📚 Effect of a Change in Income on Consumer Choice | 收入变化对消费者选择的影响
When a consumer’s income changes, the set of affordable goods and services changes, but the actual effect on consumption depends on the type of good and the consumer’s preferences. In A-Level Economics, you need to distinguish between normal goods, inferior goods, necessities and luxuries, and you must be able to show the impact using budget lines, income-consumption curves and Engel curves.
当消费者收入发生变化时，可负担的商品与服务组合会改变，但对消费的实际影响取决于商品类型和消费者偏好。在 A-Level 经济学中，你需要区分正常品、劣等品、必需品和奢侈品，并能运用预算线、收入-消费曲线和恩格尔曲线来展示其影响。
1. Income and the Consumer’s Decision | 收入与消费者决策
Consumers make choices subject to a budget constraint: they cannot spend more than their income, assuming no borrowing or saving in the basic model. An increase in income relaxes this constraint and allows the consumer to reach a higher indifference curve or afford more of at least one good.
消费者在预算约束下做出选择：在基本模型中，假设没有借贷或储蓄，支出不能超过收入。收入增加会放松这一约束，使消费者能够达到更高的无差异曲线，或至少能够购买更多一种商品。
The response to a rise in income is not the same for every product. Some goods experience an increase in quantity demanded, some experience almost no change, and some actually experience a fall in demand. This variation is central to the analysis of consumer choice.
并非所有商品对收入上升的反应都相同。有些商品的需求量会增加，有些几乎不变，有些反而下降。这种差异是消费者选择分析的核心。
2. The Budget Constraint and Parallel Shifts | 预算约束与平行移动
In a two-good model, the budget line is written as PXQX + PYQY = M, where P is price, Q is quantity and M is nominal income. The intercepts are M/PX and M/PY, and the slope is -PX/PY.
在两种商品模型中，预算线可写作 PXQX + PYQY = M，其中 P 为价格、Q 为数量、M 为名义收入。横纵截距分别为 M/PX 和 M/PY，斜率为 -PX/PY。
When income rises and prices stay constant, both intercepts increase proportionally, so the budget line shifts outward in a parallel manner. The relative price of the two goods, given by the slope, does not change. Therefore, the change in consumption is driven purely by the income effect, not by a substitution effect.
当收入上升且价格保持不变时，两个截距同比例增加，因此预算线平行向外移动。预算线斜率所表示的两种商品相对价格不变。因此，消费变化完全由收入效应引起，而不是替代效应。
3. Normal Goods and the Positive Income Effect | 正常品与正收入效应
A normal good is one for which quantity demanded increases when real income rises, ceteris paribus. The income effect is positive: at the new optimum, the consumer buys more of the good because higher purchasing power allows them to satisfy more wants.
正常品是指在实际收入上升时，需求量在其他条件不变的情况下增加的商品。其收入效应为正：在新的最优点，消费者会购买更多该商品，因为更高的购买力使他们能够满足更多需求。
Examples of normal goods include organic food, branded clothing, restaurant meals, private healthcare and overseas holidays. In an exam, a rightward shift of the budget line would be followed by a new tangency point on a higher indifference curve, with more of both goods if both are normal.
正常品的例子包括有机食品、品牌服装、餐厅用餐、私人医疗和海外度假。在考试中，预算线右移后，新的切点会位于更高的无差异曲线上；如果两种商品都是正常品，两种商品的消费量都会增加。
4. Inferior Goods and the Negative Income Effect | 劣等品与负收入效应
An inferior good is one for which demand falls as real income rises. This happens because the consumer substitutes away from the cheaper, lower-quality product once they can afford a better alternative. The income effect is negative.
劣等品是指实际收入上升时需求反而下降的商品。这是因为消费者一旦能负担更好的替代品，就会减少购买较便宜、质量较低的产品。其收入效应为负。
Common examples include supermarket value-range products, second-hand clothing, bus travel and instant noodles. Note that an inferior good is not necessarily low quality in an absolute sense; it is simply a good that consumers buy less of as they become richer.
常见例子包括超市自有平价商品、二手服装、公交车出行和方便面。需要注意的是，劣等品并不一定本身质量绝对差，它只是在消费者变得更富裕后购买量减少的商品。
5. Necessities and Luxuries | 必需品与奢侈品
Among normal goods, economists distinguish between necessities and luxuries. A necessity has a positive but small income response: demand rises by a smaller percentage than income. A luxury has an income response greater than the percentage change in income.
在正常品中，经济学家区分必需品和奢侈品。必需品的收入反应为正但较小：需求增加的百分比小于收入增加的百分比。奢侈品的收入反应则大于收入的百分比变化。
For example, spending on basic food and utilities tends to rise only modestly when income rises, while spending on fine dining, jewellery, designer fashion and premium cars tends to rise more than proportionally. This distinction is important when drawing Engel curves and interpreting income elasticity coefficients.
例如，基本食品和水电支出的增长往往低于收入增长，而高档餐饮、珠宝、名牌时装和高档汽车支出的增长往往超过收入增长。这一区别在绘制恩格尔曲线和解释收入弹性系数时非常重要。
6. The Income-Consumption Curve | 收入-消费曲线
The income-consumption curve, or ICC, connects all consumer equilibrium points as income changes while prices remain constant. It shows how the optimal bundle of two goods varies with income.
收入-消费曲线（ICC）连接收入变化而价格不变时消费者的所有均衡点。它显示两种商品的最优组合如何随收入变化而变化。
If both goods are normal, the ICC slopes upward from the origin. If one good is inferior, the ICC bends backwards, indicating that the consumer buys less of the inferior good as income rises beyond a certain level.
如果两种商品都是正常品，ICC 从原点向右上方倾斜。如果其中一种商品是劣等品，ICC 会向后弯曲，表明当收入超过一定水平后，消费者购买的劣等品数量减少。
7. Engel Curves | 恩格尔曲线
An Engel curve plots a consumer’s quantity demanded of a single good against income, holding preferences and relative prices constant. It is derived from the income-consumption curve by reading off the quantities of one good at different income levels.
恩格尔曲线描绘消费者对某一种商品的需求量与收入之间的关系，并保持偏好和相对价格不变。它由收入-消费曲线推导而来，即在不同的收入水平下读出某一种商品的数量。
For a normal necessity, the Engel curve is upward sloping but becomes steeper as income rises, because additional income leads to smaller and smaller increases in quantity demanded. For a luxury, the curve is upward sloping and becomes flatter. For an inferior good, the curve initially rises but eventually bends downward.
对正常必需品而言，恩格尔曲线向上倾斜，但随着收入增加逐渐变陡，因为额外收入带来的需求量增量越来越小。对奢侈品而言，曲线向上倾斜且逐渐变平。对劣等品而言，曲线起初上升，但最终向下弯曲。
8. Income Elasticity of Demand (YED) | 需求的收入弹性（YED）
Income elasticity of demand measures the responsiveness of quantity demanded to a change in income. The formula is:
需求的收入弹性衡量需求量对收入变化的反应程度。其公式为：
YED = % change in quantity demanded ÷ % change in income
YED values are classified as follows:
YED 的数值分类如下：
| YED value | YED 数值 | Type of good | 商品类型 |
| YED < 0 | Inferior good | 劣等品 |
| 0 < YED < 1 | Normal necessity | 正常必需品 |
| YED > 1 | Normal luxury | 正常奢侈品 |
A firm can use YED estimates to predict how demand will respond to changes in household income and to adjust its product mix accordingly.
企业可以利用 YED 估计值来预测需求如何随家庭收入变化，并相应调整产品组合。
9. Real Income and Nominal Income | 实际收入与名义收入
A change in nominal income does not always change purchasing power. Real income is nominal income adjusted for the general price level. If nominal income rises by 5% but inflation is 3%, real income rises by only about 2%.
名义收入的变化并不总能改变购买力。实际收入是名义收入根据总体价格水平调整后的结果。如果名义收入上升 5%，但通胀率为 3%，实际收入仅上升约 2%。
Consumer choice depends on real income, not nominal income. If prices rise faster than nominal income, the budget constraint actually shifts inward, even though the consumer’s monetary income has increased. This can lead to lower consumption of normal goods and higher consumption of inferior goods.
消费者选择取决于实际收入，而不是名义收入。如果价格上涨速度快于名义收入，预算约束实际上会向内移动，即使消费者的货币收入增加了。这可能导致正常品消费减少、劣等品消费增加。
10. Exam Application and Evaluation | 考试应用与评估
In a Cambridge A-Level exam, a typical question may ask you to explain how a rise in income affects the demand for a product. You should begin by distinguishing normal and inferior goods, then use a budget line diagram to show the parallel outward shift and the new consumer equilibrium.
在剑桥 A-Level 考试中，典型题目可能会要求解释收入上升如何影响某种产品的需求。你应先区分正常品和劣等品，然后用预算线图展示预算线平行外移和新的消费者均衡。
Evaluation points include the size of the income change, the time horizon, household composition, whether the good is a necessity or luxury, and the possibility that consumer preferences themselves may change as income rises. For example, a product may be normal at low income levels but become inferior at very high income levels.
评估要点包括收入变化幅度、时间范围、家庭构成、商品属于必需品还是奢侈品，以及消费者偏好本身可能随收入上升而变化。例如，某种商品在低收入水平下可能是正常品，但在极高收入水平下可能变成劣等品。
You should also mention that the analysis assumes constant prices and stable preferences. In reality, advertising, social influences and expectations can interact with income changes to produce different outcomes.
你还应指出，该分析假设价格不变且偏好稳定。现实中，广告、社会影响和预期可能与收入变化相互作用，从而产生不同结果。
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