Forty of America’s billionaires and their families pledged Wednesday to give more than half of their fortune to charity in a drive organized by Bill Gates and Warren Buffett.

The group includes CNN founder Ted Turner, New York Mayor Michael Bloomberg, Oracle co-founder Larry Ellison and Hollywood director George Lucas, as well as Microsoft mogul Gates and investment guru Buffett.

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The idea, which was announced just six weeks ago as “The Giving Pledge,” is to convince billionaires across the country to give up most of their money — 50 percent or more — to charity.

“The pledge is a moral commitment to give, not a legal contract,” a statement released Wednesday said by givingpledge.org, a drive spearheaded by Buffett and Gates.

“We’ve really just started, but already we’ve had a terrific response,” Buffett, the chief executive of the investment firm Berkshire Hathaway, said.

“At its core, the Giving Pledge is about asking wealthy families to have important conversations about their wealth and how it will be used.”