Cumulative share of income is the proportion of total income earned by the bottom x% or top x% of a population. In Intermediate Microeconomic Theory, it is used to measure and compare income inequality with tools like the Lorenz curve.
Cumulative share of income is the share of all income earned by a group of people ranked from poorest to richest in Intermediate Microeconomic Theory. You usually see it written as a percentage, like the cumulative income held by the bottom 20%, bottom 50%, or top 10% of households.
The idea only makes sense once people are sorted by income. After ranking them, you add up each group’s income share as you move through the population. That running total is the cumulative share. So if the bottom half of households earns 20% of total income, the cumulative share of income at 50% of the population is 20%.
This is a measurement tool, not a policy by itself. Economists use it to describe how income is spread across the population and to compare one distribution with another. A distribution where the bottom half gets a large share of income is more equal than one where the bottom half gets very little and the top group gets most of it.
The most common place you encounter it is on a Lorenz curve. The x-axis shows the cumulative share of the population, and the y-axis shows the cumulative share of income. If income were perfectly equal, the curve would follow the 45-degree line. When the curve bows farther below that line, income is more concentrated at the top.
A small example makes the logic easier to see. If the bottom 40% of households receives 15% of total income, then the next 40% brings the cumulative share to 60%, and the final 20% brings it to 100%. That pattern tells you a lot more about inequality than a single average income number would.
Do not confuse cumulative share with individual income. It is always about a group’s combined share of the total, which is why it works well for comparing inequality across countries, time periods, or policy changes.
Cumulative share of income is one of the basic tools economists use to turn inequality into something you can measure, graph, and compare. Average income can look fine even when a small group gets most of the gains, but cumulative shares show where the income is actually going.
In intermediate micro, that matters because market outcomes do not automatically produce equal earnings. Wages, returns to capital, education differences, and labor market structure can all shift the income distribution. When you can read cumulative shares, you can connect those outcomes back to factor markets and policy debates instead of treating inequality as a vague idea.
It also gives you the language for discussing redistribution. If a tax, transfer, or wage policy changes the income share of the bottom 50% or the top 10%, you can describe that change precisely. That is much more useful than saying income “went up” or “went down.”
The term also shows up in graphs and comparisons. If one country’s Lorenz curve sits farther from the line of equality, its cumulative income shares are more uneven, and the pattern is more unequal. That lets you compare distributions even when countries have different population sizes or different total income levels.
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Visual cheatsheet
view galleryLorenz curve
The Lorenz curve is the graph that displays cumulative share of income against cumulative share of the population. The further the curve bends away from the line of equality, the more unequal the distribution. If you know one point on the curve, like the bottom 50% earning 20% of income, you are reading a cumulative share directly.
Gini coefficient
The Gini coefficient turns the Lorenz curve into a single number that summarizes inequality. Cumulative income shares give you the raw distribution, while the Gini compresses that information into a comparison-friendly index. If you understand cumulative shares, the Gini coefficient makes more sense because it measures the same underlying pattern in a different form.
Income inequality
Income inequality is the bigger concept, and cumulative share of income is one way economists measure it. Inequality can come from wages, capital income, taxes, or transfers, but cumulative shares show the final distribution across households or individuals. That makes the term useful when you need to describe how unequal a market outcome is.
Economic Mobility
Economic mobility is about how people move between income groups over time, while cumulative share of income is a snapshot of income at one point in time. A society can have unequal cumulative shares today but still allow high mobility if people move up and down the distribution over their lifetimes. The two concepts answer different questions.
A quiz or problem set may give you a table or Lorenz curve and ask you to identify the cumulative share of income for a percentile group. You might need to read off what share the bottom 20%, bottom 50%, or top 10% earns, then compare two distributions and say which one is more unequal. If the course uses graphs, you may also explain how a movement of the Lorenz curve changes the cumulative income share at each percentile. In a short response or discussion prompt, use the term to support a claim about inequality, not just to restate the graph. For example, say that a larger income share held by the top groups means the curve bows farther from equality.
Cumulative share of income tells you what fraction of total income a ranked group of people receives.
The term is usually read alongside the Lorenz curve, where the x-axis is population share and the y-axis is income share.
Smaller cumulative shares for the bottom groups and larger shares for the top groups mean a more unequal distribution.
This is a measurement tool, so it helps you describe inequality before you talk about policy or causes.
If you can read cumulative shares, you can compare income distributions across groups, countries, or time periods.
It is the share of total income earned by a cumulative group of the population, such as the bottom 20% or bottom 50%. Economists use it to describe how income is distributed and to measure inequality. The term is easiest to read on a Lorenz curve, where it appears on the vertical axis.
Start by ranking people from lowest income to highest income, then find the share of total income earned by each cumulative population group. The bottom 40% might earn 15%, for example, which means the cumulative share of income at 40% is 15%. Points farther below the line of equality show more uneven distribution.
No. Income inequality is the broader pattern, while cumulative share of income is one way to measure that pattern. A cumulative share tells you how income is divided across groups, and that information is then used to judge how unequal the distribution is.
If the bottom 50% of households earns 20% of total income, that is the cumulative share for the bottom half. If the next 30% of households brings the total to 45%, then the cumulative share at 80% of the population is 45%. The remaining 20% would account for the last 55%.