More than three dozen of the wealthiest families and individuals in the United States have committed to giving away the majority of their wealth to charitable causes, since the "Giving Pledge" was launched by fellow billionaires Bill Gates and Warren Buffett six weeks ago.

"We've really just started, but already we've had a terrific response," said Warren Buffett, pledge co-founder and chairman and CEO of Berkshire Hathaway. "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% minimum level."

The United States has roughly 400 billionaires, about 40 percent of the world's total, according to Forbes. The 40 names that have pledged to date have a combined net worth surpassing $230 billion, reports MSNBC.

Wealthy Americans from around the country, from energy tycoon T. Boone Pickens to media tycoon Ted Turner, have taken the pledge. A full list of those 40 families and individuals, along with personal pledge letters submitted by people like Mayor Michael Bloomberg, filmmaker George Lucas, and David Rockefeller, are available online at www.givingpledge.org.

• Michael R. Bloomberg: "If you want to do something for your children and show how much you love them, the single best thing – by far – is to support organizations that will create a better world for them and their children. And by giving, we inspire others to give of themselves, whether their money or their time."

• George Lucas: "My pledge is to the process; as long as I have the resources at my disposal, I will seek to raise the bar for future generations of students of all ages. I am dedicating the majority of my wealth to improving education."

• Barron Hilton: "It is my hope that others are inspired by my father's story, and by our family's steadfast adherence to his charitable philosophy."

• Warren Buffett: "Were we to use more than 1% of my claim checks (Berkshire Hathaway stock certificates) on ourselves, neither our happiness nor our well-being would be enhanced. In contrast, that remaining 99% can have a huge effect on the health and welfare of others."

• Bill and Melinda Gates: "We have been blessed with good fortune beyond our wildest expectations, and we are profoundly grateful. But just as these gifts are great, so we feel a great responsibility to use them well."

• Jon and Karen Huntsman: "It has been clear to me since my earliest childhood memories that my reason for being was to help others."

• George B. Kaiser: "I had the advantage of both genetics (winning the "ovarian lottery") and upbringing. As I looked around at those who did not have these advantages, it became clear to me that I had a moral obligation to direct my resources to help right that balance."

• Gerry and Marguerite Lenfest: "The ultimate achievement in life is how you feel about yourself. And giving your wealth away to have an impact for good does help with that feeling."

• Lorry I. Lokey: "There's an old saying about farmers putting back in to the ground via fertilizer what they take out. So it is with money. The larger the estate, the more important it is to revitalize the soil."

• Peter G. Peterson: "As I watched and learned from my father's example, I noticed how much pleasure his giving to others gave him. Indeed, today, I get much more pleasure giving money to what I consider worthwhile causes than making the money in the first place."

• David Rockefeller: "Our family continues to be united in the belief that those who have benefitted the most from our nation's economic system have a special responsibility to give back to our society in meaningful ways."

• Sanford and Joan Weill: "Our Pledge is this: We will continue to give away all of the wealth we have been so fortunate to make except for a very small percentage allocated to our children and grandchildren between now and the time we pass because we are firm believers that shrouds don't have pockets."

The Giving Pledge, which does not involve supporting a particular set of causes or organizations, is an effort to help address society's most pressing problems by inviting the wealthiest American families and individuals to commit to giving more than half of their wealth to philanthropy or charitable causes. While it is specifically focused on billionaires, the idea takes its inspiration from other efforts that encourage and recognize givers of all financial means and backgrounds.