Measuring Income Inequality
Income inequality describes how unevenly income is spread across a population. Measuring it matters because it helps economists and policymakers track whether economic gains are being shared broadly or concentrating at the top. This section covers the main tools for measuring inequality and the forces driving it.
Distribution of Income and Key Factors
Income distribution refers to how total income in an economy is divided among individuals or households. Several factors shape where someone falls in that distribution:
- Education and skills: Higher educational attainment tends to lead to higher-paying jobs. A worker with a college degree earns significantly more on average than one with only a high school diploma.
- Technology and globalization: These forces have increased demand for high-skilled workers while reducing demand for low-skilled jobs, widening the gap between the two groups.
- Family structure: The rise in single-parent households and the trend of assortative mating (people with similar education and income levels marrying each other) both amplify inequality across households.
- Institutional factors: The decline in unionization and shifts in tax policy that favor higher-income earners have contributed to widening income gaps.
Data Visualizations
Economists rely on three main tools to measure and visualize income inequality:
Quintiles divide the population into five equal groups (each 20%) ranked by income. The lowest quintile is the poorest 20%, and the highest quintile is the richest 20%. You assess inequality by comparing each group's share of total income. For example, if the top quintile earns 50% of all income while the bottom quintile earns only 3%, that signals significant inequality.
The Lorenz curve is a graph that plots the cumulative share of income (y-axis) against the cumulative share of the population (x-axis). A 45-degree diagonal line represents perfect equality, where every group earns exactly its proportional share. The further the actual Lorenz curve bows away from that diagonal, the greater the inequality.
The Gini coefficient is a single number derived from the Lorenz curve. It equals the area between the Lorenz curve and the 45-degree line, divided by the total area under the 45-degree line. It ranges from 0 (perfect equality) to 1 (perfect inequality). The U.S. Gini coefficient is roughly 0.39, while more equal countries like Sweden sit closer to 0.27. This makes it easy to compare inequality across countries or track changes over time.
Factors Contributing to Rising Inequality
Household Composition and Wage Disparities
Changes in household composition have widened the income gap in two key ways:
- Single-parent households have become more common, and they typically have lower incomes than two-parent households because there's only one potential earner.
- Assortative mating means high earners increasingly marry other high earners, concentrating income at the top. Meanwhile, lower-income individuals also tend to partner together, reinforcing inequality at both ends.
Wage disparities have grown due to several reinforcing trends:
- Skill-biased technological change is the idea that new technologies (like automation and software) raise productivity and wages for high-skilled workers while replacing tasks done by low-skilled workers. This pushes wages apart.
- Globalization and trade allow firms to outsource low-skilled jobs to countries with lower labor costs. That increases competition for low-skilled workers domestically, putting downward pressure on their wages.
- Decline in unionization has weakened collective bargaining power, especially for low-skilled workers. With fewer unions negotiating on their behalf, these workers have less leverage to push for higher wages and better working conditions.
Together, these forces help explain why income inequality has been rising in many developed economies over the past several decades. Understanding the measurement tools and underlying causes is essential for evaluating policy proposals aimed at addressing the gap.