What did it take to turn the Earth's richest man, who rarely gave a cent to those in need, into the world's most copious donor? It took, evidently, two typically fertile conditions: love and commitment.

I'm talking about Bill Gates, the scruffy, scrappy founder of Microsoft, who didn't begin truly sharing his monumental wealth until he hooked up with Melinda French, a bright Microsoft employee with an MBA from Duke.

Only after marrying her (under pressure from his aging mother), did Bill start donating billions to causes such as treating people with AIDS and malaria. Turns out, Bill isn't so different from the rest of us.

While we don't have personal foundations with 11-figure endowments (Bill and Melinda's boasts $28.8 billion), Americans do give away lots of money: about two percent of their yearly income. About 75 percent give to at least one charitable cause, according to the American Association of Fundraising Counsel, most often to churches and schools; oddly enough, the poorer they are, the more they tend to donate.

Bill is like the rest of us in another regard also. Americans generally give away very little money when we're young, becoming regular charitable donors only as we settle down and find a home, a community, and a spouse. (Not incidentally, this is when charities tend to find us, at our finally steady addresses; being asked is a huge factor in why people give money.)

Consider the parade of couples whose names show up on any list of today's great philanthropists. Besides Bill and Melinda, there's William and Flora (the Hewletts of the Hewlett Foundation), John D. and Catherine T. (the MacArthurs), Michael and Susan (of Dell computer fame), Sandy and Joan (that's Weill of Citicorp), and more recently, Pierre and Pam Omidyar (whose big bucks came from his helping build eBay).

There's a saccharine note to the claim that love makes people better—more generous, more open-minded—but there's truth to it, too. And couples will tell you that contributing to charity is a way of communicating to each other and to the world that they share values.

Leesy T., who in her late 40s has well-worn giving habits, would be shocked if the causes she cared about didn't match up with her husband's. "I can't think of an example when John and I disagreed on a charity," she says, "because our basic values are so similar." But if they did, she would probably respect his thinking.

"My parents were both rabidly against abortion," Leesy recalls, "but when my mother died, I found out that she had been a supporter of Planned Parenthood." When her father took over the task of handling the family donations, a task long assigned to her mom, "he continued to support Planned Parenthood with a modest amount for about five years, just because it was something my mother had cared about."