FORTUNE -- Twelve billionaires -- with names like Ackman, Bronfman, and Musk -- have added their names to the Giving Pledge, the campaign started by Warren Buffett and Bill and Melinda Gates in 2010 to spur philanthropy among the superrich. That brings the number of total members, many signing jointly with their spouses, to 81.
Their commitment is to give away at least 50% of their wealth in their lifetimes or at death to charity. Buffett and the Gateses developed the idea believing that a quantified goal would help the wealthy to think through their philanthropic plans. "Obviously there are a host of people we can recruit, and certainly we'll be signing up many more," says Buffett. "But I will tell you, I would have thought the Giving Pledge a success at levels well below 81."
12 more billionaires sign on to Buffett/Gates pledge - Term Sheet
Their commitment is to give away at least 50% of their wealth in their lifetimes or at death to charity. Buffett and the Gateses developed the idea believing that a quantified goal would help the wealthy to think through their philanthropic plans. "Obviously there are a host of people we can recruit, and certainly we'll be signing up many more," says Buffett. "But I will tell you, I would have thought the Giving Pledge a success at levels well below 81."
12 more billionaires sign on to Buffett/Gates pledge - Term Sheet