At the urging of Bill Gates and Warren Buffett, forty of the world’s richest families have promised to give at least half of their fortunes to philanthropy. By taking the “Giving Pledge,” the forty families or individuals, most of whom are billionaires, are promising a collective sum of at least $125 billion to charitable causes, based on Forbes’ current estimates of their net worth and other data sources. On the whole, Americans have given more than $300 billion per year in recent years, though giving has declined since the start of the recession, according to the Giving USA Foundation. “We’re hoping that America, which is already the most generous society on Earth, becomes even more generous over time,” Buffett told reporters on a conference call today. “More philanthropy and smarter philanthropy in the future is the goal.”