Adam Looney & Michael Greenstone (both of The Hamilton Project), Just How Progressive is the U.S. Tax Code?:

[W]e examine the progressivity of the U.S. tax code and highlight two facts: the current U.S. tax system is less progressive than the tax systems of other industrialized countries, and considerably less progressive today than it was just a few decades ago.

The figure below shows how much influence taxes and transfers have in reducing inequality (measured using a common metric called the “Gini coefficient”) in various countries around the world. As indicated in the chart, the U.S. tax and transfer system does less to counteract pre-tax income inequality than the tax systems of most of our peer countries, meaning that our system is actually less progressive.

In addition to being less progressive relative to other countries, the U.S. tax system has also become less progressive over time. Over the last fifty years, tax rates for the wealthiest Americans have declined by 40 percent, while tax rates for average Americans have remained roughly constant.  This is illustrated in the figure below.