Forty billionaires have responded to the challenge by Warren Buffett and Bill and Melinda Gates six weeks ago to publicly pledge at least half of their wealth to philanthropy.

The growing list of couples or individuals signing The Giving Pledge was updated today with new names, including software mogul Larry Ellison and filmmaker George Lucas. Microsoft co-founder Paul Allen, the only other billionaire from Washington state to take the pledge, added his name last month.

Ellison, who has been criticized in the past for not giving more to philanthropy, said he intended for many years to donate 95 percent of his wealth to charity. He has donated hundreds of millions to medical research and education.

"Until now, I have done this giving quietly - because I have long believed that charitable giving is a personal and private matter," he wrote. "So why am I going public now? Warren Buffett personally asked me to write this letter because he said I would be "setting an example" and "influencing others" to give. I hope he's right."

In their letters, others talked about what they hoped their money could achieve. George B. Kaiser said he is concerned that America is failing its social contract as a land of equal opportunity.

"It is the most fundamental principle in our founding documents and it is what originally distinguished us from the old Europe. Yet, we have failed in achieving that seminal goal; in fact, we have lost ground in recent years," he wrote.

"Another distinctly American principle is a shared partnership between the public and private sectors to foster the public good. So, if the democratically-directed public sector is shirking, to some degree, its responsibility to level the playing field, more of that role must shift to the private sector."

The Gateses also published a letter discussing their motivation to improve global health and U.S. education. They have pledged to donate 95 percent of their wealth to philanthropy.

The richest haven't fit the pattern of the most generous in the past. The Chronicle of Philanthropy found that over the last decade, only 29 people on the Forbes 400 list have ever donated enough in a year to make the list of the 50 most-generous Americans.

Columnist Pablo Eisenberg said the well intentioned effort to boost philanthropy has the potential to exacerbate inequities in the nonprofit world and in society unless steps are taken to "mitigate the potential undemocratic nature of these new mega-foundations" and shift grant-making priorities to help the most disadvantaged people.

The full list is here, along with each of their pledge letters. Their estimated wealth, from a quick tally of this data from Forbes, is about $250 billion, though some have already contributed the bulk of their assets to charity.

"We've really just started, but already we've had a terrific response," said Buffett, pledge co-founder and CEO of Berkshire Hathaway. He said of the people he and Gates approached, about half agreed to sign on.

It was clear from letters written by Buffett and others that donors intend to send a message to future generations, including those around the world, to change the way people view wealth.

"If life happens to bless you with talent or treasure, you have a responsibility to use those gifts as well and as wisely as you possibly can," the Gateses wrote in their letter. "Now we hope to pass this example on to our own children."