Ron Perelman is known for a lot of things: nasty divorces, glamorous girlfriends, corporate raids. But turn the next few pages, and you’ll see he's also an exceptionally generous guy.

Perelman (see Q&A) is one of several billionaires who rank higher in generosity than wealth (he’s the 28th richest, but comes in 22nd in giving) in our Condé Nast Portfolio Generosity Index, a new effort to assess the philanthropy of the nation’s wealthiest people by taking into account recent giving, family foundations, and political donations. Eli Broad, Pierre Omidyar, and George Soros are also members of the select group whose giving punches above its wealth weight.

Of course, many of the nation’s richest people have become a lot less rich in recent months. But their largesse is taking on a heightened importance amid the current economic turmoil. So we were surprised to find that a number of the 50 wealthiest people in the country had no record of public giving in the past several years—people like Kirk Kerkorian, Steve Ballmer, and the Mars family. When we asked for details, they declined to share them. Most said their philanthropic activity is a private matter. Indeed, many people on our list could be closet donors of millions of dollars. But in a time of great economic need, the rich have the opportunity to set an example by making at least some public demonstration of charitable giving. (View a slideshow revealing how the wealthy have altered their legacies through philanthropy.)

As Perelman told us, “I think that in times like this, when the economy is having difficulties, it’s more important for individuals to step up and do stuff privately that may have historically been funded by public money. That’s going to be a task for us going into the next two years.”

How we came up with our numbers: Sources for giving data include the Chronicle of Philanthropy’s database of publicly announced gifts, I.R.S. 990 filings, the Foundation Center, GuideStar, the Federal Election Commission, and interviews with spokespeople for the 50 people on the list. Our Generosity Index is based on a formula that gives the most weight to 2007 and 2008 giving. (Figures for 2008 are as of September 19.) Donations from 2002 to 2006, family foundation assets, and political giving are given less weight. In some cases, there may be overlap between giving and foundation assets, but we wanted to make sure that maintaining a foundation as a vehicle for giving was taken into account. The wealth rankings are from the 2007 Forbes “Richest Americans” list.