More women than men around the world have become billionaires over the past 20 years, but it is Asia's rising class of female entrepreneurs - many from China - that is really driving the trend.

There are now 145 female billionaires compared with just 22 in 1995.

Asia has 25 of those female billionaires, up from only three in 2005, with most having made their fortunes in real estate, industrials and health, according to the study from Swiss bank UBS and PricewaterhouseCoopers (PwC).

It also found that Singapore's female billionaires tended to inherit their wealth, said Mr Raymond Ang, UBS Wealth Management regional market manager for Singapore.

For all the recent success of women, the billionaires' club has remained, for the most part, a men's institution.

Only 11 per cent of the 1,347 billionaires polled are female, although they have an average wealth of US$4.3 billion (S$6 billion), US$300 million more than the average male, said the report.

But wealth, as they say, is transient. Only 44 per cent, or 126 of the billionaires in 1995, remain so today, the report found. That class now represents less than 10 per cent of today's billionaires.

"Great wealth is volatile, and highly volatile," said PwC partner Michael Spellacy at a briefing yesterday.

Business failures was the top reason pushing billionaires off the list, followed by death and taxes, and finally dilution, which is when wealth is fragmented when it passes from one generation to the next.

Even so, the ultra rich are still leagues ahead of the pack: While the world's gross domestic output tripled over the last 20 years, the wealth of billionaires polled by the study jumped eightfold.