The wealth of the Forbes 400 list more than doubled from $1.27 trillion to almost $3 trillion over the past decade, and a study by University of Cal Berkeley economist Gabriel Zucman shows they actually paid less in taxes.

The big bump in assets is the result of sliding tax rates and stock market growth along with the longest economic recovery on record, and it’s sure to add more fuel to the fire in the campaigns of Sens. Elizabeth Warren and Bernie Sanders.

The Forbes 400 richest U.S. citizens makes up about 0.00025% of the population, yet it controls 3% of the country’s total wealth, more than doubling their assets while their effective tax rate dropped from 27% in 2009 to 23% in 2019. It was the first time they were actually taxed less than the U.S. working class, according to Business Insider.

Meanwhile, the tax rate for the bottom 50% of Americans paid stayed relatively the same.

Tax avoidance is quite common among the country’s most wealthy, and Zucman along with fellow Cal Berkeley economist Emmanuel Saez claim the tax burdens have shrunk due to the decisions of U.S. lawmakers on both sides of the aisle, both directly and indirectly through legislative gridlock on Capitol Hill.

The biggest savings have come through tax cuts on capital gains and estate taxes through the years, and the top income tax rates have been cut a total of six times since 1980.

Then-President Barack Obama delayed ending the tax cuts put in place by George W. Bush, which Congress let expire in 2013. And of course the mainstream media and some experts say the Tax Cuts and Jobs Act, passed by President Donald Trump and the Republican-controlled House and Senate in 2017, largely benefited the rich and big corporations.

The TCJA slashed the corporate tax rate from 35% to 21% while also reducing the tax rates for top earners. A recent report also showed that the richest 1% in the U.S. will soon hold more money than the entire middle class.

These moves have led to increasing wealth inequality in the U.S., which has never been greater, according to the U.S. Census Bureau.