Forty of the nation's wealthiest pledge fortunes to charity

The United States has about 400 billionaires, about 40 percent of the world's total, according to Forbes.

"We’ve really just started, but already we’ve had a terrific response," Buffet said Wednesday in a release. "At its core, the Giving Pledge is about asking wealthy families to have important conversations about their wealth and how it will be used. We’re delighted that so many people are doing just that — and that so many have decided to not only take this pledge but also to commit to sums far greater than the 50 percent minimum level."

The pledge is a moral commitment, not a legal contract, and it does not involve pooling money or supporting a particular set of causes or organizations. Individuals can give to the charities of their choice. 

In addition to Buffett and Gates, hotel mogul Barron Hilton, New York Mayor Michael Bloomberg, entertainment executive Barry Diller, Oracle co-founder Larry Ellison, energy tycoon T. Boone Pickens, Ted Turner, David Rockefeller, film director George Lucas and investor Ronald Perelman have all signed onto the pledge.

To see the list and pledges, click here.