????You thought that peer pressure ended in high school?

????Not so. This is the method that Warren Buffett and Bill and Melinda Gates are using to challenge America's billionaires.

????This morning, Fortune broke the story that Buffett and the Gateses (MSFT) are calling on the other wealthiest in the land to give half of their personal net worth to charity. "The biggest fundraising drive in history" is what my colleague, Carol Loomis, calls the challenge.

????Indeed. And it strikes me, reading Carol's blockbuster story (the best-kept secret at Fortune since Carol broke a related story four years ago, when Buffett disclosed that he planned to give his Berkshire Hathaway (BRKA) fortune to the Gates Foundation) and the transcript of her recent interview with Melinda Gates (which Carol shared with me this morning), it is a crowd mentality -- that "I'll do it if you do it" sensibility -- that enables this pledge drive that could change philanthropy for good.

????The mega-idea, as Carol explains in her story, was born at a May 2009 dinner in New York City. The dinner included David Rockefeller, Oprah Winfrey, Ted Turner, and New York Mayor Michael Bloomberg and led to dinners in other cities, where group-think fueled other billionaires to reconsider the final destination of their wealth.

????"You could tell the people who were still kind of on the cusp of thinking about how they were going to do this," Melinda told Carol. Melinda actually missed the first dinner but made the other two, in New York and Silicon Valley. Of her dinner companions, she said, "They were like, 'Yeah, okay. I've been kind of thinking in this direction. I knew I was going to get around to it. But wow!'

????"And so, learning from others in the group, you could see the power of the idea starting to gel with them."

????Wives were essential to get traction, says Melinda, who insisted that they be included in the dinners. "Even if he's the one that made the money, she's going to be a real gatekeeper. And she's got to go along with any philanthropic plan, because it affects her and it affects their kids."

????"It was really neat to hear in a few cases how the men let the women kind of move them," she added.

????It was a cache of California couples -- the John Doerrs, the Eli Broads, and the John Morgridges, whose billions come from Cisco (CSCO) -- who vowed early on to give at least 50% to charity. And in so doing, they are pressuring others to be "Great Givers," as Buffett decided to call them.

????As for the importance of the women, Buffett told me, back when I interviewed him for a Melinda Gates cover story in 2008, that he might not have given his wealth to the Gates Foundation if it weren't for Melinda. Today, again, Bill's other half plays the critical role of pragmatic doer.

????"One of the things I adore about Bill and Warren is," Melinda told Carol, "having a big idea but saying we don't exactly know how to execute it." As she says, "It takes some people to make it happen. So I help manage those pieces. And I enjoy that."

????She led the design and creation of the website givingpledge.org, which went up this morning. Now Melinda Gates hopes that the spirit of largess -- really big giving -- spreads beyond the super-rich.

????"My hope for this is that it takes on this momentum not only with the billionaires but that it expands out," she says, noting that young people she meets in colleges and grad schools say they want to give back to society.

????"I do think there's a crowd mentality," she adds. "It becomes the right thing to do. And so, more will because others are doing it."