Shifts in the Market Demand Curve | 市场需求曲线的移动

📚 Shifts in the Market Demand Curve | 市场需求曲线的移动

Market demand curves do not stay fixed over time. In Cambridge A-Level Economics, candidates must explain why the entire demand curve can move right or left, and how this differs from a movement along the curve caused by a price change. This article examines the main non-price determinants of demand and shows how to use them accurately in exam answers.

市场需求曲线并非长期固定不变。在剑桥A-Level经济中，考生必须解释为什么整条需求曲线会向右或向左移动，以及这与价格变化引起的沿曲线移动有何不同。本文考察需求的主要非价格决定因素，并展示如何在考试答案中准确使用这些因素。

1. Demand vs Quantity Demanded | 需求与需求量的区分

In A-Level economics, ‘demand’ refers to the entire relationship between price and quantity demanded over a given period, ceteris paribus. A change in demand means the whole demand curve shifts to a new position. By contrast, ‘quantity demanded’ is a specific point on the curve at a particular price. A change in quantity demanded is caused only by a change in the good’s own price and is shown as a movement along the existing curve. Many candidates lose marks by writing ‘demand increases’ when they mean ‘quantity demanded increases’ after a price fall.

在A-Level经济中，’需求’是指在其他条件不变的情况下，给定时期内价格与需求量之间的整体关系。需求的变化意味着整条需求曲线移动到新的位置。相反，’需求量’是特定价格下曲线上的一点。需求量的变化仅由商品自身价格变化引起，表现为沿原有曲线的移动。许多考生因为把价格下降后的’需求量增加’写成’需求增加’而失分。

2. Movement Along the Demand Curve | 沿需求曲线的移动

A movement along the demand curve is caused exclusively by a change in the good’s own price. When the price falls from P to P1, quantity demanded expands from Q to Q1; this is an extension of quantity demanded. When the price rises, quantity demanded contracts; this is a contraction. The position of the curve does not change because all non-price determinants are held constant under the ceteris paribus assumption.

沿需求曲线的移动仅由商品自身价格变化引起。当价格从P下降到P1时，需求量从Q扩张到Q1，这是需求量的扩张。当价格上升时，需求量收缩，这是需求量的收缩。曲线本身的位置不变，因为根据其他条件不变的假设，所有非价格决定因素均保持不变。

3. Shift of the Demand Curve | 需求曲线的移动

A shift of the demand curve happens when a non-price determinant changes. A rightward shift from D to D1 shows that consumers are willing and able to buy more at every possible price. A leftward shift from D to D2 shows that consumers plan to buy less at every possible price. For example, if a good becomes more fashionable, demand increases at every price, shifting the entire curve right. The demand function can be written to separate the own-price effect from the shift factors.

当非价格决定因素发生变化时，需求曲线发生移动。从D到D1的右移表明消费者在每一个可能的价格下都愿意并能够购买更多。从D到D2的左移表明消费者在每一个可能的价格下计划购买更少。例如，如果一种商品变得更时尚，则在每个价格下需求都会增加，整条曲线向右移动。需求函数可以将自身价格效应与移动因素分开表示。

Qd = f(P, Y, Ps, Pc, T, Pop, Exp, …)

In this function, Qd is quantity demanded, P is the good’s own price, Y is income, Ps and Pc are prices of substitutes and complements, T is tastes, Pop is population, and Exp is expectations. A change in P causes a movement along the curve; a change in any of the other variables causes the curve to shift.

在这个函数中，Qd是需求量，P是商品自身价格，Y是收入，Ps和Pc是替代品和互补品的价格，T是偏好，Pop是人口，Exp是预期。P的变化导致沿曲线移动；其他任何变量的变化都会使曲线移动。

4. Income: Normal and Inferior Goods | 收入与正常品、劣等品

Income affects demand through the type of good. For normal goods, a rise in income increases demand; the curve shifts right. For inferior goods, a rise in income reduces demand because consumers can now afford better substitutes; the curve shifts left. The table summarises the direction of shift.

收入通过商品类型影响需求。对于正常品，收入增加会提高需求，曲线右移。对于劣等品，收入增加会减少需求，因为消费者现在能买得起更好的替代品，曲线左移。下表总结了移动方向。