📚 Contestable Markets: Potential Competition and Market Discipline | 可竞争市场：潜在竞争与市场纪律
Traditional industrial economics argued that market power depends mainly on structure: the fewer the firms and the higher the concentration ratio, the weaker the competitive pressure. The theory of contestable markets challenges that view. It argues that what really disciplines firms is not how many rivals they currently face, but how easily a new rival could enter, take the profitable opportunities on offer and leave again. A market can therefore be dominated by one or two firms and still deliver near-competitive prices, provided that entry and exit are genuinely free.
传统产业经济学认为，市场势力主要取决于市场结构：企业数量越少、集中度越高，竞争压力就越弱。可竞争市场理论挑战了这一观点。它认为，真正约束企业的不是它们当前面对多少竞争对手，而是新对手进入、夺取现有利润机会再退出的难易程度。因此，即使一个市场由一两家企业主导，只要进入和退出真正自由，价格仍可能接近竞争水平。
This article explains the definition, the assumptions, the diagram analysis and the evaluation that A-Level Economics examiners expect. It also shows how contestability links to sunk costs, limit pricing, efficiency and real-world policy debates such as airline routes, bus services and digital platforms.
本文将系统讲解 A-Level 经济学考试所要求的定义、假设、图形分析与评价，并说明可竞争性如何与沉没成本、限制性定价、效率以及航空航线、公交线路、数字平台等现实政策争论相联系。
1. What Is a Contestable Market? | 什么是可竞争市场
A contestable market is a market in which there are no significant barriers to entry or exit, and in which the costs of entering and leaving are so low that new firms can move in quickly whenever they spot a profit opportunity. The crucial feature is not the number of firms but the freedom of movement: firms can enter, earn profit, and withdraw without losing their capital.
可竞争市场是指不存在重大进入或退出壁垒、进入和离开的成本极低，因此新企业一旦发现盈利机会就能迅速进入的市场。关键特征不是企业数量，而是流动的自由度：企业能够进入、赚取利润，并在不损失资本的情况下退出。
The theory was developed by William Baumol, John Panzar and Robert Willig in the early 1980s. It shifted the analytical focus away from the actual number of firms in an industry and towards the ease with which firms can enter and leave. In their framework, a market may be served by only one firm, yet that firm may still be forced to behave as if it faced intense competition.
该理论由 William Baumol、John Panzar 和 Robert Willig 在 20 世纪 80 年代初提出。它把分析重点从产业中企业的实际数量，转移到企业进入和退出市场的难易程度。在该框架下，一个市场可能只有一家企业提供服务，但这家企业仍可能被迫表现得仿佛面临激烈竞争。
The reason is simple. If the incumbent earns supernormal profit, new entrants will be attracted. If entry is costless, those entrants can capture profit and leave before the incumbent has time to respond. The threat of entry alone therefore reduces the scope for exploiting market power, and the long-run equilibrium profit converges on normal profit.
原因很简单。如果在位企业赚取超额利润，就会吸引新进入者。如果进入无需成本，这些进入者可以在在位企业来得及反应之前夺取利润并退出。因此，仅进入威胁本身就压缩了行使市场势力的空间，长期均衡利润收敛于正常利润。
π = TR − TC → 0 in long-run equilibrium
2. Origins of the Theory | 理论的起源
The theory emerged as a response to two older traditions. The Harvard school, built around the structure-conduct-performance paradigm, argued that concentrated market structure causes anti-competitive conduct and poor performance. The Chicago school replied that if a firm were earning excessive profit, entry would occur and erode it, so government intervention was rarely needed. Contestability theory formalised the Chicago intuition by specifying exactly what makes entry effective.
该理论是对两个更早传统的回应。以“结构—行为—绩效”范式为核心的哈佛学派认为，集中的市场结构会导致反竞争行为与不良绩效。芝加哥学派则回应说，若企业赚取过高利润，进入就会发生并将其侵蚀，因此很少需要政府干预。可竞争市场理论通过明确界定进入何以有效，把芝加哥学派的直觉形式化。
Baumol, Panzar and Willig published their book Contestable Markets and the Theory of Industry Structure in 1982. They argued that the polar cases of perfect competition and monopoly were both extreme, and that most real industries lay somewhere between. Contestability offered a more general benchmark: the competitive outcome could be achieved even in a highly concentrated industry, provided entry and exit were perfectly free.
Baumol、Panzar 和 Willig 于 1982 年出版《可竞争市场与产业结构理论》。他们认为完全竞争与垄断这两个极端情形都过于特殊，大多数现实产业处于两者之间。可竞争性提供了一个更一般的基准：只要进入和退出完全自由，即使产业高度集中，也能实现竞争性结果。
This mattered for policy. If a natural monopoly were contestable, regulators might not need to break it up or control its prices, because the threat of entry would do the job. That claim has been heavily tested and heavily criticised, which is exactly why it makes such a productive A-Level evaluation point.
这一点对政策意义重大。如果自然垄断是可竞争的，监管者也许无需拆分企业或管制价格，因为进入威胁会自动发挥作用。这一论断后来受到大量检验和批评——这正是它成为 A-Level 极佳评价点的原因。
3. Conditions for Contestability | 可竞争性的条件
For a market to be genuinely contestable, a specific set of conditions must hold. Examiners reward candidates who can list these precisely rather than simply saying "entry is easy".
要使一个市场真正具有可竞争性，必须满足一系列具体条件。考官会奖励那些能精确列出这些条件、而非笼统说“进入很容易”的考生。
- No entry barriers: there must be no legal, technical or strategic obstacles such as licences, patents, exclusive contracts or control of essential facilities.
没有进入壁垒：不存在法律、技术或策略性障碍，例如牌照、专利、排他性合同或对关键设施的控制。 - No exit barriers and no sunk costs: a firm that leaves must be able to recover the value of the assets it committed, so that failure is not punished by irreversible loss.
没有退出壁垒、没有沉没成本：退出企业必须能够收回其投入资产的价值，使失败不会以不可逆损失作为惩罚。 - Equal access to technology: potential entrants must be able to use the same production techniques and cost functions as the incumbent, so they are not permanently disadvantaged.
技术获取平等：潜在进入者必须能够使用与在位企业相同的生产技术和成本函数，从而不处于永久劣势。 - Rapid entry and exit: entrants must be able to move in, sell and withdraw quickly, before the incumbent can cut prices in response, the classic "hit-and-run" condition.
快速进入与退出：进入者必须能够迅速进入、销售并撤出，抢在在位企业降价回应之前，这就是经典的“打了就跑”条件。 - Perfect information for consumers: buyers must know about the entrant's price and be willing to switch, otherwise entry does not take market share.
消费者的完全信息：买方必须了解进入者的价格并愿意转换，否则进入无法夺取市场份额。 - No retaliation with commitment value: the incumbent must be unable to make a credible, irreversible price cut or capacity commitment that deters entry.
不存在具有承诺价值的报复：在位企业必须无法做出可信的、不可逆的降价或产能承诺来阻止进入。
If all of these hold, the industry is perfectly contestable. If only some hold, the industry is imperfectly contestable, which is the realistic and examinable case for most markets.
如果这些条件全部成立，该产业就是完全可竞争的。如果只满足部分条件，则属于不完全可竞争——这正是大多数市场的现实情况，也是考试中可展开讨论的情形。
4. Hit-and-Run Entry and Potential Competition | “打了就跑”进入与潜在竞争
The central mechanism of contestability is hit-and-run entry. Suppose an incumbent charges a price well above average cost. An entrant observes this, enters the market, undercuts the incumbent slightly, serves as much demand as it can at that price, and then exits before the incumbent can react by cutting prices.
可竞争性的核心机制是“打了就跑”式进入。假设在位企业定价远高于平均成本，进入者观察到这一点，进入市场并以略低价格抢占需求，尽可能多地服务消费者，然后在位企业降价回应之前就退出。
Because the entrant can recover its capital on exit, entry is a low-risk strategy. The incumbent cannot punish it, because by the time prices fall the entrant is already gone. The incumbent therefore knows that any price above average cost is a standing invitation to entry, and rationally chooses to price at or near average cost in the first place.
由于进入者在退出时能收回资本，进入是一种低风险策略。在位企业无法惩罚它，因为等到价格下降时，进入者早已离场。因此在位企业清楚：任何高于平均成本的价格都是对进入的公开邀请，理性选择就是在最初就把价格定在平均成本附近。
The key implication is that potential competition does the work of actual competition. Even a pure monopolist can be disciplined, provided the market is contestable. This is why contestability is often described as a theory of "competition for the market" rather than competition within the market.
其核心含义是：潜在竞争发挥了实际竞争的作用。即使面对纯垄断者，只要市场是可竞争的，它同样会被约束。正因如此，可竞争性常被描述为“为市场而竞争”，而非“在市场内竞争”的理论。
Enter if P > AC; stay out if P ≤ AC
5. Sunk Costs: The Real Barrier | 沉没成本才是真正的壁垒
Sunk costs are costs that have already been incurred and cannot be recovered if the firm exits. They are the decisive variable in contestability theory. Entry barriers such as licences and patents matter, but even without them, high sunk costs will make a market uncontestable.
沉没成本是已经发生、且企业退出时无法收回的成本。它们是可竞争性理论中具有决定意义的变量。牌照、专利等进入壁垒固然重要，但即使没有这些壁垒，高昂的沉没成本也会使市场失去可竞争性。
Advertising to build brand awareness, research and development, specialised machinery with no second-hand market, and non-refundable regulatory fees are all sunk. If an entrant must spend heavily on these and cannot resell them on exit, then entry becomes a gamble rather than a low-risk arbitrage.
为建立品牌认知而支出的广告费、研发投入、没有二手市场的专用设备，以及不可退还的监管费用，都属于沉没成本。如果进入者必须大量投入这些成本且退出时无法转售，那么进入就变成赌博，而不是低风险的套利行为。
The entry decision can be expressed as a simple expected-profit test. An entrant will move in only if the profit it expects to earn exceeds the sunk cost it would lose on exit.
进入决策可以用一个简单的预期利润检验来表达。只有当进入者预期获得的利润超过退出时将损失的沉没成本时，它才会进入。
Enter if (P − AC) × Q > S, where S = sunk cost
This explains why industries such as airlines, coach services and freight haulage are often cited as relatively contestable, while industries such as pharmaceuticals, electricity transmission and heavy manufacturing are not. In the former, aeroplanes, coaches and lorries can be sold or redeployed; in the latter, the capital is often highly specific and hard to recover.
这就解释了为什么航空、长途客运和货运常被视为相对可竞争的行业，而制药、输电和重工业则不是。前者的飞机、客车和货车可以出售或转用；后者的资本往往高度专用，难以收回。
6. Contestability Compared with Other Market Structures | 可竞争市场与其他市场结构的比较
Contestability is a property of a market, not a separate market structure. It can overlap with monopoly and oligopoly, and it is therefore best understood as a dimension along which markets vary.
可竞争性是一种市场属性，而不是一种独立的市场结构。它可以与垄断和寡头并存，因此最好把它理解为市场差异化的一个维度。
| Market type | Firms | Entry conditions | Long-run price | Long-run profit |
| Perfect competition | Many | Completely free | P = MC = minimum AC | Normal |
| Contestable natural monopoly | One | Free, no sunk costs | P = AC > MC | Normal |
| Classic monopoly | One | Blocked | P > AC | Supernormal |
| Non-contestable oligopoly | Few | Blocked or costly | P > AC | Supernormal |
The comparison shows the essential insight: perfect competition requires both many firms and free entry, whereas contestability requires only free entry. This makes contestability a weaker and therefore more widely applicable condition than perfect competition.
这一比较揭示了核心洞见：完全竞争既要求企业数量众多，也要求自由进入，而可竞争性只要求自由进入。这使可竞争性成为比完全竞争更弱、因而适用范围更广的条件。
7. Equilibrium Pricing: Limit Pricing and Normal Profit | 均衡定价：限制性定价与正常利润
In a perfectly contestable market the incumbent sets price equal to average cost. Any price above average cost would attract hit-and-run entry; any price below average cost would generate losses in the long run. The only sustainable price is therefore the break-even price, sometimes called the sustainable price.
在完全可竞争市场中，在位企业会把价格定在等于平均成本的水平。任何高于平均成本的价格都会吸引“打了就跑”式进入；任何低于平均成本的价格在长期都会造成亏损。因此唯一可持续的价格就是盈亏平衡价格，有时称为可维持价格。
This outcome is a form of limit pricing, but not in the predatory sense. The incumbent is not temporarily sacrificing profit to drive a rival out; it is permanently accepting normal profit because it knows entry would otherwise occur. It is deterrence through credible passivity rather than aggressive retaliation.
这一结果属于限制性定价的一种形式，但并非掠夺性含义。在位企业不是在暂时牺牲利润以驱逐对手，而是永久接受正常利润，因为它知道若不如此进入就会发生。这是通过可信的“不激进行动”实现遏制，而非通过激进的报复。
The result matters for welfare. Consumers pay a price equal to average cost rather than the monopoly price, and output is higher. However, price still exceeds marginal cost whenever average cost is falling, so allocative inefficiency is not fully eliminated.
这一结果对福利有重要意义。消费者支付等于平均成本的价格，而不是垄断价格，产量也更高。然而，当平均成本下降时，价格仍然高于边际成本，因此配置无效率并未被完全消除。
P = AC > MC → normal profit but residual allocative inefficiency
8. Diagram Analysis | 图形分析
In the standard exam diagram for a contestable natural monopoly, the average cost curve slopes downward throughout the relevant range, reflecting substantial economies of scale. The marginal cost curve lies below the average cost curve and is also downward sloping. The average revenue, or demand, curve slopes downward.
在标准的可竞争自然垄断图形中，平均成本曲线在相关产出范围内持续向下倾斜，反映显著的规模经济。边际成本曲线位于平均成本曲线下方，同样向下倾斜。平均收益即需求曲线向下倾斜。
An unregulated monopolist equates marginal revenue with marginal cost, producing Qₘ and charging Pₘ, where Pₘ > AC. The shaded area between price and average cost represents supernormal profit.
不受管制的垄断者令边际收益等于边际成本，产量为 Qₘ，价格为 Pₘ，且 Pₘ > AC。价格与平均成本之间的阴影面积即代表超额利润。
In a perfectly contestable market, the relevant equilibrium is where average revenue meets average cost, at output Q꜀ and price P꜀. At this point the firm earns only normal profit, because any price above P꜀ invites entry.
在完全可竞争市场中，相关均衡点是平均收益与平均成本相交之处，产量为 Q꜀，价格为 P꜀。此时企业仅获得正常利润，因为任何高于 P꜀ 的价格都会招致进入。
The comparison is therefore: output rises from Qₘ to Q꜀, price falls from Pₘ to P꜀ MC, the firm would make a loss because P < AC, and would require a subsidy to survive.
因此比较结果是：产量从 Qₘ 增至 Q꜀，价格从 Pₘ 降至 P꜀，超额利润被消除。若监管者改为实施边际成本定价 P = MC，由于 P < AC，企业将发生亏损，必须依靠补贴才能生存。
Pₘ > P꜀ = AC > MC and Qₘ < Q꜀
9. Efficiency Implications | 效率含义
Contestability affects each type of efficiency differently, and a strong answer distinguishes them explicitly rather than treating efficiency as a single concept.
可竞争性对各类效率的影响各不相同，优秀的答案会明确区分它们，而不是把效率当作单一概念。
Allocative efficiency tive efficiency is not achieved. Contestability improves on monopoly pricing but does not reach the competitive optimum.
配置效率要求 P = MC。在可竞争自然垄断中，P = AC > MC，因此并未实现配置效率。可竞争性优于垄断定价，但未达到竞争性最优。
Productive efficiency requires production at minimum average cost. Because a single firm supplies the whole market in a contestable natural monopoly, it can exploit economies of scale and may be more productively efficient than a fragmented market with many small firms each operating above minimum efficient scale.
生产效率要求在最低平均成本处生产。在可竞争自然垄断中，由一家企业供应整个市场，可以充分利用规模经济，可能比众多小企业各自在最低有效规模之上运营的分散市场更具生产效率。
Dynamic efficiency is more ambiguous. The absence of supernormal profit reduces the internal funds available for investment and innovation, which may weaken dynamic efficiency. On the other hand, the constant threat of entry forces the incumbent to keep costs low and to innovate in order to preserve its position.
动态效率则较为模糊。超额利润的缺失减少了可用于投资和创新的内部资金，可能削弱动态效率。另一方面，持续的进入威胁迫使在位企业压低成本、进行创新，以维持自身地位。
X-inefficiency is likely to be reduced. Managers in a contestable market know that slack, overstaffing and wasteful spending would raise average cost above the level at which entry becomes attractive, so organisational inefficiency is punished by the market itself.
X 效率
