News that Gina Rinehart may be the richest woman in the world highlights one of the oddities of today’s wealth: why aren’t there more women billionaires?

According to Forbes , 104 of the world’s 1,226 billionaires are women. That’s about 8.5 percent, even though women make up half of the total population.

The ranks of self-made rich women are even smaller. The vast majority of women billionaires around the world made their fortune through marriage (and divorce) or inheritance, according to various lists.

In the United States, only about a half-dozen of the richest 400 are self-made women – or slightly more than one percent. They include Oprah, Meg Whitman, Lynn Tilton and Sara Blakely, the founder of Spanx. The Gap’s Doris Fisher and ABC Supply’s Diane Hendricks are also on the list, since they co-founded their companies with their husbands. (Facebook’s Sheryl Sandberg may also soon be added to the list.)

The big question is why so few women reach the rarified heights of billionaire-dom. They are climbing the ladder in every other segment of the economy – education, jobs, income. They are even gaining ground among millionaires, accounting for somewhere between 37 and 50 percent of American millionaires.

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