A new book-length study on the tax burden of the ultrarich begins with a startling finding: In 2018, for the first time in history, America's richest billionaires paid a lower effective tax rate than the working class.

"The Triumph of Injustice," by economists Emmanuel Saez and Gabriel Zucman of the University of California at Berkeley, presents a first-of-its kind analysis of Americans' effective tax rates since the 1960s. It finds that in 2018 the average effective tax rate paid by the richest 400 families in the country was 23 per cent, a full percentage point lower than the 24.2 per cent rate paid by the bottom half of American households.

In 1980, by contrast, the 400 richest had an effective tax rate of 47 per cent. In 1960, their tax rate was as high as 56 per cent. The effective tax rate paid by the bottom 50 per cent, by contrast, has changed little over time.

The tipping point came in 2017 with the passage of Donald Trump's tax cuts. They were a windfall for the wealthy. Credit:AP

The analysis differs from many other published estimates of tax burdens by encompassing the totality of taxes Americans pay: not just federal income taxes but also corporate taxes, as well as taxes paid at the state and local levels. It also includes the burden of about $US250 billion of what Saez and Zucman call "indirect taxes," such as licenses for motor vehicles and businesses.