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I compute the share of U.S. household giving accounted for by the American tax units donating the largest amounts over the 1960–2012 period from repeated cross-sectional samples of federal income tax returns. The share of donations accounted for by a minority of top donors rose sharply over this period. Donor concentration has risen both because the largest gifts have grown larger and because more households give little or nothing in any given year.
Nicolas Duquette (Mon,) studied this question.