The Gini coefficient is a number that summarizes how unequal a distribution is, most often income or wealth. It helps economists, governments, and citizens compare inequality across countries, regions, or time periods. A Gini coefficient of 0 means perfect equality, while a value closer to 1 means greater inequality.
In personal finance and public policy, it helps show whether economic growth is being shared widely or concentrated among a smaller group.
The Gini coefficient is based on the Lorenz curve, a graph that compares the cumulative share of the population with the cumulative share of income. If everyone had the same income, the Lorenz curve would lie on the diagonal line of perfect equality. The more the curve bows below that diagonal, the more unequal the income distribution is.
The Gini coefficient equals the area between the diagonal and the Lorenz curve divided by the total area under the diagonal.
Key Facts
- Gini coefficient ranges from 0 to 1, where 0 means perfect equality and 1 means maximum inequality.
- The Lorenz curve plots cumulative share of population on the x-axis and cumulative share of income on the y-axis.
- Perfect equality line: cumulative share of income = cumulative share of population.
- Gini coefficient = Area between equality line and Lorenz curve / Total area under equality line.
- If using areas A and B on the Lorenz diagram, Gini = A / (A + B).
- A higher Gini coefficient means income or wealth is more concentrated among fewer people.
Vocabulary
- Gini Coefficient
- A numerical measure of inequality that ranges from 0 for perfect equality to 1 for maximum inequality.
- Lorenz Curve
- A graph that shows the cumulative share of income received by the cumulative share of the population.
- Line of Perfect Equality
- The diagonal line on a Lorenz curve graph where each share of the population receives the same share of income.
- Income Distribution
- The way total income is divided among individuals or households in a group.
- Cumulative Share
- A running total expressed as a percentage of the whole, such as the bottom 40 percent of people earning 20 percent of income.
Common Mistakes to Avoid
- Confusing the Gini coefficient with average income. The Gini coefficient measures inequality, not how rich or poor a society is overall.
- Thinking a lower Gini always means everyone is wealthy. A country can have low inequality but still have low average income.
- Reading one point on the Lorenz curve as one person's income. Each point represents cumulative groups, such as the bottom 60 percent of the population and their total share of income.
- Assuming the Gini coefficient explains the causes of inequality. It measures the size of inequality but does not by itself show whether it comes from wages, wealth, taxes, education, or other factors.
Practice Questions
- 1 A country has area A = 0.18 between the equality line and the Lorenz curve, and area B = 0.32 under the Lorenz curve. Calculate the Gini coefficient using Gini = A / (A + B).
- 2 In a simple economy, the bottom 20 percent of people earn 5 percent of total income, the bottom 40 percent earn 15 percent, the bottom 60 percent earn 35 percent, the bottom 80 percent earn 65 percent, and all people earn 100 percent. Plot these points on a Lorenz curve and describe whether inequality appears low, moderate, or high.
- 3 Two countries have the same average income, but Country A has a Gini coefficient of 0.25 and Country B has a Gini coefficient of 0.55. Explain which country has more equal income distribution and what that means for how income is shared.