Similarly, most of the 40 billionaires who have signed the Giving Pledge have already been active in philanthropy. And while a few have promised to significantly increase their donations to reach the 50 percent mark, the initiative may not lead to an immediate jump in total giving. In today's harsh economic climate, Americans as a whole gave about $304 billion to charity in 2009, down from $315 billion in 2008, according to the Giving USA Foundation and the Center on Philanthropy at Indiana University.
Still, the stature of Buffett and Gates in the world of commerce is such that their challenge could have an enormous effect on philanthropy in the years ahead.
"I think there is a certain amount of peer pressure when you do have people of great wealth who are very visible in their giving," says Hilton, who disburses the proceeds of the family's nearly $2 billion foundation, including the $1.5 million Hilton Humanitarian Prize, one of the largest annual prizes of any sort. "Specifically, when you look at what Warren Buffett and Bill Gates have been doing, because they are at the top of their game in the business world and are looked up to by hundreds of millions of people ... there's no doubt in my mind that that has great influence on everyone, especially those who are in their same peer group." (Hilton's father, Barron, is a signatory to the Giving Pledge, though he, too, had already promised most of his $2.5 billion fortune to the Hilton Foundation.)