📚 Income Changes and Consumer Choice | 收入变化对消费者选择的影响
In consumer theory, an individual's choice of goods and services is determined by three factors: preferences, the prices of goods, and the consumer's income. When income rises or falls, the budget set changes; given unchanged preferences and prices, the consumer re-optimises, buying more of some goods, less of others, and possibly abandoning some entirely. This article examines the mechanisms, diagrams and elasticity measures used in CIE A-Level Economics to explain how income changes shape consumer choice.
在消费者理论中，个人对商品和服务的选择由三个因素决定：偏好、商品价格和消费者收入。当收入上升或下降时，预算集合发生改变；在偏好与价格不变的前提下，消费者会重新优化选择，增加某些商品的购买、减少另一些商品，甚至完全放弃某些商品。本文考察 CIE A-Level 经济学中用于解释收入变化如何影响消费者选择的机制、图形与弹性指标。
1. The Budget Constraint and Income Changes | 预算约束与收入变化
A consumer's budget constraint shows all combinations of two goods, X and Y, that can be purchased with a given money income M. If the prices of X and Y are Px and Py, the budget line is written as:
消费者的预算约束表示在既定货币收入 M 下，能够购买到的两种商品 X 和 Y 的所有组合。若 X 和 Y 的价格分别为 Px 和 Py，预算线可写为：
Px·X + Py·Y = M
The vertical intercept is M/Py and the horizontal intercept is M/Px; the slope of the line is −Px/Py. When income M increases while prices remain constant, both intercepts move outward and the budget line shifts to the right in a parallel fashion. Conversely, a fall in income shifts the budget line inward, parallel to the original line, because relative prices — and therefore the slope — are unchanged.
纵轴截距为 M/Py，横轴截距为 M/Px，预算线的斜率为 −Px/Py。当收入 M 增加而价格不变时，两个截距均向外移动，预算线平行向右移动；反之，收入下降使预算线向内平行移动，因为相对价格（进而斜率）并未改变。
For example, if M = $100, Px = $5 and Py = $10, the intercepts are 20 units of X and 10 units of Y. If income rises to $150, the intercepts become 30 and 15: the consumer's opportunity set expands, but the trade-off between X and Y is unchanged.
例如，若 M = 100 美元，Px = 5 美元，Py = 10 美元，则横轴截距为 20 单位 X，纵轴截距为 10 单位 Y
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