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Almost every argument about inequality blurs two distinct ideas. Income is a flow — what a household earns over a year, from wages, a business, or returns on assets. Wealth is a stock — what it owns at a moment in time, minus what it owes. They move together but are not the same, and the wealth gap is far wider than the income gap. A nurse and a retired founder can report similar income in a given year while owning wildly different amounts. Most public data — and most of the century-long story — starts with income, because tax records capture it. Wealth is harder to measure and even more concentrated.
The century-long record comes from federal income-tax data, first reconstructed by Thomas Piketty and Emmanuel Saez and now maintained in the World Inequality Database. Because tax returns go back to 1913, economists can trace top income shares across the whole modern era — the single deepest baseline in this Dot. The Census and CBO add survey- and tax-based measures of the full distribution, including after taxes and transfers.
Wealth is harder. The Federal Reserve’s Distributional Financial Accounts give a quarterly read since 1989, tying detailed household surveys to the national balance sheet. The triennial Survey of Consumer Finances adds depth on assets, debt, and race. For the very long run, the Saez-Zucman capitalized-income series estimates top wealth shares back to 1913 — the source for the deep wealth U on the next tab.