The total wealth held by the world’s billionaires rose by 17 percent in 2016 to $6 trillion, and the uber-rich are concerned that growing inequality could lead to society turning against them.

There was a 10 percent rise in the number of billionaires around the world to 1,542, according to the UBS/PwC Billionaires 2017 report. And for the first time ever, the number of Asian billionaires has overtaken those in the United States.

The growth in 2016 means that the world’s richest people now control the highest concentration of wealth since the period known as the Gilded Age in the United States, where business families built vast empires in the late 19th century, the Guardian reported.

The Gilded Age ultimately ended in society pushing back against the wealthy. President Theodore Roosevelt came to power in 1901 and split up monopolies; and the early 20th century was known as the Progressive Era, a period of activism and reform partially aimed at bettering the lot of the common person and taking down huge companies.

According to Josef Stadler, the lead author of the report, modern billionaires are worried that a similar phenomenon could occur today.

“Wealth concentration is as high as in 1905; this is something billionaires are concerned about. The problem is the power of interest on interest—that makes big money bigger—and the question is to what extent is that sustainable, and at what point will society intervene and strike back?” Stadler, the global head of ultra-high net worth at UBS, told the Guardian.

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But while billionaires may be fearing a backlash from the middle and lower classes, the report points out that the super-rich do a lot of good for society. The world’s billionaires employ at least 27.7 million people around the world—approximately equivalent to the U.K.’s working population—and Stadler said that 98 percent of their wealth found its way back into society.

The arts and sports in particular have benefited recently from the generosity of billionaires, as the wealthy seek to invest their fortunes in things that the wider public can enjoy. Seventy-two of the world’s top 200 art collectors are billionaires, and the report said that “public museums are also receiving more funding than ever before.”

The rich have also ploughed their money into sports clubs: More than 140 of the world’s top sports clubs in the world are owned by just 109 billionaires, with 60 coming from the United States. Two-thirds of NBA and NFL teams—and half of the MLB teams—are owned by billionaires, while nine out of 20 clubs in the English Premier League have billionaire owners, like Malcolm Glazer at Manchester United and Sheikh Mansour bin Zayed al-Nahyan at Manchester City.

The report also estimated that with more than one-third of billionaire wealth belonging to people aged 70 or older, $2.4 trillion of wealth would be transferred over the next 20 years. While most of this will likely go to the heirs of billionaires, there will also be a “huge windfall” for charities, the report said.

There is debate as to whether global inequality is growing. Some have argued that income inequality and poverty have reduced dramatically in the past few decades, while others have claimed that the global economic system disproportionately benefits the rich. In a 2016 report, Oxfam claimed that the richest 1 percent of people in the world control more wealth than the rest of the world combined.

Earlier in October, the International Monetary Fund said that raising tax rates on the rich would help to reduce inequality and would not have a negative impact upon economic growth.