A-Level Economics: Income Elasticity of Demand Explained | A-Level 经济：需求的收入弹性解析

📚 A-Level Economics: Income Elasticity of Demand Explained | A-Level 经济：需求的收入弹性解析

Income elasticity of demand (YED) measures how much the quantity demanded of a good responds to a change in consumers’ income. It is a key tool in A-Level Economics for distinguishing between normal and inferior goods, and for understanding how households adjust their spending as their financial situation changes.

需求的收入弹性（YED）衡量的是商品需求量对消费者收入变化的反应程度。它是 A-Level 经济学中一个关键工具，用于区分正常品与劣等品，并理解家庭在财务状况变化时如何调整其支出结构。

1. Definition and Formula | 1. 定义与公式

YED is defined as the percentage change in quantity demanded divided by the percentage change in income. The formula is essential knowledge for CIE A-Level candidates.

YED 的定义是：需求量的百分比变化除以收入的百分比变化。这个公式是 CIE A-Level 考生的必备知识。

YED = %ΔQd ÷ %ΔY

where %ΔQd is the percentage change in quantity demanded and %ΔY is the percentage change in household income.

其中 %ΔQd 是需求量的百分比变化，%ΔY 是家庭收入的百分比变化。

Unlike price elasticity of demand, YED focuses only on income as the determinant of demand, holding all other factors constant (ceteris paribus).

与需求的价格弹性不同，YED 仅关注收入这一需求决定因素，并保持所有其他因素不变（即其他条件相同）。

2. Normal Goods vs Inferior Goods | 2. 正常品与劣等品

Income elasticity of demand can be positive or negative, and this sign is the first thing you should identify when working with YED.

需求的收入弹性可以是正值或负值，正负号是处理 YED 时首先要判断的内容。

A positive YED means that as income increases, the quantity demanded also increases. This applies to normal goods, which make up the majority of consumer purchases.

正值的 YED 意味着当收入增加时，需求量也增加。这适用于正常品，即大多数消费者的购买行为。

A negative YED means that as income rises, demand for the good falls. These are known as inferior goods, such as cheap instant noodles or second-hand clothing, which consumers abandon as they can afford better alternatives.

负值的 YED 意味着当收入上升时，该商品的需求量反而下降。这类商品被称为劣等品，例如廉价方便面或二手衣物，当消费者能够负担更好的替代品时，就会放弃这些商品。

3. Necessities and Luxuries | 3. 必需品与奢侈品

Within normal goods, economists further divide goods based on whether their YED is less than or greater than one.

在正常品之中，经济学家会依据 YED 是小于 1 还是大于 1 进一步划分商品类别。

If 0 < YED < 1, the good is a necessity. Examples include bread, milk, and basic utilities. Demand for necessities grows more slowly than income because the good is already consumed in sufficient quantity.

如果 0 < YED < 1，该商品就是必需品，例如面包、牛奶和基本水电。必需品的需求增长比收入增长更慢，因为人们对该商品的消费量已经基本饱和。

If YED > 1, the good is classified as a luxury, or a superior good. Examples include restaurant meals, foreign travel, and designer goods. A 10% rise in income could lead to a 20% rise in demand for such goods.

如果 YED > 1，该商品则被归类为奢侈品或高档品，例如外出就餐、出境旅游和名牌商品。收入增加 10% 可能会导致此类商品的需求增加 20%。

4. Worked Calculation Example | 4. 计算例题

Let us apply the formula with a simple scenario to ensure you can compute YED accurately in the exam.

下面用一个简单场景来应用公式，确保你在考试中能准确计算 YED。

Example: A consumer’s income increases from £2,000 to £2,200 per month. As a result, their monthly spending on restaurant meals rises from £100 to £130.

示例：某消费者的月收入从 2,000 英镑增加到 2,200 英镑。结果，他每月外出就餐的支出从 100 英镑上升到 130 英镑。

Step 1: Calculate the percentage change in income. Income increased by £200, and £200 ÷ £2,000 × 100 = 10%.

第一步：计算收入变动百分比。收入增加了 200 英镑，200 ÷ 2,000 × 100 = 10%。

Step 2: Calculate the percentage change in quantity demanded. Spending rose by £30, and £30 ÷ £100 × 100 = 30%.

第二步：计算需求变动百分比。支出增加了 30 英镑，30 ÷ 100 × 100 = 30%。

Step 3: Substitute into the formula. YED = 30% ÷ 10% = +3.

第三步：代入公式。YED = 30% ÷ 10% = +3。

The YED of +3 means that restaurant meals are a luxury for this consumer, because the demand increased by three times the rate of income growth.

YED 为 +3 说明对该消费者而言，外出就餐属于奢侈品，因为需求增长率是收入增长率的 3 倍。

5. Interpreting YED Values | 5. 解读 YED 数值

A summary table helps you visualise all possible YED ranges and their corresponding goods classifications.

下表汇总了所有可能的 YED 区间及其对应的商品分类，帮助你直观掌握。

YED Value	Type of Good	Relationship with Income
YED < 0	Inferior good	Demand falls as income rises
YED = 0	Perfectly income inelastic	Demand remains constant
0 < YED < 1	Normal necessity	Demand rises by a smaller % than income
YED > 1	Luxury / superior good	Demand rises by a larger % than income

Note that a YED of exactly zero is rare in practice, but salt and certain medicines are often cited as approximate examples because we buy them in fixed quantities regardless of income.

需要注意，YED 恰好为 0 的情况在实际中非常少见，但食盐和某些药品常被引用为近似例子，因为无论收入高低，我们都会购买固定数量。

6. Determinants of YED | 6. 影响 YED 的因素

Several factors determine whether a good has a high or low income elasticity. These explain why YED is not a fixed number but varies across markets and time periods.

多个因素决定了商品收入弹性的高低。这些因素解释了为什么 YED 不是一个固定数字，而会因市场和时期而异。

The degree of necessity: goods that satisfy basic needs, such as housing and food, tend to have low YED values, whereas discretionary goods have high YED values.

必需程度：满足基本需求的商品（如住房和食品）往往具有较低的 YED 值，而可选择性消费的商品则具有较高的 YED 值。
The time period considered: in the short run, income elasticities may be lower because consumers cannot immediately adjust their spending habits; over time, they can, so YED becomes more elastic.

考虑的时间跨度：在短期内，由于消费者无法立即调整消费习惯，收入弹性可能较低；随着时间推移，消费者能够调整，因此 YED 会变得更有弹性。
Consumer preferences and advertising: a widely promoted luxury brand can sustain a high YED, while a product that consumers see no reason to buy more of will have a low YED.

消费者偏好与广告：大力推广的奢侈品牌能够维持较高的 YED，而消费者没有理由更多购买的产品则 YED 较低。
The income level of the consumer: a good that is a luxury for a low-income person, such as a car, might be a necessity for a high-income person. Thus, YED values depend on who is being considered.

消费者本身的收入水平：对于低收入者而言是奢侈品的商品（如汽车），对高收入者可能已变成必需品。因此，YED 值取决于考察对象是谁。

7. YED and Business Strategy | 7. YED 与企业策略

Firms study income elasticity carefully when making production plans and marketing decisions.

企业在制定生产计划和营销决策时会仔细研究收入弹性。

During periods of economic growth, incomes rise. A firm selling luxury goods can expect sales to grow rapidly and should therefore expand capacity, raise prices, and increase marketing budgets.

在经济扩张期，居民收入上升。销售奢侈品的企业可以预期销售额快速增长，因此应扩大产能、提高价格并增加营销预算。

During a recession, incomes fall. Suppliers of inferior goods, such as budget supermarkets and discount retailers, tend to see an increase in demand, while luxury brands may suffer a disproportionate drop in revenue.

在经济衰退期，收入下降。劣等品的供应商（如平价超市和折扣零售店）往往需求增长，而奢侈品牌则可能遭受不成比例的收入下滑。

Knowing YED also helps firms with product diversification. A multinational company may sell both luxury and necessity product lines so that its overall revenue remains stable across the economic cycle.

了解 YED 还有助于企业进行产品多样化。跨国公司可以同时经营奢侈品和必需品产品线，以使整体收入在整个经济周期中保持稳定。

8. YED and Government Policy | 8. YED 与政府政策

Governments use income elasticity data to design effective tax and welfare policies.

政府利用收入弹性数据来设计有效的税收和福利政策。

If the government wants to raise stable tax revenue, it may focus on taxing necessities, which have low YED and therefore a steady tax base. However, taxing necessities can be regressive and hit the poor hardest, so policy makers must balance efficiency with equity.

如果政府希望获得稳定的税收收入，可能会重点对必需品征税，因为必需品的 YED 较低，税基稳定。但对必需品征税具有累退性，对穷人的打击最大，因此决策者必须兼顾效率与公平。

YED also informs subsidy decisions. Because luxuries have high positive income elasticity, governments often impose higher taxes on them to discourage over-consumption or to redistribute income. On the other hand, goods with low or negative YED may be subsidised to protect low-income households from price changes.

YED 也为补贴决策提供依据。由于奢侈品具有较高的正收入弹性，政府通常对其征收较高税率以抑制过度消费或再分配收入。相反，对于 YED 较低或为负的商品，政府可能提供补贴，以保护低收入家庭免受价格变动的影响。

9. Limitations of YED | 9. YED 的局限性

Although YED is a useful analytical tool, it has several limitations that you should mention in essays to access higher-level evaluation marks.

尽管 YED 是一个有用的分析工具，但具有若干局限性，在你的论文中提及这些局限可以获得更高的评估分。

YED measures quantity demanded, not actual sales, and most statistics are estimates based on household surveys, so calculations are only approximate.

YED 衡量的是需求量，而非实际销售量，而且大多数统计数据是基于家庭调查的估算，因此计算结果只是近似值。
The value of YED changes over time and across different income groups, so a single number does not fully describe a good’s nature.

YED 的数值会随时间和不同收入群体而变化，因此单一数值并不能完全描述商品的性质。
The ceteris paribus assumption is difficult to satisfy in the real world because income changes are usually accompanied by changes in tastes, prices, and other economic conditions.

其他条件相同的假设在现实中难以成立，因为收入变化通常伴随着偏好、价格和其他经济环境的变化。

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A-Level Economics: Income Elasticity of Demand Explained | A-Level 经济：需求的收入弹性解析