First, the progressivity of the U.S. federal tax system at the top of the income
distribution has declined dramatically since the 1960s. For example, the top
0.01 percent of earners paid over 70 percent of their income in federal taxes in
1960, while they paid only about 35 percent of their income in 2005. Average
federal tax rates for the middle class have remained roughly constant over time.
This dramatic drop in progressivity at the upper end of the income distribution is
due primarily to a drop in corporate taxes and to a lesser extent estate and gift
taxes, both of which fall on capital income, combined with a sharp change in the
composition of top incomes away from capital income and toward labor income.
The reduction in top marginal individual income tax rates has contributed only
marginally to the decline of progressivity of the federal tax system, because with
various deductions and exemptions, along with favored treatment for capital gains,
the average tax rate paid by those with very high income levels has changed much
less over time than the top marginal rates. 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
https://eml.berkeley.edu/~saez/piketty-saezJEP07taxprog.pdf