📚 Oligopoly: Key Exam Points for IB & CIE Economics | IB & CIE 经济学：寡头考点精讲
Oligopoly is one of the most challenging market structures in IB and CIE Economics, defined by a few large firms dominating the market, high barriers to entry, and mutual interdependence. Mastering this topic requires clear understanding of game theory, collusive and non-collusive models, price rigidity, and welfare implications. This article covers all the key concepts, diagrams, and evaluation points you need for top marks.
寡头是 IB 和 CIE 经济学中最具挑战性的市场结构之一，其特点是少数大企业主导市场、进入壁垒高以及企业间的相互依存。掌握这一主题需要清晰理解博弈论、合谋与非合谋模型、价格刚性以及福利影响。本文涵盖所有关键概念、图形和评估要点，助你冲刺高分。
1. Key Characteristics of Oligopoly | 寡头的关键特征
An oligopoly is a market structure with a small number of large firms that dominate the industry. The main characteristics include high concentration ratios, product differentiation (or homogeneity), significant barriers to entry and exit, and mutual interdependence among firms.
寡头是一种由少数大企业主导行业的市场结构。其主要特征包括高集中度、产品差异化（或同质化）、显著的进入和退出壁垒以及企业之间的相互依存。
Firms in oligopoly may produce differentiated products (e.g., cars, smartphones) or homogeneous products (e.g., steel, oil). Barriers to entry such as economies of scale, high capital requirements, brand loyalty, and legal restrictions protect existing firms from new competitors.
寡头企业可能生产差异化产品（如汽车、智能手机）或同质化产品（如钢铁、石油）。规模经济、高资本要求、品牌忠诚度以及法律限制等进入壁垒保护现有企业免受新竞争者的威胁。
Mutual interdependence means that each firm’s actions directly affect its rivals, and firms must anticipate competitors’ responses when making pricing or output decisions. This strategic behaviour is the core of oligopoly analysis.
相互依存意味着每家企业的行为会直接影响其竞争对手，企业在制定价格或产量决策时必须预测竞争对手的反应。这种策略性行为是寡头分析的核心。
2. Concentration Ratios and Market Power | 集中度与市场势力
The concentration ratio (CR) measures the combined market share of the largest firms, typically the top 3, 4, or 5 firms. A high CR (e.g., CR4 > 60%) indicates an oligopolistic market. The higher the ratio, the greater the market power of the leading firms and the weaker the competitive pressure.
集中度（CR）衡量最大几家企业的市场份额之和，通常取前 3、4 或 5 家。高集中度（如 CR4 > 60%）表明市场为寡头结构。比率越高，领先企业的市场势力越大，竞争压力越弱。
IB and CIE exam questions often ask you to calculate and interpret concentration ratios. For example, if the four largest firms have sales of $40bn, $30bn, $20bn and $10bn out of a total market of $150bn, the CR4 = (40+30+20+10)/150 = 66.7%, indicating a concentrated oligopoly.
IB 和 CIE 考试常要求你计算并解读集中度。例如，若前四家企业销售额分别为 400 亿、300 亿、200 亿和 100 亿美元，市场总额为 1500 亿美元，则 CR4 = (40+30+20+10)/150 = 66.7%，属于集中型的寡头市场。
However, CR has limitations: it ignores the distribution within the top firms and the role of foreign competition. A market with a high CR may still be contestable if entry barriers are low.
但集中度也有局限性：它忽略了头部企业内部的分布情况以及国外竞争的作用。如果进入壁垒较低，高集中度的市场仍可能具有可竞争性。
3. Interdependence and Strategic Behaviour | 相互依存与策略行为
Interdependence distinguishes oligopoly from other market structures. Any change in price, output, or advertising by one firm triggers reactions from rivals, creating a strategic environment analysed through game theory.
相互依存使寡头有别于其他市场结构。一家企业改变价格、产量或广告策略，都会引发竞争对手的反应，形成一个可通过博弈论进行剖析的策略环境。
Firms must consider the reactions of others when making decisions. If a firm raises its price, competitors may not follow, leading to a loss of market share. If it lowers price, rivals are likely to match the cut to protect their sales, resulting in a price war that erodes profits for all.
企业在决策时必须考虑其他企业的反应。如果一家企业提价，对手可能不跟进，导致其市场份额流失；如果降价，对手很可能同步降价以保销量，引发价格战，使所有企业利润受损。
This strategic interdependence is the foundation of collusion and non-collusive behaviour explored in the following sections.
这种策略性的相互依存是接下来合谋与非合谋行为分析的基础。
4. Game Theory and the Prisoner’s Dilemma | 博弈论与囚徒困境
Game theory models the strategic interactions between firms. The prisoner’s dilemma is a classic example showing why firms in an oligopoly may not cooperate even when it seems mutually beneficial.
博弈论为企业的策略互动建立模型。囚徒困境是一个经典例子，说明为什么寡头中的企业即使在看似对双方都有利的情况下也不会合作。
Consider two firms, A and B, deciding whether to charge a high price or a low price. The payoff matrix (profits in £m) is:
假设两家企业 A 和 B，需要决定收取高价还是低价。收益矩阵（利润，百万英镑）如下：
| B charges High | B charges Low | |
|---|---|---|
| A charges High | (10, 10) | (2, 15) |
| A charges Low | (15, 2) | (5, 5) |
If both firms charge a high price, they each earn £10m. However, each has an incentive to cheat and undercut the other to capture more market share and earn £15m (if the rival keeps price high). The Nash equilibrium is (Low, Low) with profits of (5,5), which is worse than the cooperative outcome. This explains price wars and the instability of cartels.
如果双方都定高价，各自获利 1000 万。但双方都有动机通过降价抢夺市场份额，若对手维持高价则可获利 1500 万。纳什均衡为（低价，低价），双方利润均为 500 万，比合作结果更差。这解释了价格战和卡特尔的不稳定性。
5. Nash Equilibrium and Dominant Strategies | 纳什均衡与优势策略
A Nash equilibrium occurs when no player can improve their outcome by changing their strategy, given the other player’s strategy. In the above matrix, ‘Low’ is a dominant strategy for both firms because it yields higher payoffs regardless of the rival’s choice. The intersection of dominant strategies gives the Nash equilibrium.
纳什均衡指在对手策略既定的情况下，没有参与者能通过改变自身策略来改善结果。在上述矩阵中，“低价”是双方的优势策略，因为无论对手如何选择，低价带来的收益都更高。优势策略的交点就是纳什均衡。
Students must be able to identify dominant strategies and Nash equilibria in payoff matrices. Be aware that a Pareto superior outcome (High, High) is not chosen due to lack of trust and the fear of being undercut. This market failure can justify government intervention or regulation.
学生必须能够在收益矩阵中识别优势策略和纳什均衡。需要注意，帕累托更优的结果（高价，高价）由于缺乏信任以及担心被降价打击而没有被选择。这种市场失灵为政府干预或监管提供了理由。
6. Collusion and Cartels | 合谋与卡特尔
Collusion occurs when firms in an oligopoly cooperate to reduce competition and increase joint profits. It can be overt (explicit cartels like OPEC) or tacit (implicit understanding without formal agreements), the latter being more common where cartels are illegal.
合谋指寡头企业合作以减少竞争并提高联合利润。合谋可以是公开的（如 OPEC 这类明示卡特尔），也可以是默契的（没有正式协议的隐含谅解），后者在卡特尔的非法性普遍存在的地区更为常见。
Cartels act like a monopoly by agreeing to restrict output and raise prices to maximize group profits. They allocate production quotas among members. However, cartels are inherently unstable due to the incentive to cheat: each member can profit by secretly increasing output above its quota and selling at the high cartel price, as the prisoner’s dilemma predicts.
卡特尔像垄断者一样，通过协议限制产量、提高价格以最大化集体利润，并在成员间分配产量配额。然而，卡特尔本质上不稳定，因为存在背叛动机：正如囚徒困境所预测，每个成员都可能通过暗中增产并以高价出售来获利。
Exam questions often ask you to evaluate the conditions for successful collusion: small number of firms, similar costs and demand, high entry barriers, effective punishments for cheats, and stable demand. Real-world examples like OPEC and the breakdown of airline cartels are valuable.
考试常要求你评估合谋成功的条件：企业数量少、成本与需求相似、高进入壁垒、对背叛者有效的惩罚机制以及稳定的需求。OPEC 和航空业卡特尔的瓦解等现实案例极具价值。
7. The Kinked Demand Curve and Price Rigidity | 弯折的需求曲线与价格刚性
The kinked demand curve model explains why prices in non-collusive oligopolies tend to be stable. It assumes that if a firm raises its price, rivals do not follow (elastic demand above the kink), but if it lowers its price, rivals match the cut (inelastic demand below the kink). This creates a kink at the current price P*.
弯折的需求曲线模型解释了为何非合谋寡头市场中的价格往往具有刚性。该模型假设：若一家企业提价，对手不会跟随（弯折点以上的需求富有弹性）；若降价，对手会同步跟进（弯折点以下的需求缺乏弹性）。这便在当前价格 P* 处形成了一个弯折。
The corresponding marginal revenue (MR) curve has a discontinuous vertical gap at the kink’s output level. As long as the marginal cost (MC) curve passes through this gap, profit-maximising output and price remain unchanged. Thus, changes in costs do not lead to price changes, reinforcing price rigidity.
相应的边际收益（MR）曲线在弯折点产量处出现垂直间断。只要边际成本（MC）曲线穿过该间断区域，利润最大化的产量和价格就保持不变。因此，成本变动不会引发价格变化，进一步增强了价格刚性。
This model is useful for explaining sticky prices in industries like banking, petrol retail, or supermarkets. However, it does not explain how the initial price P* is determined, which is a common evaluation criticism.
该模型有助于解释银行业、汽油零售或超市等行业中的价格粘性。但它无法解释最初的价格 P* 是如何确定的，这是一个常见的评价性批评。
8. Non-price Competition | 非价格竞争
Because price wars can be disastrous, oligopolistic firms often compete through non-price means: advertising, branding, product innovation, loyalty schemes, packaging, after-sales service, and product differentiation. This shifts the demand curve to the right and makes demand more price inelastic, raising both price and profit margins.
由于价格战可能带来灾难性后果，寡头企业往往通过非价格手段竞争：广告、品牌建设、产品创新、忠诚度计划、包装、售后服务以及产品差异化。这会使需求曲线右移且变得更缺乏价格弹性，从而提高价格和利润率。
Non-price competition can benefit consumers through better quality and variety, but it may also waste resources on excessive advertising and create artificial brand loyalty, leading to higher prices and allocative inefficiency.
非价格竞争可通过提升质量和多样性使消费者受益，但也可能因过度广告浪费资源并制造人为的品牌忠诚度，导致价格上升和配置效率损失。
9. Evaluation: Efficiency and Welfare | 评价：效率与福利
Oligopoly is often associated with market failure. Compared to perfect competition, oligopoly tends to set higher prices and lower output, leading to a loss of allocative efficiency (P > MC). Productive inefficiency may also arise if firms do not produce at the minimum average cost due to X-inefficiency or spare capacity.
寡头往往与市场失灵联系在一起。与完全竞争相比，寡头倾向于定高价、低产出，导致配置效率损失（价格高于边际成本）。由于 X-非效率或产能
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