More than 900 billionaires have been created worldwide over the past two decades, but a new report says the billionaire boom may be about to end. The global wealth wave that has minted most of the world's billionaires over the past 20 years could be ending in the next decade or two, said the UBS-PwC Billionaire Report released Tuesday. Weaker economic growth—especially in emerging markets—and government responses to inequality with higher taxes and regulations could slow the creation of new billionaires, it said. Another factor is that billionaires are an aging population, with most over 60 years old. (Tweet This)

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"The rich have been getting richer over the last 20 years," said John Mathews, head of Private Wealth Management at UBS. "But we don't think these things happen in a straight line. It's more of an S-curve type of phenomena."

That S-curve for billionaires started in the Gilded Age between 1870 and 1910, where new industries like steel, railroads and industrialization created vast new fortunes and an explosion of extreme wealth. But after the Great Depression and World War II, billionaire growth took a sharp downward turn, with few billionaires and multimillionaires created between 1930 and 1980.

During that 50-year billionaire drought, caused by the depression, higher taxes and the emergence of giant companies, "the 'entrepreneurial age' yielded to a 'managerial economy' phase," the report said. "In this time, a few big, established companies dominated economies leaving little room for entrepreneurialism." Read More$100M new benchmark for trophy homes

Now, the report said, we may be headed for another downward slope. The latest boom has lasted more than 20 years, with more than 900 of the world's 1,300 billionaires added since 1995. But the report said a number of factors could stop the billionaire juggernaut. Emerging market economies, especially China and the other BRIC countries, are slowing. Government responses to inequality could lead to higher taxes on the wealthy (estate taxes and income taxes) as well as more regulation against dominant companies. The era of low interest rates and rising asset prices around the world could change if central banks decide to tighten, which could lead to asset-price deflation.