(Bloomberg) -- The rapid rate of wealth creation by billionaire entrepreneurs will probably level off over the next two decades as economic growth slows in emerging markets and governments act against income inequality, a study found.

"Just as economies grow in cycles, so the opportunities that led to this most recent exceptional period of wealth generation are likely to end," according to a report from UBS Group and PwC released Tuesday. "Tax, social equality initiatives, asset price deflation and geopolitical tensions" such as civil wars "will likely have significant consequences for great wealth creation," it said.

UBS, the world's largest manager of money for the rich, and PwC found that 917 self-made billionaires accumulated $3.6 trillion of wealth in the past 20 years. The authors compared the period straddling the end of the 20th century with the "gilded age," when a few businessmen built fortunes from cars, steel and electricity 100 years earlier.

Most growth in recent years occurred in the U.S., where fortunes were made in finance and technology, and Asia, where the consumer industry and real estate pushed the number of billionaires in the region past Europe. Since 1995, the portion of billionaires' total wealth held by entrepreneurs increased to two-thirds from less than half, according to the study, which focused on the 14 richest billionaire markets.

'PRESERVATION STRATEGY'

Banks including UBS and Credit Suisse Group have their sights on winning more clients from the growing ranks of the very rich. The fact that two-thirds of billionaires are nearing the age when they will have to transfer their fortunes to the next generation may act as a brake on wealth creation, according to the study.

What the majority of billionaires who are more than 60 years old choose to do with their wealth is crucial, "as fortunes will quickly dilute without a sensible preservation strategy and governance," the report said.

Apart from being coveted clients for the banks, many of the self-made billionaires covered by the report work in financial services. In the U.S., 30% of self-made billionaires come from the financial industry, compared with 27.3% from the field of technology. Tech billionaires are richer, however, with fortunes averaging $7.8 billion, compared with $4.5 billion for financiers, the study found.

By contrast, half of Europe's and 20% of Asia's self-made billionaires built their fortunes in the consumer industry in the last two decades.

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