
Lowest 20 percent	1.1	5.1
Second 20 percent	7.9	11.1
Third 20 percent	15.5	16.5
Fourth 20 percent	24.7	23.8
Highest 20 percent	50.7	43.5

The table above shows the distribution of income in Country X in 2010 before and after taxes and transfer payments. Which of the following can be concluded about the effect of the government’s tax and transfer policies on income distribution in Country X?

The distribution of income is significantly less equal after accounting for the impact of government policies.
The distribution of income is about the same after accounting for the impact of government policies.
The largest gainers from government policies are the third and fourth quintile groups.
The only quintile to benefit from government policies is the lowest quintile.
The major transfer of income is from the highest quintile to the two lowest quintiles.

各组份额变化（百分点）：最低 +4.0，第二 +3.2，第三 +1.0，第四 −0.9，最高 −7.2。最高 20% 少了 7.2，最低两组合计多了 7.2 → 主要是从最高组转移到最低两组。分配明显更平等了，(A)(B) 错。

T202024 AP Micro Q28

Suppose a country has a single person with all the income while all other people have no income. What is the Gini coefficient for that country?

−1
−0.5
0
0.5
1

一个人拿走全部收入 = 完全不平等 → 基尼系数 = 1。完全平等是 0，基尼系数不可能是负数。

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FRQFree-Response Practice 自由问答

前三道是 AP 真题，评分标准依照 College Board 官方 Scoring Guidelines 整理；第四道是依照 AP 题型原创的综合题。先在纸上完整作答（包括画图），再展开对照。

FRQ 1 · 2021 AP Micro FRQ Set 2 Q2 · 5 分 · 铜矿的污染真题

Copper is produced in a perfectly competitive market with an upward-sloping supply curve and a downward-sloping demand curve. Assume the production of copper results in liquid waste, which seeps into local rivers. The contaminated river water causes human illnesses and crop failures downstream. The marginal external cost from producing copper is constant across all quantities of copper produced.

(a) Draw a correctly labeled graph of the copper market with the marginal social benefit (MSB), marginal private benefit (MPB), marginal social cost (MSC), and marginal private cost (MPC) curves, and show each of the following.
  (i) The market equilibrium quantity, labeled Q_M
  (ii) The socially efficient quantity, labeled Q_S
(b) Suppose the demand for copper decreases. On your graph in part (a), show the deadweight loss at the new market equilibrium, shaded completely.
(c) Suppose the government is considering levying a tax on copper.
  (i) What per-unit tax level would achieve the socially optimal quantity?
  (ii) Explain why a lump-sum tax on producers will not achieve the socially optimal quantity in the short run.

Point	得分要求与解析
(a) 1 pt	正确标注的图：向上倾斜的供给标 MPC，向下倾斜的需求标 MPB，市场均衡数量 Q_M 在 MPB = MPC 处。
(a) 1 pt	MSC 在每个产量上都高于 MPC，社会有效数量 Q_S 在 MSB = MSC 处。（边际外部成本不变，MSC 与 MPC 平行；没有消费外部性，MSB 与 MPB 是同一条线。）
(b) 1 pt	需求左移，并把新的 DWL 整块涂满：新需求以上、MSC 以下，在新需求与 MSC 的交点和新需求与 MPC 的交点之间。
(c)(i) 1 pt	每单位税 = 边际外部成本（MSC − MPC）。
(c)(ii) 1 pt	定额税不改变边际成本，所以生产者的产量不变。

(a) 参考图：MPB = MSB 向下倾斜，MPC 向上倾斜，MSC 在 MPC 上方且平行（边际外部成本不变）。Q_M 在 MPB = MPC，Q_S 在 MSB = MSC，Q_S < Q_M。

(b) 参考图：需求左移到 D₂。新的 DWL 是 Q_S2 与 Q_M2 之间、MSC 下方、新需求上方的三角形，要整块涂满。

常见失分：(b) 只涂了新需求和 MPC 之间的部分，或者把原来的 DWL 也涂上；(c)(i) 只写“征税”或写一个具体数字，没有说“等于边际外部成本”。

FRQ 2 · 2018 AP Micro FRQ Q2 · 6 分 · 文字处理软件真题

Modern-day word processing software gives consumers the ability to create and save documents in different file formats that can then be accessed by multiple computer operating systems. The graph below depicts a perfectly competitive market for word processing software. In the graph, MSB is the marginal social benefit, MPB is the marginal private benefit, MPC is the marginal private cost, and MSC is the marginal social cost.

按原题重绘

(a) Identify the type of market failure illustrated by the graph. Explain.
(b) Using the numbers on the graph, identify the market equilibrium price and quantity.
(c) Using the labeling on the graph, identify the area representing the deadweight loss at the quantity identified in part (b).
(d) Suppose the government is considering granting a subsidy to correct the market failure. What is the dollar value of the per-unit subsidy that would achieve the socially optimal quantity?
(e) Suppose the government does not grant the subsidy and instead imposes a price floor at $8.
(i) How many units will consumers and producers exchange at the price floor?
(ii) Does the price floor correct the market failure? Explain.

Point	得分要求与解析
(a) 1 pt	正外部性，并解释：MSB > MPB，或市场均衡数量少于社会最优数量。
(b) 1 pt	市场均衡在 MPB = MPC 的 E 点：价格 $6，数量 16。
(c) 1 pt	DWL = DEF（16 到 24 之间、MSB 以下、MSC 以上）。
(d) 1 pt	每单位补贴 = $4（MSB 与 MPB 的竖直距离，例如在 24 处 $8 − $4）。
(e)(i) 1 pt	8 单位：价格 $8 时消费者只买 8（MPB 上的 B 点），成交量由较小的需求量决定。
(e)(ii) 1 pt	No，理由任选其一：成交量 8 少于社会最优的 24；在 8 单位处 MSB > MSC；DWL 比原来更大。

市场 16 已经生产不足，价格下限让成交量再降到 8，DWL 从 DEF 扩大成 A、F、C 之间的大三角形（ACF）——“干预”不等于“纠正”。

FRQ 3 · 2017 AP Micro FRQ Q3 · 7 分 · 有负外部性的垄断真题

The graph below shows the marginal social cost (MSC), marginal private cost (MPC), marginal social benefit (MSB), demand (D), and marginal revenue (MR) curves for a monopoly.

按原题重绘

(a) Identify the monopolist's
  (i) profit-maximizing quantity
  (ii) profit-maximizing price
(b) What information in the graph indicates that there is a negative externality?
(c) Identify the socially optimal quantity.
(d) In the case in which the government imposes a per-unit tax equal to the marginal external cost, identify each of the following.
  (i) The dollar value of the tax, using the price labels from the graph
  (ii) The profit-maximizing quantity associated with the tax
(e) Given the monopoly facing the negative externality, would the deadweight loss increase, decrease, or stay the same as a result of imposing the per-unit tax? Explain.

Point	得分要求与解析
(a)(i) 1 pt	利润最大的产量 Q₃（MR = MPC）。
(a)(ii) 1 pt	利润最大的价格 P₄（从 Q₃ 往上读需求）。
(b) 1 pt	MSC > MPC（MSC 曲线在 MPC 上方）。
(c) 1 pt	社会最优数量 Q₃（MSB = MSC）。
(d)(i) 1 pt	税额 = P₄ − P₁（Q₃ 处 MSC 与 MPC 的距离）。
(d)(ii) 1 pt	征税后企业的边际成本变成 MSC，在 MR = MSC 处生产 → Q₂。
(e) 1 pt	DWL 增加：征税前垄断产量正好等于社会最优数量 Q₃，征税后产量 Q₂ 低于社会最优。

这道题的关键：垄断本身让产量偏少，负外部性让产量偏多，两种扭曲在这里正好抵消。这时再征庇古税，反而把产量压到最优以下。政府干预要先看清楚市场离最优有多远（CED POL-4.A.4）。

FRQ 4 · 原创 · 6 分 · 松湖镇 Pine Lake 原创

Pine Lake is a fishing lake. Anyone may fish in the lake at no charge, and every fish caught by one person cannot be caught by anyone else.

(a) Identify the type of good that the fish in Pine Lake represent. Explain using the concepts of rivalry and excludability.
(b) Explain why, without government intervention, fishers will catch more than the socially optimal quantity of fish.
(c) The town is also considering a free fireworks show over the lake that everyone in town can watch. Explain why a private firm would provide less than the socially optimal quantity of fireworks shows.
(d) The table below shows the percentage of the town's total income received by each quintile of households before and after a new tax-and-transfer program.

Quintile	Before the Program (%)	After the Program (%)
Lowest 20 percent	4	7
Second 20 percent	9	12
Third 20 percent	15	16
Fourth 20 percent	22	21
Highest 20 percent	50	44

(i) Calculate the cumulative percentage of income received by the lowest 40 percent of households after the program. Show your work.
(ii) As a result of the program, will the Gini coefficient increase, decrease, or stay the same? Explain.
(e) Under the new program, Maya earns $40,000 per year and pays $4,000 in income tax, and Ben earns $100,000 per year and pays $15,000 in income tax. Is the income tax progressive, regressive, or proportional? Explain using numbers.

Point	评分要点与解析（原创题，按 AP 评分方式整理，非官方）
(a) 1 pt	公共资源（common resource）：竞争——一个人钓走的鱼别人就钓不到；非排他——任何人都可以免费钓。
(b) 1 pt	每个钓鱼的人只考虑自己的成本和收益，不考虑自己多钓一条让别人以后能钓的鱼变少（给别人带来的成本）→ MSC > MPC → 过度捕捞（公地悲剧）。
(c) 1 pt	烟花表演非排他（且非竞争）：不付钱也能看，人们会搭便车，私人企业收不到足够的收入，所以提供得比社会最优少。
(d)(i) 1 pt	7% + 12% = 19%。
(d)(ii) 1 pt	基尼系数下降：最低三组的份额上升、最高两组下降，洛伦兹曲线向完全平等线靠近，收入分配更平等。
(e) 1 pt	累进：Maya 的平均税率 $4,000/$40,000 = 10%，Ben 的平均税率 $15,000/$100,000 = 15%，收入越高税率越高。

参考图：用累计份额画洛伦兹曲线。实施后（蓝）比实施前（红）更靠近完全平等线 → 收入更平等，基尼系数下降。（AP 不要求画洛伦兹曲线或计算基尼系数，这张图只帮助理解。）

FRQ 答题提醒

外部性图：四条线都要标名字（MPB、MSB、MPC、MSC），哪两条重合就写“MPB = MSB”或“MPC = MSC”。
Q_M 读私人曲线的交点，Q_S 读社会曲线的交点，两个都要用虚线引到横轴。
DWL 要“shaded completely”：从 Q_S 到 Q_M，夹在两条社会曲线（或社会曲线与需求）之间的整个三角形。
税额、补贴额用“边际外部成本 / 收益”或图上的价格标签作答（如 P₄ − P₁），不要写成面积。
“Explain”题要说出机制：定额税 → 不改变 MC；公共物品 → 非排他 → 搭便车；累进 → 用数字算平均税率。

接下来上一单元：→ 打开 Unit 5 Factor Markets（与本文件放在同一文件夹）。
Unit 6 是 AP 微观的最后一个单元。复习时可以把 Unit 2 的消费者剩余、生产者剩余和 DWL 与本单元的外部性、税收图放在一起对照。

JL · AP Microeconomics · Unit 6 Market Failure and the Role of Government · 题目标签：真题历年 AP 考试原题（含 College Board 练习卷、国际卷），答案均对照官方答案键并重新验算（2024 年题目的答案为自行验算） · 原创依照 AP 题型原创