Investments from charitable foundations may have been key in helping Bernard Madoff sustain his Ponzi investment scheme for so many years, writes a columnist in Fortune magazine.

Mitchell Zuckoff, professor of journalism at Boston University and the author of Ponzi’s Scheme: The True Story of a Financial Legend, says most Ponzi deals collapse rather quickly, as withdrawals from investors exceed money coming in.

But because most charitable foundations spend no more than the federally required 5 percent of their funds each year, Mr. Zucoff suggests, Mr. Madoff could count on foundations not to withdraw large amounts from his fund, ensuring that there wouldn’t be a “run on the bank” that would cause his investment structure to collapse.