Gabriel Zucman and Emmanuel Saez in the Boston Review:
Wealth is power. An extreme concentration of wealth means an extreme concentration of power: the power to influence government policy, the power to stifle competition, the power to shape ideology. Together, these amount to the power to tilt the distribution of income to one’s advantage. This is the core reason why the extreme wealth of some can reduce what remains for the rest—why part of the income of today’s superrich can be earned at the expense of the rest of society. That’s what earned John Astor, Andrew Carnegie, John Rockefeller, and other Gilded Age industrialists their epithet of “Robber Barons.”
In much of the twentieth century, the U.S. tax system protected against such extreme disparities. But far from curbing this trend, the tax system in the last four decades has instead reinforced it. The three traditional progressive taxes—the individual income tax, the corporate income tax, and the estate tax—have all weakened. The top marginal federal income tax rate has fallen dramatically, from more than 70 percent every year between 1936 and 1980—in fact, often higher, peaking at 94 percent during the final years of World War II—to 37 percent in 2018.