There's been a lot of discussion about how progressive taxes are in the U.S. and how the progressivity of the tax code has changed over time. In particular, the Wall Street Journal editorial page, Eddie Lazear, and others have claimed that the Bush tax cuts have made U.S. taxes more progressive mainly because of changes at the top of the income distribution. Thomas Piketty and Emmanuel Saez, the leading experts on long-term trends in inequality, look at this question and conclude the opposite, that progressivity at the top of the income distribution has declined dramatically since 1960:
How Progressive is the U.S. Federal Tax System? A Historical and International Perspective, by Thomas Piketty and Emmanuel Saez, NBER WP 12404 Issued, August 2006: Abstract This paper provides estimates of federal tax rates by income groups in the United States since 1960, with special emphasis on very top income groups. We include individual and corporate income taxes, payroll taxes, and estate and gift taxes. The progressivity of the U.S. federal tax system at the top of the income distribution has declined dramatically since the 1960s. This dramatic drop in progressivity is due primarily to a drop in corporate taxes and in estate and gift taxes combined with a sharp change in the composition of top incomes away from capital income and toward labor income. The sharp drop in statutory top marginal individual income tax rates has contributed only moderately to the decline in tax progressivity. ...
In pictures, here's total federal taxes by income group in 1970 and 2004. Starting in 1960 instead does not change the picture much:
What caused this change between 1970 and 2004? Following Piketty and Saez and focusing on the top income groups (the following graphs collapse the income groups below the 90th percentile into one group), was it income taxes?:
Some, but not so much. How about payroll taxes?:
Nope, that's not it, those go the wrong way, though the overall percentage changes aren't that large. Corporate taxes perhaps?
Bingo. And how have estate taxes changed?:
That reinforces the corporate tax change. Thus, as noted in the abstract, the change in progressivity is due primarily to changes in corporate taxes with an assist from estate taxes, as well as a compositional effect not shown in these graphs.
From the conclusion to the paper, contrary to what others have said:
These large reductions in tax progressivity since the 1960s took place primarily during two periods: the Reagan presidency in the 1980s and the Bush administration in the early 2000s. The only significant increase in tax progressivity since 1960 took place in the early 1990s during the first Clinton administration.