收入的市场分配 — AP 微观经济学
AP 微观经济学 · 第5单元：要素市场 · 14 min read
区分个人收入分配与功能收入分配
构建并解释收入分配的洛伦兹曲线
计算并解释基尼系数作为不平等程度的衡量指标
运用边际生产率理论解释市场收入分配
识别并解释收入不平等的常见来源
1. 收入分配的核心框架 ★☆☆☆☆ ⏱ 2 min 收入的市场分配描述了市场体系中要素所有者获得的总收入如何在单个家庭之间或各生产要素之间分配。它研究的是市场经济中谁获得多少收入的问题，而非总产出的生产规模。
**个人收入分配**：收入如何按收入水平排序后分配到各个家庭
**功能收入分配**：收入如何分配到四种生产要素（劳动、资本、土地、企业家才能）
根据AP CED大纲，本主题占考试总分的2-4%，通常会出1-2道选择题，偶尔会结合要素市场概念出现在多部分自由作答题中。
2. 衡量不平等：洛伦兹曲线 ★★☆☆☆ ⏱ 3 min 洛伦兹曲线是衡量个人收入不平等的主要图形工具。它在纵轴绘制总收入的累计份额，横轴绘制（从最穷到最富排序的）人口累计份额。
完全平等线
洛伦兹曲线图中从(0,0)到(1,1)的45度线。这条线上的任意一点都意味着x%的人口拥有x%的总收入，对应所有家庭收入完全相等的情况。
如果收入不是完全平等分配的，洛伦兹曲线会向45度线下方弯曲。曲线离45度线越远，收入分配越不平等。
一个小村庄有5户家庭，年收入分别为：\$15,000, \$25,000, \$35,000, \$45,000, \$180,000。计算绘制洛伦兹曲线所需的累计人口和累计收入份额。
计算年总收入:
15000 + 25000 + 35000 + 45000 + 180000 = 300000每个家庭占总人口的20% (1/5)，因此累计人口份额为 20%, 40%, 60%, 80%, 100%。
计算各家庭的收入份额:
\frac{15000}{300000} = 5\%, \quad \frac{25000}{300000} \approx 8.33\%, \quad \frac{35000}{300000} \approx 11.67\%, \quad \frac{45000}{300000} = 15\%, \quad \frac{180000}{300000} = 60\%计算累计收入份额: 20% of population = 5% cumulative income, 40% = 13.33%, 60% = 25%, 80% = 40%, 100% = 100%.
3. 基尼系数 ★★☆☆☆ ⏱ 3 min 基尼系数是直接从洛伦兹曲线推导出来的收入不平等数值汇总指标。它将图形信息浓缩为0到1之间的一个数字，可用于比较不同时期或不同地区的不平等程度。
基尼系数的基本公式为:
G = \frac{A}{A + B}
Where $A$ = the area between the 45-degree line of perfect equality and the actual Lorenz curve, and $B$ = the area under the Lorenz curve. Since the total area under the 45-degree line is $A+B = 0.5$, the formula simplifies to:
G = 2A
A Gini coefficient of 0 means perfect equality, while a Gini of 1 means perfect inequality (one household holds all income). Real-world Gini coefficients almost always fall between 0.2 (very equal) and 0.5 (very unequal).
The area between the line of perfect equality and the Lorenz curve for Country Y is 0.18. What is the Gini coefficient for Country Y, and does this country have more or less inequality than a country with a Gini of 0.4?
Use the simplified Gini formula $G = 2A$, which applies when $A$ is measured as a share of the total area under the 45-degree line.
Substitute $A=0.18$:
G = 2(0.18) = 0.36Higher Gini values mean more inequality. 0.36 is less than 0.4, so Country Y has less inequality than the comparison country.
Gini measures relative inequality, not absolute income. Never assume a higher Gini means the poorest households have lower absolute income — a rich unequal country can still have poorer households with higher absolute income than a poor equal country.
4. 收入分配的边际生产率理论 ★★★☆☆ ⏱ 3 min 边际生产率理论是解释市场经济为何会产生不平等收入分配的核心经济学理论。该理论指出，在竞争市场中，每种生产要素获得的报酬等于其边际收益产品（MRP），即雇佣的最后一单位该要素带来的额外收入。
A competitive coffee shop sells lattes for \$4 each. The marginal product of a senior barista is 18 lattes per day, and the marginal product of a new trainee barista is 5 lattes per day. According to marginal productivity theory, what is the difference in daily wages between the two workers?
For a firm in a competitive output market, $MRP = P \times MP$, and the equilibrium wage equals MRP.
Calculate the senior barista's equilibrium wage:
W_S = 4 \times 18 = \$72 \text{ per day}Calculate the trainee barista's equilibrium wage:
W_T = 4 \times 5 = \$20 \text{ per day}The wage difference is $\$72 - \$20 = \$52$ per day, explained entirely by the higher marginal productivity of the senior barista.
AP often asks if marginal productivity theory predicts equal income in competitive markets: the answer is no. It predicts income is distributed according to marginal contribution, not equal shares.
5. Common Sources of Income Inequality ★★☆☆☆ ⏱ 3 min Beyond differences in marginal productivity, AP Microeconomics frequently tests these key sources of income and wage inequality:
Differences in human capital: accumulated education, training, and experience increase productivity and wages
Ability differences: natural talent for high-productivity tasks leads to higher income
Compensating differentials: higher wages for dangerous or unpleasant jobs
Market power: unions raise member wages, monopoly profits increase business owner income
Discrimination: exclusion from high-paying jobs reduces income for affected groups
Inheritance: inherited wealth generates capital income independent of personal productivity
Which of the following changes will increase income inequality: (a) a decrease in the top marginal income tax rate, or (b) an increase in need-based college scholarships for low-income students?
A decrease in the top marginal income tax rate increases the after-tax income of high-income households, increasing the gap between top and bottom incomes. This increases inequality.
An increase in need-based college scholarships for low-income students allows more low-income people to accumulate human capital, increasing their future wages and reducing the wage gap. This decreases inequality.
Only the decrease in the top marginal income tax rate increases income inequality.
On FRQ, always tie your explanation of an inequality source back to either differences in marginal revenue product of owned factors, or market imperfections that cause factor payments to deviate from MRP to earn full points.
Common Pitfalls
Why: Students mix up direction because they forget the population is sorted from poorest to richest, not the reverse
Why: Students forget that $A+B = 0.5$, so dividing by 0.5 equals multiplying by 2
Why: Students confuse equal opportunity with equal outcome
Why: The terms sound similar but describe different concepts
Why: Students confuse relative distribution with total income level
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