Income inequality describes how unevenly income is distributed across people or households in an economy. This cheat sheet helps students compare inequality using income shares, Lorenz curves, and the Gini coefficient. These tools are important because inequality affects living standards, opportunity, social mobility, and public policy debates.
Students need a clear reference to connect graphs, formulas, and real-world interpretations.
The core idea is to compare an actual income distribution with perfect equality, where every group receives the same share of total income. A Lorenz curve shows cumulative population share on the x-axis and cumulative income share on the y-axis. The Gini coefficient measures the area between the line of equality and the Lorenz curve as a number from 0 to 1.
Higher Gini values mean greater inequality, while taxes, transfers, education, and labor market policies can change the distribution of income.
Key Facts
- Perfect income equality means each percentage of the population receives the same percentage of total income, such as 20% of people receiving 20% of income.
- A Lorenz curve plots cumulative share of population on the x-axis and cumulative share of income on the y-axis.
- The line of equality is the 45-degree line where cumulative population share equals cumulative income share.
- The Gini coefficient is calculated as Gini = A / (A + B), where A is the area between the equality line and the Lorenz curve and B is the area under the Lorenz curve.
- A Gini coefficient of 0 means perfect equality, and a Gini coefficient of 1 means one person or household receives all income.
- If the Lorenz curve bows farther away from the line of equality, the Gini coefficient increases.
- Income share for a group is calculated as income share = group income / total income x 100%.
- Redistribution through progressive taxes and transfer payments can reduce after-tax income inequality.
Vocabulary
- Income Inequality
- Income inequality is the uneven distribution of income among individuals, households, or groups in an economy.
- Lorenz Curve
- A Lorenz curve is a graph that shows the cumulative share of income received by the cumulative share of the population.
- Gini Coefficient
- The Gini coefficient is a numerical measure of income inequality ranging from 0 for perfect equality to 1 for maximum inequality.
- Line of Equality
- The line of equality is a 45-degree line showing a situation where each share of the population receives the same share of income.
- Quintile
- A quintile is one of five equal groups, each containing 20% of the population, often used to compare income shares.
- Redistribution
- Redistribution is the use of taxes, transfers, or public programs to change the distribution of income after market incomes are earned.
Common Mistakes to Avoid
- Confusing income with wealth is wrong because income is money earned over a period, while wealth is the total value of assets owned at a point in time.
- Reading the Lorenz curve as individual income is wrong because each point shows cumulative population share and cumulative income share, not one person's earnings.
- Thinking a higher Gini coefficient means more equality is wrong because Gini values rise as the Lorenz curve moves farther from the line of equality.
- Ignoring taxes and transfers is wrong because market income inequality and after-tax income inequality can be very different.
- Comparing Gini coefficients without context is misleading because countries may differ in data quality, household size, cost of living, and public services.
Practice Questions
- 1 In a country, the bottom 20% of households receive 6% of total income. What income share does this quintile receive, and how does it compare with perfect equality?
- 2 A Lorenz curve has area A = 0.18 between the equality line and the curve, and area B = 0.32 under the curve. Calculate the Gini coefficient using Gini = A / (A + B).
- 3 The richest 20% of households receive 50% of total income, while the poorest 20% receive 4%. By how many percentage points does the richest quintile's share exceed the poorest quintile's share?
- 4 Explain why two countries with the same Gini coefficient might still have different levels of poverty or different standards of living.
Understanding Income Inequality & Gini Coefficient
A country can have the same average income while giving people very different daily lives. Imagine total income divided by the number of residents. That average may look comfortable, yet it can hide low pay for many families and very high earnings for a small group.
Median income is often useful beside the average. It identifies the income at the middle of the distribution.
Half of people earn less and half earn more. When very high incomes pull the average upward, the median gives a clearer picture of what a typical household receives.
Students should notice which kind of income a statistic uses. Market income includes earnings from work, business income, investments, and sometimes pensions before government action. Disposable income is what remains after direct taxes, plus cash benefits such as unemployment support or child payments.
Inequality is often lower for disposable income because tax systems and transfers shift resources. Noncash public services matter too.
Public schools, health care, housing support, and transport subsidies can improve living conditions even when they do not appear as cash income. Different definitions can produce different Gini values for the same country.
Household size changes the meaning of income data. A household earning fifty thousand with one person has more resources per person than a household earning fifty thousand with four people. Researchers often adjust income to reflect the number of adults and children sharing costs.
This is called equivalised income. It does not assume every person needs exactly the same amount of money, since people can share rent, heating, and food.
When comparing groups, check whether data refer to individuals, workers, tax units, or households. A comparison is only fair when the measurement method is similar.
The Gini coefficient is valuable, but it cannot tell the whole story. Two places can have similar Gini values while one has severe poverty and the other has high incomes across the distribution. The number does not show whether inequality comes mainly from a very rich top group, a struggling bottom group, or differences throughout the middle.
Income is not wealth either. Wealth includes savings, homes, land, shares, and debt. It is usually more unevenly distributed than yearly income.
When reading a graph or headline, look for the year, the population measured, whether figures are before or after taxes, and whether the source reports income or wealth. These details prevent misleading conclusions.