第6章推导了竞争性企业的供给曲线：在 $P = MC$ 处生产。但这一结果假设企业是价格接受者——相对于市场而言太小，无法影响价格。许多现实市场违反了这一假设。单一卖方（垄断者）自行定价。少数大型企业（寡头垄断者）必须考虑竞争对手的反应。本章描绘市场结构的谱系，并引入博弈论作为战略互动的语言。
前置知识：第6章（成本曲线、利润最大化、拉格朗日乘数法）。
在第6章中，我们证明了竞争性企业在 $P = MC$ 处实现利润最大化。在长期中，自由进入和退出导致进一步的结果。
经济利润为零并不意味着企业遭受损失。这意味着它们获得了正常回报——恰好覆盖所有成本，包括资本的机会成本。会计利润仍然为正。
其中 $P(Q)$ 是反需求函数——它给出垄断者要销售 $Q$ 单位必须设定的价格。与竞争企业（以价格为给定）不同，垄断者认识到增加销量需要降低价格。
这说明了什么： The monopolist chooses how much to produce by balancing two forces: producing more means more revenue from additional sales, but it also means lowering the price on every unit. Profit is total revenue minus total cost, and the monopolist picks the quantity where the gap is largest.
为什么这很重要： Unlike a competitive firm that simply takes the market price and decides how much to make, the monopolist controls the price through its output decision. This single difference -- that the firm faces the entire demand curve rather than a flat price line -- is what generates all of monopoly theory: restricted output, higher prices, and deadweight loss.
什么发生变化： If costs rise, the monopolist produces less and charges more. If demand shifts outward (more consumers, higher willingness to pay), the monopolist produces more but also charges more -- pocketing much of the increase as profit rather than passing it through as lower prices.
In Full Mode, Eq. 6.2 states the formal optimization problem.边际收益由两部分构成：
对于向下倾斜的需求曲线，$dP/dQ < 0$，所以 $MR < P$。对于线性需求 $P = a - bQ$：$TR = aQ - bQ^2$，所以 $MR = a - 2bQ$。MR曲线与需求曲线有相同的截距但斜率是两倍。
垄断者永远不会在 $MR < 0$ 处生产（因为减少产量反而能增加收入），因此垄断者只在需求的弹性区间运营，即 $|\arepsilon_d| > 1$。
利润最大化条件：
这说明了什么： Marginal revenue is the extra revenue from selling one more unit. For a monopolist facing a downward-sloping demand curve, MR is always less than price because lowering the price to sell one more unit reduces revenue on all existing units.
为什么这很重要： The gap between price and MR is why monopolists restrict output — they stop producing before the competitive quantity because each additional unit erodes revenue on previous sales. The firm maximizes profit by producing until MR exactly equals MC.
什么发生变化： When demand becomes more elastic (consumers more price-sensitive), MR gets closer to price and the monopolist behaves more like a competitive firm. When demand is inelastic, MR is actually negative — the monopolist would never produce in the inelastic portion of demand.
In Full Mode, Eqs. 6.3–6.4 derive MR from the revenue function and show the profit-maximization condition.这说明了什么： A monopolist faces a dilemma that competitive firms do not: to sell one more unit, it must lower the price on every unit, not just the last one. So the extra revenue from selling one more unit (marginal revenue) is always less than the price. The monopolist produces where MR = MC and charges a markup. The Lerner Index measures that markup: it equals the inverse of demand elasticity. If customers have few alternatives (inelastic demand), the monopolist charges a bigger markup.
为什么这很重要： This is why monopolies restrict output and raise prices — not out of villainy, but because the math of facing a downward-sloping demand curve makes it profitable to sell less at a higher price. The deadweight loss comes from units that consumers value more than they cost to produce, but the monopolist withholds because selling them would require cutting the price on all other units.
什么发生变化： When demand becomes more elastic (consumers have more substitutes), the Lerner index falls and the monopolist's markup shrinks — the price moves closer to marginal cost. When demand is very inelastic (few alternatives), the monopolist can charge a much larger markup. This is why pharmaceutical companies with patented drugs charge far more above cost than, say, a local cable company facing satellite competition.
In Full Mode, Eq. 6.5 derives the Lerner index from the MR = MC condition.边际成本之上的加价等于需求价格弹性（绝对值）的倒数。需求弹性越大意味着市场力量越小。
需求：$P = 100 - 2Q$。成本：$TC = 20Q$（常数 $MC = 20$）。
$TR = 100Q - 2Q^2$，$MR = 100 - 4Q$。
$MR = MC$：\$100 - 4Q = 20 \implies Q_M = 20$，$P_M = 60$。
$\Pi = (60 - 20)(20) = 800$。
竞争结果：$P = MC = 20$，$Q_C = 40$。
$DWL = \frac{1}{2}(60 - 20)(40 - 20) = 400$。
勒纳指数：$(60 - 20)/60 = 2/3$。验证：$\varepsilon_d = (dQ/dP)(P/Q) = (-1/2)(60/20) = -1.5$，所以 $1/|\varepsilon_d| = 2/3$。✓
调整边际成本，观察垄断者的最优价格、产量、利润和无谓损失如何变化。切换竞争结果叠加层进行比较。
图 6.2.垄断者将产量限制在MR = MC处，定价高于边际成本。蓝色矩形是垄断利润；黄色三角形是无谓损失。切换竞争叠加层可以看到有效结果。
Lina Khan was a 28-year-old law student when she published "Amazon's Antitrust Paradox" — an argument so influential it got her appointed chair of the FTC. Her claim: the consumer welfare standard that has governed antitrust since the 1980s is blind to Amazon's power because it only looks at prices. Amazon keeps prices low, so the standard says there's no problem. Khan says the standard is broken. By the Lerner index you just learned, she's making a radical claim — that market power can exist even when $(P - MC)/P$ is near zero.
中级企业对每个消费者收取其最高支付意愿。这提取了全部消费者剩余。产量是有效的（$Q = Q_C$）——没有无谓损失——但所有剩余归企业所有。
企业提供不同的定价方案（数量折扣、捆绑销售、版本定价）让消费者自行选择。例如：机票（商务舱与经济舱）、软件（基础版与专业版）、批量定价。
企业识别具有不同弹性的群体，对每个群体收取不同的价格：
由于 $MR = P(1 - 1/|\varepsilon|)$（来自MR与弹性的关系），各市场上MR相等意味着需求弹性较小的群体（替代品较少、转换成本较高）必须被收取更高的价格。最优价格比满足 $P_1/P_2 = (1 - 1/|\varepsilon_2|)/(1 - 1/|\varepsilon_1|)$。
这说明了什么： A price-discriminating firm sets marginal revenue equal across all markets and equal to marginal cost. This means the firm charges higher prices to customers who are less price-sensitive (more inelastic demand) and lower prices to those who are more price-sensitive.
为什么这很重要： This is the logic behind student discounts, senior pricing, regional pricing, and surge pricing. The firm is not being charitable to students -- it is extracting more total revenue by charging different prices to groups with different willingness to pay. Airlines do this with extraordinary precision: business travelers pay more because they have less flexibility.
什么发生变化： If the elasticity gap between markets narrows (both groups become equally price-sensitive), the optimal prices converge and discrimination becomes unprofitable. If arbitrage becomes possible (students resell to adults), the price discrimination collapses to a single price.
In Full Mode, the MR-elasticity relation shows exactly how the price ratio depends on the elasticity ratio.需求弹性更低的群体支付更高的价格。
一家剧院面对两个市场。成人需求：$P_A = 20 - Q_A$。学生需求：$P_S = 12 - Q_S$。$MC = 2$。
成人：$MR_A = 20 - 2Q_A = 2 \implies Q_A = 9$，$P_A = 11$。
学生：$MR_S = 12 - 2Q_S = 2 \implies Q_S = 5$，$P_S = 7$。
总利润：$(11-2)(9) + (7-2)(5) = 81 + 25 = 106$。
两个需求弹性不同的市场。调整MC，观察每个市场的最优价格和产量如何变化。
市场A（成人）： $P_A = 20 - Q_A$
市场B（学生）： $P_S = 12 - Q_S$
短期：企业可能获得正利润或负利润。长期：进入和退出驱动经济利润归零。每家企业在其需求曲线与平均成本曲线相切处生产——而非平均成本的最低点。
In long-run equilibrium, each firm produces where its demand curve is tangent to its AC curve. The tangency condition imposes two simultaneous requirements:
Because the firm faces a downward-sloping demand curve, the tangency point occurs to the left of the AC minimum — firms produce below the efficient scale.
这说明了什么： In the long run, monopolistic competition produces a distinctive outcome: firms earn zero economic profit (free entry competed away the profits), but they still charge above marginal cost (product differentiation gives each firm a small monopoly on its particular variety). The firm operates below the scale that minimizes average cost.
为什么这很重要： This is the "price of variety." Having 50 different restaurants instead of 50 identical cafeterias means each restaurant serves fewer customers and operates below its most efficient scale. Whether this is truly inefficient depends on how much consumers value the differentiation itself.
什么发生变化： If products become less differentiated (more substitutable), each firm's demand curve becomes more elastic, the markup shrinks, and the outcome approaches perfect competition. If entry barriers increase, firms can sustain positive profit in the long run — moving the outcome toward monopoly.
In Full Mode, Eq. 6.8 shows the tangency condition that pins down the long-run equilibrium.这意味着垄断竞争相对于完全竞争有两种"低效"：
这些是否真的低效是有争议的。Dixit-Stiglitz框架表明消费者重视多样性——拥有50家不同的餐厅比50家相同的餐厅更有价值，即使相同的餐厅更便宜。边际成本之上的加价是"多样性的价格"。
在第2章中，比较优势在完全竞争下为自由贸易做出了干净的论证。你现在掌握了垄断竞争和策略性互动。下面说明不完全竞争如何使那个故事复杂化。
在垄断竞争下（Krugman 1980），贸易允许更多产品种类并利用规模经济——带来超越比较优势的贸易收益。各国贸易不是因为彼此不同，而是因为消费者看重种类、企业受益于更大的市场。但在古诺寡头下（Brander-Spencer 1985），对本国企业的政府补贴可以把纳什均衡向其有利方向移动，从外国对手那里攫取租金。幼稚产业论也获得了形式化基础：如果生产涉及边做边学（成本随累积产出下降），临时保护可以让企业沿成本曲线下移，使其在长期具有竞争力。战略性贸易理论说，在不完全竞争下，贸易政策可以在国家之间转移利润——自由贸易不再自动最优。
反对战略性贸易：它要求政府挑选赢家——识别哪些产业具有合适的市场结构和学习曲线使干预奏效。政府失灵（游说、腐败、信息问题）使这在实践中很危险。有益战略性贸易的理论条件是刀尖上的：政府必须了解需求弹性、成本结构以及对手政府的反应。反对幼稚产业：历史记录不一——许多"幼稚"产业从未长大。保护为政治上有关系的企业制造租金，而非真正的学习。而且一旦授予了保护，取消保护的政治经济学是残酷的——受益者会游说使其永久化。
中国冲击文献之后主流观点发生了转变。2010年之前，共识强烈支持自由贸易，以再分配作为配套政策。2010年之后，学界承认贸易的调整成本比此前假定的更大、更持久、在地理上更集中（Autor, Dorn & Hanson 2013, 2016）。本应补偿输家的贸易调整援助项目规模小、效果差。克鲁格曼本人——部分因为证明了不完全竞争下的贸易收益而获得诺奖——承认分配效应被低估了数十年。
自由贸易在大多数时候对大多数国家仍是净正向的——第2章的比较优势逻辑是稳健的，克鲁格曼的垄断竞争模型增加了来自种类和规模的进一步收益。但无条件的论据已经变弱。分配效应比学界数十年承认的更大，补偿机制已经失败。战略性贸易和幼稚产业论在理论上有价值，但在实践中危险——政府失灵是具有约束力的制约。诚实的答案：自由贸易是正确的默认，策略性干预可以奏效但通常不会，贸易的输家需要真正的补偿，而非承诺。
此处的模型是静态的——它们比较一个均衡与另一个均衡。在一个有供应链依赖（半导体、稀土、能源）的世界里，我们该如何思考贸易？保护的经济安全论据与效率论据不同。而且宏观经济维度完全缺失：贸易逆差、资本流动和汇率都影响这个故事。请在第17章（§17.1–17.7）回来看，那里开放经济宏观框架把国际收支核算、不可能三位一体和全球失衡加入画面。
“关税人”说关税让美国更富。经济学家说这是对美国消费者征税。2018年贸易战提供了现代数据中的首个重大检验案例。
中级企业同时选择产量。每家企业的最优产量取决于其他企业的产量。
设定。两家企业，需求 $P = a - b(q_1 + q_2)$，两家的边际成本均为常数 $c$。
企业1的最优反应函数：
Firm 1 maximizes $\Pi_1 = [a - b(q_1 + q_2) - c] \cdot q_1$. Taking the first-order condition:
Solving for $q_1$ gives the best response function:
古诺-纳什均衡（联立求解）：
这说明了什么： Each firm picks its quantity by asking: "Given what my rival produces, what quantity maximizes my profit?" The best response function captures this strategic interdependence -- if my rival produces more, I should produce less (since total output drives the price down). The equilibrium is where both firms are simultaneously best-responding: neither wants to change. Each duopolist produces one-third of the competitive output; together they produce two-thirds.
为什么这很重要： Cournot shows that oligopoly outcomes fall between monopoly and perfect competition. More firms push the market closer to the competitive outcome. This is the formal basis for antitrust intuitions about market concentration: fewer firms means higher prices and more deadweight loss.
什么发生变化： When a rival expands production, the best response is to contract -- the reaction functions slope downward. Adding more firms to the market shrinks each firm's share and pushes the price toward marginal cost. With 2 firms, the industry produces 2/3 of competitive output; with 5 firms, 5/6; with 20 firms, the market is essentially competitive. Higher marginal costs shift the equilibrium toward lower output and higher prices for all firms.
In Full Mode, Eqs. 6.7-6.10 derive the best response functions and solve for the Cournot-Nash equilibrium.对称的 $n$ 家企业，$q_i = (a-c)/((n+1)b)$ 且当 $n \to \infty$ 时 $P \to c$。
这说明了什么： As the number of firms grows, each firm's share of the market shrinks, and the total output rises. With enough firms, the oligopoly outcome becomes indistinguishable from perfect competition: price equals marginal cost, economic profit vanishes, and deadweight loss disappears.
为什么这很重要： This is the Cournot convergence result -- it provides the bridge between monopoly (one firm, maximum market power) and perfect competition (many firms, zero market power). It gives precise meaning to the idea that "more competition is better": each additional firm moves the price closer to cost.
什么发生变化： With 2 firms, the markup is substantial. With 5 firms, it is much smaller. With 20 firms, the market is essentially competitive. The speed of convergence depends on cost structure: when marginal costs are high relative to demand, fewer firms suffice to drive the market toward competition.
In Full Mode, the n-firm Cournot formula shows the exact relationship between the number of firms and the market outcome.需求：$P = 100 - Q$，$c = 10$。最优反应：$q_i^* = 45 - q_j/2$。
均衡：$q_1^C = q_2^C = 30$。$Q^C = 60$，$P^C = 40$。$\Pi_i = 900$。
| 结构 | 产量 | 价格 | 行业利润 | 无谓损失 |
|---|---|---|---|---|
| 竞争 | 90 | 10 | 0 | 0 |
| 古诺双寡头 | 60 | 40 | 1,800 | 450 |
| 垄断 | 45 | 55 | 2,025 | 1,012.5 |
将企业数量从1（垄断）滑动到20。观察总产量上升、价格下降、无谓损失趋近于零——市场趋向完全竞争。
图 6.3a。随着N增加，古诺结果趋向完全竞争。N=1时为垄断。柱状图展示关键指标如何随市场结构变化。
调整每家企业的边际成本，观察反应函数如何移动以及均衡点如何变化。不对称成本导致不对称产出。
图 6.3b。每家企业的反应函数向下倾斜：对手产量增加会降低最优反应产量。交叉点是古诺-纳什均衡。拖动成本滑块可以看到不对称成本如何移动反应函数和均衡点。
在第2章，竞争模型给出了干净的答案：高于均衡的最低工资会造成失业。你现在掌握了垄断、寡头，以及对市场势力建模的工具。下面是当劳动市场并非竞争性时会发生什么。
把§6.2的垄断框架应用到劳动市场，但方向反过来：不是考虑拥有市场势力的单一卖方，而是考虑劳动的单一买方——买方垄断者。企业面对向上倾斜的劳动供给曲线 $w(L)$，$w' > 0$。劳动的边际成本超过工资：$MC_L = w + w' \cdot L$。企业在 $MC_L = MRP_L$ 处雇用，工资低于竞争水平，就业也低于竞争水平。现在在买方垄断工资与竞争工资之间施加一个最低工资。企业的劳动边际成本在最低工资处变得平坦（在一定范围内），这意味着它会雇用更多而非更少的工人。最低工资可以同时提高就业和收入。在竞争工资之上，标准的失业预测又回来了。
即使单个企业拥有一些劳动市场势力，工人也能在雇主、产业和城市之间流动。劳动流动性在长期限制买方垄断势力。经验上相关的问题是实际存在多少买方垄断势力——这在不同部门、地区和工人类型之间差异巨大。小镇上的快餐可能接近买方垄断；旧金山的科技招聘则接近竞争性。"新买方垄断"文献（Manning 2003）认为搜寻摩擦和迁移成本即使在有许多雇主时也制造买方垄断势力——但其程度，以及因此最低工资的就业效应，仍然是一个仅凭理论无法裁决的经验问题。
主流早就把买方垄断作为一种理论可能性吸收进来——Joan Robinson在1933年将其形式化。但在Card和Krueger 1994年的标志性研究之前，学界把买方垄断视为经验上罕见，而竞争模型的失业预测是主导结果。"新买方垄断"文献把概念从"公司镇上的唯一雇主"拓展到"由于搜寻摩擦、迁移成本和信息不对称，雇主拥有一定的工资设定权"——这比教科书的买方垄断要常见得多。
理论现在很清楚：最低工资的效应取决于买方垄断势力的程度。"总是导致失业"和"从不导致失业"作为一般性主张都是错的。正确的理论答案是"取决于市场结构"——而市场结构因劳动市场而异。§6.5的古诺模型提供了类比：正如寡头的福利效应取决于企业数量和市场势力程度，最低工资的就业效应也取决于劳动市场的结构。竞争模型和买方垄断模型是一个谱的两端。
理论给出一个有条件的预测：就业效应取决于市场结构。但哪种市场结构在经验上相关？我们需要数据来裁决。请在第10章（§10.4）回来看，那里用双重差分分析Card和Krueger的自然实验——这一计量方法开启了竞争与买方垄断预测之间30年的经验之战。
买方垄断模型说适度上调可以提高就业。但旧金山的 \$15 与密西西比乡村的 \$15 非常不同。答案取决于当地的工资咬合度——以及当地雇主市场势力的程度。
中级在伯特兰模型中，企业同时选择价格（而非产量）。在产品相同且边际成本相等时：
仅有两家企业，价格竞争就复现了完全竞争结果。这就是伯特兰悖论：古诺模型说需要很多企业才能实现竞争；伯特兰模型说两家就够了。
这说明了什么： When two firms sell identical products and compete on price, a relentless undercutting logic drives the price all the way down to marginal cost. If Firm A charges \$20 and Firm B charges \$19.99, every customer goes to Firm B. So Firm A cuts to \$19.98, then Firm B cuts to \$19.97 -- and this continues until neither can go lower without losing money. The result: the competitive outcome with just two firms.
为什么这很重要： This is the Bertrand paradox -- it says the number of firms is not what determines market power. What matters is how firms compete. Quantity competition (Cournot) preserves market power with few firms; price competition (Bertrand) destroys it immediately. The real-world question is which model better fits a given industry.
什么发生变化： The paradox dissolves when products are differentiated (a small price cut does not steal the entire market), when firms have capacity constraints (they cannot serve everyone), when firms interact repeatedly (enabling tacit collusion), or when consumers face search costs (they do not instantly switch). Most real markets have some combination of these frictions, which is why we rarely see pure Bertrand outcomes.
In Full Mode, the undercutting argument is stated precisely: any P > MC is not a Nash equilibrium because a rival can profitably deviate.悖论消解的条件：
两家企业销售差异化产品。企业 $i$ 的需求：$q_i = 100 - 2p_i + p_j$（产品是替代品但非完全相同）。边际成本：$c = 10$。
企业1最大化：$\Pi_1 = (p_1 - 10)(100 - 2p_1 + p_2)$。
一阶条件：\$100 - 4p_1 + p_2 + 20 = 0 \implies p_1^*(p_2) = \frac{120 + p_2}{4} = 30 + p_2/4$。
由对称性：$p^* = 30 + p^*/4 \implies p^* = 40$。
每家企业：$q^* = 100 - 80 + 40 = 60$。$\Pi^* = 30 \times 60 = 1{,}800$。
在差异化产品下，均衡价格（\$40$）超过边际成本（\$10$）。伯特兰悖论消解了，因为小幅降价不再能夺取整个市场。
在施塔克尔伯格模型中，一家企业（领导者）先行动，选择其产量。跟随者观察领导者的选择后进行优化。领导者将跟随者的反应函数内部化。
Step 1 (Follower's problem): The follower observes $q_1$ and maximizes $\Pi_2 = [a - b(q_1 + q_2) - c] \cdot q_2$. This yields the same best-response function as Cournot: $q_2^*(q_1) = \frac{a - c}{2b} - \frac{q_1}{2}$.
Step 2 (Leader's problem): The leader substitutes the follower's best response into its own profit function: $\Pi_1 = [a - b(q_1 + q_2^*(q_1)) - c] \cdot q_1$. Maximizing gives:
这说明了什么： When one firm moves first, it can commit to a large quantity, forcing the follower to accommodate by producing less. The leader produces half the competitive output (the monopoly quantity); the follower produces only half of what the leader does. Total output exceeds Cournot, so the price is lower.
为什么这很重要： Commitment has strategic value. By going first and locking in a large quantity, the leader effectively says "I am flooding the market -- adjust accordingly." This is the formal logic behind first-mover advantages in industries where capacity decisions are hard to reverse (factories, infrastructure, spectrum licenses).
什么发生变化： If the leader's cost advantage grows, it produces even more and squeezes the follower further. If commitment becomes less credible (the leader can easily reverse its decision), the game reverts toward the Cournot outcome because the follower no longer needs to accommodate. The asymmetry depends entirely on the irreversibility of the leader's move.
In Full Mode, Eqs. 6.12-6.13 derive the Stackelberg quantities via backward induction.领导者生产垄断产量，跟随者生产其一半。总产量超过古诺；价格更低。先行者优势来自于在跟随者选择之前承诺大产量。
$P = 100 - Q$，$c = 10$：
$q_1^S = 45$，$q_2^S = 22.5$。$Q^S = 67.5$，$P^S = 32.5$。
$\Pi_1 = 1{,}012.5$（领导者），$\Pi_2 = 506.25$（跟随者）。
领导者利润超过古诺（\$1{,}012.5 > 900$）。跟随者境况更差（\$506.25 < 900$）。
在同时博弈（古诺）和序贯博弈（施塔克尔伯格）之间切换，使用 $P = 100 - Q$、$c = 10$ 比较产量和利润。
图 6.4.比较古诺（对称）和施塔克尔伯格（领导者优势）。在反应函数图上，施塔克尔伯格均衡位于古诺的右下方：领导者产量更多，跟随者产量更少。
这说明了什么： A Nash equilibrium is a situation where every player is doing the best they can, given what everyone else is doing. Nobody can improve their outcome by changing their own strategy alone. Think of it as a "no regrets" outcome -- once you see what everyone else chose, you would not change your choice.
为什么这很重要： Nash equilibrium is the central solution concept in game theory and applies far beyond economics -- to politics, biology, and any situation with strategic interaction. It does not mean the outcome is good for society (the Prisoner's Dilemma shows it can be terrible), just that it is self-enforcing: no individual has an incentive to deviate.
什么发生变化： When payoffs change, equilibria shift. If the penalty for defection increases (stronger enforcement, higher fines), cooperation becomes easier to sustain. If a new strategy becomes available, old equilibria may dissolve. Some games have multiple Nash equilibria (coordination games), some have exactly one (Prisoner's Dilemma), and some have none in pure strategies -- requiring randomization (mixed strategies).
In Full Mode, Eq. 6.14 states the formal condition: no player can improve their payoff by unilateral deviation.每个参与者都在对其他人做最优反应。在其他人的行为给定的情况下，没有人有理由偏离。
| 参与者2：合作 | 参与者2：背叛 | |
|---|---|---|
| 参与者1：合作 | (3, 3) | (0, 5) |
| 参与者1：背叛 | (5, 0) | (1, 1) |
占优策略：无论对方如何选择，背叛都是最优的。纳什均衡：(背叛, 背叛)，收益为(1, 1)。双方都比相互合作(3, 3)更差，但都无法单方面改善。
这说明了什么： The Prisoner's Dilemma captures a fundamental tension: what is rational for each individual leads to a bad outcome for everyone. Each player reasons: "No matter what the other does, I am better off defecting." But when both think this way, they end up with (Defect, Defect) -- worse for both than if they had cooperated.
为什么这很重要： This structure appears everywhere in economics and beyond. Firms in a cartel each have an incentive to secretly increase output. Countries each want to free-ride on others' carbon reductions. Arms race participants each prefer to build weapons while the other disarms. The core insight: markets, institutions, and enforcement mechanisms exist precisely to solve Prisoner's Dilemmas -- converting individual incentives toward socially better outcomes.
什么发生变化： If the temptation payoff (defecting while the other cooperates) shrinks -- through penalties, reputation effects, or social norms -- cooperation becomes easier. If the game is repeated, future punishment can sustain cooperation (see repeated games below). If communication is allowed, players can coordinate -- but only if commitments are enforceable.
囚徒困境为何重要：
输入2×2博弈的任意收益。工具自动识别占优策略、纳什均衡和帕累托最优结果。绿色单元格为纳什均衡；蓝色边框标记帕累托最优结果。
| 参与者2：L | 参与者2：R | |
|---|---|---|
| 参与者1：U | (,\n ) | (,\n ) |
| 参与者1：D | (,\n ) | (,\n ) |
蓝色 = 参与者1的收益 | 红色 = 参与者2的收益
协调博弈：
| B：左 | B：右 | |
|---|---|---|
| A：左 | (2, 2) | (0, 0) |
| A：右 | (0, 0) | (1, 1) |
两个纳什均衡：(左, 左)和(右, 右)。挑战在于协调，而非冲突。
性别之战：
| B：歌剧 | B：足球 | |
|---|---|---|
| A：歌剧 | (3, 1) | (0, 0) |
| A：足球 | (0, 0) | (1, 3) |
两个纯策略纳什均衡，每个参与者有不同的偏好结果。
两家企业选择是否投放广告(A)或不投放(N)：
| 企业2：A | 企业2：N | |
|---|---|---|
| 企业1：A | (4, 4) | (7, 2) |
| 企业1：N | (2, 7) | (5, 5) |
第一步——检查占优策略。
企业1：如果企业2选择A，企业1获得4(A)对2(N) → A更好。如果企业2选择N，企业1获得7(A)对5(N) → A更好。因此A是企业1的占优策略。由对称性，A也是企业2的占优策略。
第二步——找出纳什均衡。
唯一的纳什均衡是(A, A)，收益为(4, 4)。两家企业都投放广告，尽管(N, N) = (5, 5)帕累托占优。这是一个囚徒困境：投放广告的个体激励导致了集体更差的结果。
当囚徒困境被重复进行（且参与者有耐心）时，合作可以维持。未来惩罚（回归背叛）的威胁使当前合作具有自我执行力。这就是无名氏定理。
Under the grim trigger strategy (cooperate until the other defects, then defect forever), cooperation is sustainable if:
where $\pi_C$ is the per-period cooperation payoff, $\pi_D$ is the one-shot deviation payoff, and $\pi_N$ is the Nash (punishment) payoff. With standard prisoner's dilemma payoffs (CC=3, DC=5, DD=1): $\delta \geq (5-3)/(5-1) = 1/2$.
这说明了什么： Cooperation in a repeated game is a cost-benefit calculation: the short-run temptation to cheat (the one-time gain from defecting while the other cooperates) versus the long-run punishment (being stuck in mutual defection forever). If players are patient enough (high discount factor), the future punishment outweighs the immediate gain, and cooperation is self-enforcing.
为什么这很重要： This explains why cartels, arms agreements, and trade deals can work even without external enforcement. The threat of retaliation (price wars, tariff escalation, arms races) sustains cooperation -- as long as the relationship is expected to continue. It also explains why cooperation breaks down when firms are impatient, when the game has a known end date, or when cheating is hard to detect.
什么发生变化： Higher discount factor (more patience) makes cooperation easier. Larger temptation payoff makes it harder. If the punishment is mild (Nash payoff close to cooperation payoff), cooperation requires more patience. This is why OPEC struggles to maintain output quotas: the temptation to overproduce is large, detection is slow, and punishment is weak.
In Full Mode, Eq. 6.15 derives the critical discount factor from the grim trigger strategy.直觉是：今天的合作维持了关系。欺骗带来短期收益但永远触发惩罚。如果折现因子 $\delta$ 足够高，惩罚的长期成本超过短期收益。
在标准囚徒困境（收益：CC=3, CD=0, DC=5, DD=1）中，通过冷酷触发策略实现合作需要折现因子 $\delta$ 超过某个门槛值。滑动 $\delta$ 查看合作是否可持续。
图 6.5.水平线表示维持合作所需的最低折现因子 $\delta^*$。当 $\delta > \delta^*$ 时，合作的长期价值超过一次性背叛的诱惑。图表比较了永久合作的现值与背叛一次然后永远受罚的现值。
| 市场结构 | 企业数量 | 价格 | 产量 | 利润 | 无谓损失 | 战略性？ |
|---|---|---|---|---|---|---|
| 完全竞争 | 多 | $P = MC$ | 最高 | 零（长期） | 无 | No |
| 垄断竞争 | 多 | $P > MC$ | 低于竞争 | 零（长期） | 小 | No |
| 古诺寡头垄断 | Few | $MC < P < P_M$ | 介于之间 | 正 | 中等 | 是（Q） |
| 施塔克尔伯格 | Few | 低于古诺 | 更高 | 领导者 > 古诺 | 更少 | 是（序贯） |
| 伯特兰（同质） | Two | $P = MC$ | 竞争水平 | 零 | 无 | 是（P） |
| 垄断 | One | 最高 | 最低 | 最高 | 最大 | No |
竞争对手内特在街对面开了一个柠檬水摊。两人有相同的成本结构。社区需求为 $P = 5 - (Q_M + Q_N)/20$，$MC = 1.50$。
古诺均衡： $Q_M^* = Q_N^* = 23.3$ 杯。$P = 2.67$。玛雅的利润：\$17.2$/天（仅材料成本）。
施塔克尔伯格（玛雅为领导者）： $Q_M^S = 35$，$Q_N^S = 17.5$。$P = 2.375$。玛雅的利润：\$10.6$/天——由于先行者优势略高。
内特进入市场后，玛雅的产量从45杯降至23.3杯，价格从\$1.75降至\$1.67。
| 标签 | 公式 | 描述 |
|---|---|---|
| 式 6.1 | $P = MC = AC_{min}$, $\Pi = 0$ | 长期竞争均衡 |
| 式 6.2 | $\max \Pi = P(Q)Q - TC(Q)$ | 垄断者的问题 |
| 式 6.3 | $MR = P + Q(dP/dQ)$ | 边际收益 |
| 式 6.4 | $MR = MC$ | 垄断利润最大化条件 |
| 式 6.5 | $(P-MC)/P = 1/|\varepsilon_d|$ | 勒纳指数 |
| 式 6.6 | $MR_1 = MR_2 = MC$ | 三级价格歧视 |
| 式 6.7–6.8 | 最优反应函数 | 古诺反应函数 |
| 式 6.9 | $q_i^C = (a-c)/(3b)$ | 古诺对称均衡 |
| 式 6.10 | $P^C = (a+2c)/3$ | 古诺价格 |
| 式 6.11 | $P^B = c$ | 伯特兰均衡（同质产品） |
| 式 6.12–6.13 | $q_1^S = (a-c)/(2b)$, $q_2^S = (a-c)/(4b)$ | 施塔克尔伯格产量 |
| 式 6.14 | $u_i(s_i^*, s_{-i}^*) \geq u_i(s_i, s_{-i}^*)$ 对所有 $s_i$ 成立 | 纳什均衡 |
| Eq. 6.15 | $\delta \geq (\pi_D - \pi_C)/(\pi_D - \pi_N)$ | Cooperation threshold (grim trigger) |
| B: X | B: Y | |
|---|---|---|
| A: X | (3, 3) | (1, 4) |
| A: Y | (4, 1) | (2, 2) |
第三部分预告：宏观经济学把尺度从企业提升到国家。
