A-Level经济 寡头垄断 博弈论 纳什均衡

A-Level经济 寡头垄断 博弈论 纳什均衡

Introduction to Market Structures 市场结构简介

In A-Level Economics, understanding market structures is essential. After studying perfect competition and monopoly, students encounter oligopoly ： a market where a few large firms dominate. 在A-Level经济学中，理解市场结构至关重要。学完完全竞争和垄断之后，学生会遇到寡头垄断：一种由少数几家大企业主导的市场结构。Oligopoly sits between monopoly and monopolistic competition, and it is the most common market structure in modern economies. 寡头垄断介于垄断和垄断竞争之间，是现代经济中最常见的市场结构。

What Is an Oligopoly 什么是寡头垄断

An oligopoly is a market dominated by a small number of large firms, typically between two and five. 寡头垄断是由少数几家大企业（通常两到五家）主导的市场。These firms are interdependent ： the actions of one firm directly affect the others. 这些企业相互依存：一家企业的行为直接影响其他企业。Classic examples include the petrol industry, supermarkets, mobile phone networks, and the airline industry. 经典例子包括石油行业、超市、移动通信网络和航空业。In the UK, the Big Four supermarkets ： Tesco, Sainsbury’s, Asda, and Morrisons ： control over two-thirds of the grocery market, a textbook case of oligopoly. 在英国，四大超市：Tesco、Sainsbury’s、Asda和Morrisons：控制着超过三分之二的食品杂货市场，是寡头垄断的教科书案例。The key feature of oligopoly is strategic interdependence, which makes the behaviour of firms far more complex than in other market structures. 寡头垄断的关键特征是战略相互依存，这使得企业行为比其他市场结构复杂得多。

Characteristics of Oligopoly 寡头垄断的特征

There are several defining characteristics of oligopolistic markets. 寡头垄断市场有以下几个决定性特征。First, there are high barriers to entry, such as large capital requirements and economies of scale enjoyed by incumbents. 首先，进入壁垒高，例如大量资本需求和现有企业享有的规模经济。Second, firms produce branded products with some degree of differentiation. 其次，企业生产品牌产品，具有一定程度的差异化。Third, firms are price makers but they must anticipate rivals’ reactions before changing prices. 第三，企业是价格制定者，但在改变价格之前必须预测竞争对手的反应。

Concentration Ratios 集中度比率

Economists measure the degree of market concentration using concentration ratios. 经济学家使用集中度比率来衡量市场集中程度。The most common is the N-firm concentration ratio, which sums the market share of the top N firms. 最常见的是N企业集中度比率，将前N家企业的市场份额相加。For example, a five-firm concentration ratio of 80 percent means the top five firms control 80 percent of the market ： a highly concentrated market. 例如，五企业集中度比率为80%意味着前五家企业控制着80%的市场：这是一个高度集中的市场。Another measure is the Herfindahl-Hirschman Index (HHI), which squares each firm’s market share and sums them, giving more weight to larger firms. 另一种衡量标准是赫芬达尔-赫希曼指数（HHI），将每家企业的市场份额平方后求和，赋予大企业更大的权重。An HHI above 2000 indicates a highly concentrated market. HHI超过2000表明市场高度集中。

Game Theory and Strategic Behaviour 博弈论与策略行为

Game theory is the mathematical study of strategic interactions between interdependent decision-makers. 博弈论是研究相互依存的决策者之间策略互动的数学学科。In the context of oligopoly, game theory helps explain why firms may not compete aggressively even when they could gain short-term profits from doing so. 在寡头垄断背景下，博弈论有助于解释为什么企业即使可能从中获得短期利润，也不会激烈竞争。The most famous concept in game theory is the prisoner’s dilemma. 博弈论中最著名的概念是囚徒困境。

The Prisoner’s Dilemma 囚徒困境

Two prisoners are interrogated separately. If both stay silent, each gets one year. If one confesses and the other stays silent, the confessor goes free while the silent one gets ten years. If both confess, each gets five years. 两个囚犯分别接受审讯。如果两人都保持沉默，各判一年。如果一人招供而另一人沉默，招供者获释而沉默者判十年。如果两人都招供，各判五年。Although the best joint outcome is for both to stay silent, each prisoner has a dominant strategy to confess ： leading to a worse outcome for both. 虽然最佳联合结果是两人都保持沉默，但每个囚犯都有占优策略去招供：导致对两人都更糟的结果。

Applying the Prisoner’s Dilemma to Oligopoly 将囚徒困境应用于寡头垄断

In an oligopoly, firms face a similar dilemma when deciding whether to set high or low prices. 在寡头垄断中，企业在决定设定高价还是低价时面临类似的困境。If both charge high prices, both earn high profits. But if one cheats by lowering prices, it captures market share at the rival’s expense. 如果双方都定高价，双方都获得高额利润。但如果一方通过降价作弊，它就会以牺牲竞争对手为代价获取市场份额。The dominant strategy for each firm is to charge a low price, which leads to lower profits for all ： a suboptimal Nash equilibrium. 每个企业的占优策略是定低价，这导致所有企业利润降低：一个次优的纳什均衡。

Nash Equilibrium 纳什均衡

A Nash equilibrium is a situation where no player can improve their outcome by unilaterally changing their strategy, given the strategies of all other players. 纳什均衡是指在其他所有参与者的策略给定的情况下，没有任何参与者可以通过单方面改变策略来改善自己结果的状况。In the prisoner’s dilemma, mutual confession is a Nash equilibrium because neither prisoner can do better by changing strategy alone. 在囚徒困境中，相互招供是纳什均衡，因为任何一个囚犯都无法通过单独改变策略来做得更好。The concept was developed by John Nash, who won the Nobel Prize in Economics for this work. 这一概念由约翰·纳什提出，他因此获得了诺贝尔经济学奖。

Collusion and Cartels 合谋与卡特尔

To escape the prisoner’s dilemma, firms in an oligopoly may collude ： agreeing to coordinate prices, output, or market sharing. 为了摆脱囚徒困境，寡头垄断中的企业可能合谋：达成协议协调价格、产量或市场分配。A cartel is a formal agreement between firms to act as a monopoly, maximising joint profits by restricting output and raising prices. 卡特尔是企业之间正式达成的、像垄断企业一样行动以最大化联合利润的协议，通过限制产量和提高价格来实现。The most famous cartel is OPEC, which coordinates oil production among member countries. 最著名的卡特尔是欧佩克，它协调成员国之间的石油产量。However, cartels are inherently unstable because each member has an incentive to cheat by selling more than their agreed quota, especially when the cartel price is well above marginal cost. 然而，卡特尔本质上是不稳定的，因为每个成员都有动力通过超出约定配额的销售来作弊，尤其是当卡特尔价格远高于边际成本时。This temptation to cheat is why many cartels eventually collapse. 这种作弊的诱惑正是许多卡特尔最终崩溃的原因。

Tacit Collusion 默契合谋

Where formal collusion is illegal ： as it is in most countries ： firms may engage in tacit collusion. 在正式合谋非法的情况下：大多数国家都是如此：企业可能进行默契合谋。This involves firms coordinating their behaviour without explicit communication, often through price leadership. 这涉及企业在没有明确沟通的情况下协调行为，通常通过价格领导来实现。For example, one dominant firm announces a price change, and other firms follow. 例如，一家主导企业宣布价格变动，其他企业随之调整。Tacit collusion is difficult for regulators to prove but can result in similar outcomes to a formal cartel. 默契合谋对监管者来说难以证明，但可能产生与正式卡特尔类似的结果。

The Kinked Demand Curve 弯折的需求曲线

The kinked demand curve model explains why prices tend to be stable in oligopolistic markets. 弯折的需求曲线模型解释了为什么寡头垄断市场的价格往往稳定。The demand curve is kinked at the current price because rivals will match a price cut but ignore a price rise. 需求曲线在当前价格处弯折，因为竞争对手会跟随降价但忽略提价。Above the kink, demand is elastic ： a price rise causes a large fall in quantity demanded as customers switch to competitors. 在弯折点上方，需求是有弹性的：提价会导致需求量大幅下降，因为客户转向竞争对手。Below the kink, demand is inelastic ： a price cut is matched by rivals so there is little gain in market share. 在弯折点下方，需求缺乏弹性：降价被竞争对手匹配，因此市场份额收益很小。This creates a discontinuity in the marginal revenue curve, which means marginal cost can change within a range without affecting the profit-maximising price and output. 这在边际收益曲线中产生了间断，意味着边际成本可以在一定范围内变化而不影响利润最大化的价格和产量。This helps explain the observed price rigidity in many oligopolistic industries. 这有助于解释许多寡头垄断行业中观察到的价格刚性。

Price Wars 价格战

When collusion breaks down, oligopolistic markets can experience price wars. 当合谋破裂时，寡头垄断市场可能经历价格战。Firms cut prices aggressively to gain market share, driving prices down to or even below average cost. 企业大幅降价以获取市场份额，将价格压低至平均成本甚至更低。While consumers benefit in the short run from lower prices, price wars can lead to firms exiting the market and eventually higher prices when the survivors regain market power. 虽然消费者短期内从低价中受益，但价格战可能导致企业退出市场，最终当幸存者重新获得市场力量时价格上升。

Evaluation of Oligopoly 寡头垄断的评估

Oligopoly has both advantages and disadvantages. 寡头垄断既有优点也有缺点。On one hand, large firms in oligopoly can achieve significant economies of scale, invest heavily in research and development, and offer consumers a degree of product choice and innovation. 一方面，寡头垄断中的大企业可以实现显著的规模经济、大力投资研发并为消费者提供一定程度的产品选择和创新能力。On the other hand, prices tend to be higher and output lower than under perfect competition, and collusion harms consumer welfare by reducing choice and raising prices above competitive levels. 另一方面，价格往往比完全竞争下更高、产量更低，合谋通过减少选择和将价格提高到竞争水平之上损害了消费者福利。Regulators such as the UK’s Competition and Markets Authority (CMA) therefore face a difficult task in distinguishing beneficial competition from harmful collusion. 因此，英国竞争与市场管理局（CMA）等监管者在区分有益竞争与有害合谋方面面临艰巨任务。

Exam Tips for A-Level Economics 考试技巧

When answering exam questions on oligopoly, always define key terms clearly. 在回答有关寡头垄断的考试题目时，务必清晰地定义关键术语。Use diagrams such as the kinked demand curve to support your analysis. 使用弯折需求曲线等图表来支持你的分析。Provide real-world examples ： UK supermarkets, mobile networks, and airlines are always relevant. 提供现实世界的例子：英国超市、移动网络和航空公司总是相关的。Remember to evaluate: discuss both the pros and cons of oligopoly, and consider the effectiveness of government intervention. 记住要进行评估：讨论寡头垄断的利弊，并考虑政府干预的有效性。A strong conclusion should weigh the evidence and make a reasoned judgement. 有力的结论应该权衡证据并做出理性的判断。