Mr Gates' wealth grew by $3bn (£1.9bn) to $59bn, despite his philanthropic（仁慈的） efforts, as Microsoft's stock climbed.

 

Mr Buffett, who also continued giving his wealth to good causes, dropped $11bn to $39bn as the value of his Berkshire Hathaway firm slid.

 

Fellow investor George Soros also entered the top 10, with $22bn.

 

 

Both investors have been notable in giving vocal backing to left-leaning policies in the US.

 

The Hungarian-born Mr Soros is a major donor to the Democrats, and outside the US he sponsors "Open Society Foundations" that seek to promote human rights and liberal democracy.

 

Mr Buffett meanwhile gave his backing over the summer to higher taxes on high-earning Americans, in particular an increase in the tax rate on capital gains that he claimed allowed him to pay a lower tax rate in the US than his secretary.

 

President Barack Obama happily took up the idea in his latest job creation proposal, naming the proposal the "Buffett Rule", much to the consternation（惊慌失措） of anti-tax Republicans.

 

However, the right-wing also has its own supporters in the top 10.

 

Brothers Charles and David Koch - joint owners of a vast industrial empire - are each worth $25bn, according to Forbes magazine.

 

And they have thrown their weight behind the populist Republican Tea Party movement.

 

However, another wealthy Republican - New York mayor Michael Bloomberg - has dropped out of the top 10.

 

Meanwhile, the Waltons - owners of giant retail chain Wal-Mart - remain the wealthiest family in the US.

 

Four family members are in the top 20, with a collective wealth of $87bn.