📚 Monopoly: Market Power, Efficiency and Policy | 垄断：市场势力、效率与政策
In A-Level Economics, monopoly is one of the core market structures examined by Cambridge International. It sits at the opposite end of the spectrum from perfect competition and is essential for understanding market failure, pricing behaviour, efficiency losses and the rationale for government regulation.
在 A-Level 经济学中，垄断是剑桥国际考试考查的核心市场结构之一。它与完全竞争处于市场谱系的两端，对于理解市场失灵、定价行为、效率损失以及政府监管的理由至关重要。
1. Defining Monopoly | 垄断的定义
A pure monopoly exists when a single firm supplies the entire market output, meaning the firm is the industry and there are no close substitutes.
纯粹垄断指一家企业供给整个市场的全部产量，即企业就是行业，且没有近似替代品。
In practice, Cambridge Economics defines a monopoly more broadly as any firm with dominant market power, often indicated by a market share of 25% or above, and the ability to act as a price maker rather than a price taker.
在实践中，剑桥经济学将垄断更宽泛地定义为任何拥有主导市场势力的企业，通常以 25% 或以上的市场份额为标志，并能够作为定价者而非价格接受者行事。
This distinction matters because few true pure monopolies exist, but many firms behave like monopolists within their specific markets.
这一区分很重要，因为真正的纯粹垄断很少存在，但许多企业在其特定市场中表现得像垄断者。
A legal monopoly may also be created by the state, for example through a patent or an exclusive licence, even if the firm does not supply the whole market.
法定垄断也可能由国家创造，例如通过专利或独家许可证，即使该企业并未供给整个市场。
The relevant market must be defined carefully. A firm may have high market share in one narrow market but face strong competition in a broader market.
相关市场必须谨慎界定。一家企业可能在某个狭窄市场拥有高份额，但在更广泛市场中面临激烈竞争。
2. Characteristics and Barriers to Entry | 特征与进入壁垒
Monopolistic markets are identified by several key characteristics: a single dominant seller, high barriers to entry, price-making ability, imperfect information and profit-maximising behaviour.
垄断市场可由几个关键特征识别：单一主导卖方、高进入壁垒、定价能力、信息不完全以及利润最大化行为。
Barriers to entry are the structural, legal or strategic obstacles that prevent new firms from entering the industry.
进入壁垒是阻止新企业进入行业的结构性、法律性或策略性障碍。
- Legal barriers such as patents, licences and copyrights give an exclusive right to produce.
法律壁垒，如专利、许可证和版权，赋予独家生产权。 - Economies of scale can make it inefficient for more than one firm to supply the market, creating a natural monopoly.
规模经济可能使多家企业供给市场变得低效，从而形成自然垄断。 - Control of key inputs, such as raw materials or distribution networks, can block rivals.
控制关键投入品，如原材料或分销网络，可以阻挡竞争对手。 - Brand loyalty and advertising raise sunk costs and make consumer switching harder.
品牌忠诚和广告提高沉没成本，使消费者转换更难。 - Strategic barriers include limit pricing, predatory pricing and mergers.
策略性壁垒包括限制性定价、掠夺性定价和并购。 - Network effects can make a large incumbent more valuable to users, reinforcing its dominance.
网络效应可使大型在位企业对用户更有价值，从而强化其支配地位。
These barriers allow the monopolist to sustain supernormal profit in the long run, unlike firms in perfect competition.
这些壁垒使垄断者能够在长期维持超额利润，这与完全竞争企业不同。
Sunk costs are especially important: if entering the market requires large irreversible spending, potential rivals are deterred.
沉没成本尤其重要：如果进入市场需要大量不可逆支出，潜在竞争对手就会被吓退。
3. Market Power, Demand and Revenue | 市场势力、需求与收入
Because the monopolist is the industry, the firm faces the downward-sloping market demand curve. To sell more output, it must lower price.
由于垄断者就是行业，企业面对向下倾斜的市场需求曲线。要销售更多产量，它必须降低价格。
This means price is greater than marginal revenue. Average revenue is the demand curve, while marginal revenue lies below it and falls more steeply.
这意味着价格大于边际收益。平均收益即需求曲线，而边际收益位于其下方并下降更陡。
For a linear demand curve, the marginal revenue curve has twice the slope of the average revenue curve.
对于线性需求曲线，边际收益曲线的斜率是平均收益曲线的两倍。
P > MR, and AR = P
The monopolist is a price maker, but it is still constrained by consumer demand and the possibility of substitutes.
垄断者是定价者，但仍受消费者需求及替代品可能性的约束。
If demand is price inelastic at the current price, the firm may raise price and increase total revenue, but it must accept lower quantity sold.
如果当前价格下需求缺乏价格弹性，企业可能提价并增加总
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