📚 IB Edexcel Economics: Oligopoly Key Points Review | IB Edexcel 经济：寡头考点精讲
Oligopoly is a fascinating and highly examinable market structure in both IB and Edexcel Economics. It sits between perfect competition and monopoly, and it is characterised by a few dominant firms, strategic interdependence, and the potential for both fierce rivalry and collusive behaviour. This article summarises the key concepts, diagrams, game theory applications, and evaluation points you need to master for top marks.
寡头是 IB 和 Edexcel 经济学中非常迷人且高度可考的市场结构。它介于完全竞争与垄断之间，由少数主导企业构成，具有战略依赖的特点，既可能激烈竞争也可能共谋。本文概括了核心概念、图表、博弈论应用以及需要掌握的评价要点，助你冲击高分。
1. Definition of Oligopoly | 寡头的定义
An oligopoly is a market structure in which a small number of large firms dominate the industry, each holding significant market power. The actions of any one firm directly affect the profits and decisions of its rivals, making strategic behaviour inevitable. Typical examples include the automobile, airline, and smartphone markets.
寡头是一种由少数几家大企业主导整个行业的市场结构，每家企业都掌握着显著的市场力量。任何一家企业的行为都会直接影响其竞争对手的利润与决策，因此策略性行为不可避免。常见的例子包括汽车、航空和智能手机市场。
2. Key Characteristics of Oligopoly | 寡头的关键特征
Few dominant firms: The market is concentrated in the hands of a small number of sellers. This can be measured by concentration ratios (e.g., a five-firm concentration ratio above 60%).
少数主导企业：市场集中在少数卖方手中，可用集中度比率衡量（例如五企业集中度比率超过60%）。
High barriers to entry: Substantial obstacles, such as economies of scale, brand loyalty, legal patents, and high sunk costs, prevent new firms from easily entering the market.
高进入壁垒：规模经济、品牌忠诚度、专利法及高沉没成本等重大阻力，使得新企业难以轻易进入市场。
Interdependence: Each firm’s pricing and output decisions are shaped by anticipated reactions of rivals. This mutual dependence is the defining feature of oligopoly.
相互依赖性：每家企业的定价和产量决策都建立在对竞争对手反应的预期之上。这种相互依存是寡头的标志性特征。
Product differentiation: Products may be homogeneous (e.g. steel, oil) or differentiated (e.g. cars, soft drinks). Differentiation often drives non-price competition.
产品差异化：产品可以是同质的（如钢铁、石油）或差异化的（如汽车、软饮料）。差异化常常推动非价格竞争。
Non-price competition: Advertising, branding, after-sales service, and product innovation are prevalent because price wars can be mutually destructive.
非价格竞争：广告、品牌、售后服务与产品创新非常普遍，因为价格战可能两败俱伤。
3. Concentration Ratio | 集中度比率
The n-firm concentration ratio measures the combined market share of the largest n firms in an industry. A common version is the five-firm concentration ratio CR₅. The formula is:
n企业集中度比率衡量一个行业中前 n 家最大企业的市场份额之和。常用的是五企业集中度比率 CR₅。其公式为：
CRₙ = (Combined sales of top n firms / Total market sales) × 100%
In the UK, a market with a CR₅ above 60% is often regarded as an oligopoly. However, the concentration ratio does not reveal the distribution of market shares among the top firms; a market with one large firm and four small ones can have the same CR₅ as one with five equal-sized firms.
在英国，CR₅ 超过60%的市场常被视作寡头市场。但集中度比率无法反映领先企业之间的份额分布；一家超大配四家微小的市场可能与五家均分的市场拥有相同的 CR₅。
4. Interdependence and Strategic Behaviour | 相互依赖与策略行为
Because there are few sellers, every oligopolist must think strategically. A firm knows that if it raises price, it may lose significant sales if rivals hold theirs constant; if it cuts price, rivals are likely to match the cut to protect market share. This interdependence makes the outcomes of decisions highly uncertain and introduces game theory as the natural analytical tool.
由于卖方数量很少，每个寡头都必须进行策略性思考。企业知道，如果它提价而对手维持原价，就可能损失大量销售；如果它降价，对手很可会跟随降价以保住市场份额。这种相互依赖使得决策结果高度不确定，也让博弈论成为天然的分析工具。
5. Game Theory: The Prisoner’s Dilemma | 博弈论：囚徒困境
Game theory analyses situations where the payoff to one player depends on the actions of all players. The classic prisoner’s dilemma captures the tension between cooperation and self-interest. Two firms face a choice: charge a high collusive price or undercut with a low price. The payoff matrix (profits in £m) below illustrates typical outcomes:
博弈论分析的是参与者的收益取决于所有人行动的情形。经典的囚徒困境展现了合作与自利之间的张力。两家企业面临选择：收取高价的共谋价格，还是低价抢客。下面的收益矩阵（利润单位为百万英镑）展示了典型结果：
| Firm B: High Price | Firm B: Low Price | |
|---|---|---|
| Firm A: High Price | (10, 10) | (2, 15) |
| Firm A: Low Price | (15, 2) | (5, 5) |
Each firm has a dominant strategy to charge a low price. As a result, the Nash equilibrium is (Low Price, Low Price) with joint profits of 10, even though both could do better by cooperating at (10, 10). This explains why price wars break out and collusion is attractive.
每家企业都有收取低价的主导策略。最终，纳什均衡为（低价，低价），合计利润仅为10，尽管双方都可通过合作达到（10,10）而获得更多利润。这就解释了为何价格战会爆发，以及共谋为何诱人。
6. Nash Equilibrium and Dominant Strategies | 纳什均衡与主导策略
A Nash equilibrium occurs when each player’s strategy is the best response to the strategies chosen by others, so no one has an incentive to unilaterally change. A dominant strategy is one that yields a higher payoff regardless of the opponent’s choice. In the matrix above, Low Price is dominant for both, leading to the unique Nash equilibrium. However, in many real oligopolistic games, no dominant strategy exists; firms must rely on conjecture and repeated interaction to reach an equilibrium.
当每个参与者的策略都是对其他参与者所选策略的最优反应，没有人有动机单方面改变时，就实现了纳什均衡。主导策略是不论对手作何选择都能带来更高收益的策略。在上面的矩阵中，低价对双方都是主导策略，从而导向唯一的纳什均衡。但在许多现实中的寡头博弈里，不存在单一主导策略；企业必须依靠推测和重复互动来达成均衡。
7. Kinked Demand Curve and Price Rigidity | 折弯需求曲线与价格刚性
The kinked demand curve model offers an explanation for price stability in oligopoly. The demand curve has a kink at the prevailing market price P*. Above P*, the demand is elastic because rivals are assumed not to follow a price rise; the firm would lose many customers. Below P*, demand is inelastic because rivals match price cuts to avoid losing market share, so the firm gains few additional sales.
折弯需求曲线模型为寡头市场中的价格稳定提供了一种解释。需求曲线在现行市场价格 P* 处有一个折弯。在 P* 之上，需求富有弹性，因为假设对手不会跟随提价，企业将失去大量顾客；在 P* 之下，需求缺乏弹性，因为对手会同步降价以守住市场份额，企业只能获得很少的额外销量。
The corresponding marginal revenue (MR) curve has a vertical discontinuity (a gap) directly below the kink. Even if marginal cost (MC) rises or falls within this gap, the profit-maximising output and price remain unchanged. This yields price rigidity — prices stay fixed for long periods despite changes in costs.
相应的边际收益（MR）曲线在折弯的正下方出现垂直间断（一个缺口）。只要边际成本（MC）在这个缺口的范围内升降，利润最大化的产量与价格就保持不变。这便产生了价格刚性——尽管成本变化，价格仍长久不动。
8. Non-Price Competition | 非价格竞争
Because price cuts are quickly matched and erode profits, oligopolists often compete through non-price methods. Extensive advertising builds brand loyalty and makes demand less price-sensitive. Product innovation, loyalty cards, extended warranties, and after-sales service can differentiate a firm’s offering. While such competition can benefit consumers, it may also create wasteful expenditure and reinforce barriers to entry.
因为降价会迅速被跟进并侵蚀利润，寡头企业经常通过非价格方式竞争。大量广告能建立品牌忠诚度，使需求对价格不那么敏感。产品创新、积分卡、延长保修和售后服务可以差异化企业的产品。这类竞争能为消费者带来好处，但也可能造成浪费性支出并强化进入壁垒。
9. Collusion and Cartels | 共谋与卡特尔
Collusion occurs when firms coordinate their actions to reduce uncertainty and maximise joint profits. It can be tacit (implicit understanding, e.g. price leadership) or overt (explicit agreement). A cartel is a formal collusive agreement, often involving output quotas at the monopoly price. The OPEC oil cartel is a famous example. Collusion allows firms to earn supernormal profits, but it is illegal in many jurisdictions because it harms consumers through higher prices and restricted output.
共谋是指企业协调行动以减少不确定性并最大化联合利润。它可以是默契的（隐性理解，如价格领导制）或公开的（明确协议）。卡特尔是一种正式的共谋协议，通常涉及垄断价格下的产量配额。欧佩克石油卡特尔是著名例子。共谋让企业获得超额利润，但在许多法域是非法的，因为它通过抬高价格、限制产量而损害消费者。
Factors facilitating collusion: small number of firms, similar cost structures, high entry barriers, frequent interaction, transparent pricing, ineffective competition policy.
促进共谋的因素：少量企业、相似的成本结构、高进入壁垒、频繁互动、透明的定价以及无效的竞争政策。
Obstacles to collusion: falling market demand, product differentiation, asymmetric costs, secret price cuts to increase market share, and new entry threats.
阻碍共谋的因素：市场需求的下降、产品差异化、成本不对称、为增加份额而暗中降价，以及新进入者的威胁。
10. Evaluation: Efficiency and Competition Policy | 评价：效率与竞争政策
Oligopoly often leads to allocative inefficiency because price exceeds marginal cost (P > MC) and productive inefficiency since firms do not operate at minimum average cost. The deadweight welfare loss is smaller than under pure monopoly but can still be significant. However, oligopolists may achieve dynamic efficiency: supernormal profits can be reinvested in research and development (R&D), driving innovation and long-run economic growth. Furthermore, non-price competition can widen consumer choice and raise quality.
寡头常常导致配置无效率，因为价格高于边际成本（P > MC），也会产生生产无效率，因为企业并不在最低平均成本处运营。其所造成的无谓福利损失虽小于纯垄断，但仍可能相当可观。然而，寡头有可能实现动态效率：超额利润可以再投资于研发（R&D），推动创新与长期经济增长。此外，非价格竞争可以扩大消费者选择并提升品质。
Governments use competition policy to prevent abusive practices. This includes breaking up cartels, blocking anti-competitive mergers, and investigating price-fixing. Regulators may also promote contestability by reducing barriers to entry. The balance between intervention and allowing the benefits of large-scale production is a key policy tension.
政府运用竞争政策来防止滥用行为，包括解散卡特尔、阻止反竞争的合并以及调查操纵价格。监管机构还可能通过降低进入壁垒来提升市场的可竞争性。在干预与允许大规模生产的好处之间寻求平衡，是一个关键的政策张力所在。
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