📚 The Definition of Demand and Its Determinants | 需求的定义与影响因素
In economics, demand is one of the most fundamental concepts you will encounter in your IGCSE course. Understanding what demand truly means ‘ and the factors that influence it ‘ is essential for analysing how markets work. This article breaks down the definition of demand, the law of demand, and the key determinants that shift the demand curve, all tailored to the CIE IGCSE Economics syllabus.
在经济学中，需求是IGCSE课程中最基本的概念之一。真正理解需求的含义以及影响需求的因素，对于分析市场如何运作至关重要。本文根据CIE IGCSE经济学大纲，详细讲解需求的定义、需求定律以及使需求曲线移动的关键影响因素。
1. What Is Demand? | 什么是需求
Demand is defined as the quantity of a good or service that consumers are willing and able to purchase at a given price over a given period of time. The key words here are ‘willing’ and ‘able’. A consumer may want a luxury sports car, but if they cannot afford it, their want does not count as demand. Similarly, if they have the money but are not willing to spend it on that product, it is not demand either.
需求的定义是：在给定价格下，消费者在一定时期内愿意并且能够购买的商品或服务的数量。这里的关键词是”愿意”和”能够”。消费者可能想要一辆豪华跑车，但如果负担不起，这种欲望就不构成需求。同样，如果消费者有钱但不愿意花钱购买该产品，那也不构成需求。
2. Effective Demand vs. Wants | 有效需求与欲望
It is important to distinguish between ‘wants’ and ‘demand’. A want is simply a desire for a product, while demand is a want backed by the ability and willingness to pay. Economists call this ‘effective demand’. For example, everyone might want a private jet, but only a tiny number of people have the purchasing power to translate that want into demand.
区分”欲望”与”需求”非常重要。欲望仅仅是对产品的渴望，而需求则是以支付能力和支付意愿为支撑的欲望。经济学家称之为”有效需求”。例如，也许每个人都想要一架私人飞机，但只有极少数人拥有将这种欲望转化为需求的购买力。
3. The Law of Demand | 需求定律
The law of demand states that there is an inverse relationship between the price of a good and the quantity demanded, ceteris paribus (all other things being equal). When the price of a product rises, the quantity demanded falls; when the price falls, the quantity demanded rises. This occurs for two main reasons: the income effect and the substitution effect.
需求定律指出，在其他条件不变的情况下（ceteris paribus），商品价格与需求量之间存在反向关系。当产品价格上升时，需求量下降；当价格下降时，需求量上升。这主要归因于两个原因：收入效应和替代效应。
Price ↑ → Quantity Demanded ↓
Price ↓ → Quantity Demanded ↑
4. The Demand Curve | 需求曲线
The demand curve is a graphical representation of the relationship between price and quantity demanded. It slopes downward from left to right, reflecting the law of demand. Price is plotted on the vertical (y) axis and quantity demanded on the horizontal (x) axis. A demand curve is normally drawn as a straight line in IGCSE economics for simplicity.
需求曲线是价格与需求量之间关系的图形表示。它从左向右向下倾斜，反映了需求定律。价格标在纵轴（y轴）上，需求量标在横轴（x轴）上。在IGCSE经济学中，为简化起见，需求曲线通常画成一条直线。
5. Movements Along the Demand Curve | 沿着需求曲线的移动
A movement along the demand curve occurs when there is a change in the price of the good itself. If price rises from P₁ to P₂, the quantity demanded falls from Q₁ to Q₂, and we move from point A to point B along the same curve. This is called a contraction of demand. Conversely, a fall in price causes an extension of demand (movement from B to A).
当商品本身的价格发生变化时，会发生沿着需求曲线的移动。如果价格从P₁上升到P₂，需求量会从Q₁下降到Q₂，我们从同一条曲线上的A点移动到B点。这被称为需求的收缩。相反，价格下降则引起需求的扩展（从B点移动到A点）。
6. Shifts of the Demand Curve | 需求曲线的移动
A shift of the demand curve occurs when a factor other than the price of the good itself changes. An increase in demand shifts the whole curve to the right (from D₁ to D₂), meaning consumers are willing to buy more at each price. A decrease in demand shifts the curve to the left (from D₁ to D₃), meaning consumers buy less at every price. This distinction between a movement along and a shift of the curve is a classic exam question.
当商品本身价格以外的因素发生变化时，需求曲线会发生移动。需求增加使整条曲线向右移动（从D₁到D₂），意味着在每个价格水平上消费者都愿意购买更多。需求减少则使曲线向左移动（从D₁到D₃），意味着在每个价格水平上消费者购买得更少。区分沿曲线的移动与曲线的移动是经典考试题目。
7. Determinant 1: Income | 影响因素一：收入
A change in consumers’ income affects demand. For normal goods, an increase in income leads to an increase in demand, shifting the curve to the right. For inferior goods, however, an increase in income leads to a decrease in demand, shifting the curve to the left. Inferior goods are cheaper alternatives, such as own-brand supermarket products or instant noodles, which consumers buy less of once they can afford better options.
消费者收入的变化会影响需求。对于正常商品而言，收入增加导致需求增加，曲线向右移动。但对于低档商品，收入增加反而导致需求减少，曲线向左移动。低档商品是较便宜的替代选择，例如超市自有品牌产品或方便面，一旦消费者能够负担更好的选择，就会减少购买这些商品。
8. Determinant 2: Price of Substitutes and Complements | 影响因素二：替代品与互补品的价格
The prices of related goods also affect demand. Substitutes are goods that satisfy similar needs, such as tea and coffee. If the price of coffee rises, the demand for tea increases as consumers switch to the cheaper alternative. Complements are goods that are used together, such as cars and petrol. If the price of cars rises, the demand for petrol will also fall because fewer cars are being purchased and driven.
相关商品的价格也会影响需求。替代品是满足类似需求的商品，例如茶和咖啡。如果咖啡价格上涨，对茶的需求会增加，因为消费者会转向更便宜的替代品。互补品是一起使用的商品，例如汽车和汽油。如果汽车价格上涨，对汽油的需求也会下降，因为购买和驾驶的汽车数量减少了。
Substitutes: Price of Coffee ↑ → Demand for Tea ↑
Complements: Price of Cars ↑ → Demand for Petrol ↓
9. Determinant 3: Tastes and Preferences | 影响因素三：口味与偏好
Changes in consumer tastes, fashion, and advertising can shift the demand curve. If a product becomes more fashionable, demand increases and the curve shifts right. For example, a celebrity endorsement can boost the demand for a particular brand of trainers. Conversely, negative news reports about a product’s health effects will reduce demand and shift the curve to the left.
消费者口味、时尚潮流和广告的变化会使需求曲线移动。如果某产品变得更时尚，需求会增加，曲线向右移动。例如，名人代言可以提升某个特定品牌运动鞋的需求。相反，关于某产品健康影响的负面新闻报道会减少需求，使曲线向左移动。
10. Determinant 4: Population and Demographics | 影响因素四：人口与人口结构
The size and composition of the population affect demand. A growing population increases overall demand for most goods and services, shifting demand curves to the right. Changes in the age structure also matter: an ageing population increases demand for healthcare and retirement services, while a young population increases demand for education and toys. Changes in income distribution can also affect which types of goods are demanded.
人口的规模和结构会影响需求。人口增长会提高大多数商品和服务的总体需求，使需求曲线向右移动。年龄结构的变化也很重要：人口老龄化会增加对医疗保健和养老服务的需求，而年轻人口则增加对教育和玩具的需求。收入分配的变化也会影响对哪些类型商品的需求。
11. Determinant 5: Expectations of Future Prices | 影响因素五：未来价格预期
If consumers expect the price of a good to rise in the future, they are likely to buy more now, shifting current demand to the right. Similarly, if a price fall is expected, consumers may delay their purchases, reducing current demand and shifting the curve to the left. This is particularly common for durable goods like cars, furniture, and electronic appliances.
如果消费者预期某商品未来价格会上涨，他们现在可能会购买更多，使当前需求向右移动。同样，如果预期价格会下降，消费者可能会推迟购买，从而减少当前需求并使曲线向左移动。这种现象在耐用品中尤为常见，如汽车、家具和电子电器。
12. Summary Table | 总结表格
The table below summarises the key determinants of demand and the direction of the shift in the demand curve.
下表总结了影响需求的关键因素以及需求曲线的移动方向。
| Determinant 影响因素 | Increase 增加 | Decrease 减少 |
| Income (normal goods) 收入（正常商品） | Shift right 右移 | Shift left 左移 |
| Price of substitutes 替代品价格 | Shift right 右移 | Shift left 左移 |
| Price of complements 互补品价格 | Shift left 左移 | Shift right 右移 |
| Tastes and preferences 口味与偏好 | Shift right 右移 | Shift left 左移 |
| Population 人口 | Shift right 右移 | Shift left 左移 |
| Future price expectations 未来价格预期 | Shift right 右移 | Shift left 左移 |
In exam questions, always state whether there is a movement along the demand curve or a shift of the curve, and clearly justify your answer with reference to the relevant determinant. Drawing a correctly labelled diagram will help you earn full marks.
在考试题目中，务必说明是沿着需求曲线的移动，还是需求曲线的移动，并参考相关影响因素清晰论证你的答案。画出正确标注的图形将帮助你获得满分。
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