Image caption Despite his efforts to give his money away, Bill Gates increased his wealth and extended his lead

Microsoft founder Bill Gates has extended his lead over investor Warren Buffett at the top of the Forbes list of rich Americans.

Mr Gates' wealth grew by $3bn (£1.9bn) to $59bn, despite his philanthropic 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.

Money and politics

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 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.