When the Bezoses finalized the most expensive divorce ever in April, MacKenzie Bezos walked away with what’s now worth $36.6 billion. Her newfound individual fortune catapulted her to into the top echelon of the world’s richest people, where few women exist. She’s currently No. 22. Only three women on earth—Francoise Bettencourt Meyers, Alice Walton, and Jacqueline Mars—are worth more, according to Bloomberg’s Billionaire Index.

At the time of the divorce, I wrote about why her money mattered. First, there is inherent power that accompanies such extraordinary sums of money, so it’s worth pointing out the rare occasions when it ends up in women’s hands. (By Forbes’ count earlier this year, there were 2,153 billionaires in the world; of them, just 244 were women. Of those female billionaires, only about a quarter are self-made.) Second, in managing their wealth, women are more socially-minded; they’re more interested than men in ‘sustainable’ investing, according to a Morgan Stanley survey, and “making the greatest impact” is the top factor in their charitable giving, according to research by The Economist Intelligence and RBC Wealth Management. (Men’s is tax benefits.)

In April, MacKenzie Bezos didn’t indicate what she’d do with her money. “Excited about my own plans,” she said in announcing that she was giving ex-husband Jeff her interests in the Washington Post and Blue Origin and 75% of the couple’s Amazon stock. But on Tuesday she lived up to that earlier analysis by signing up to Warren Buffett’s Giving Pledge, which commits the uber-rich to giving away at least half of their wealth.

“We each come by the gifts we have to offer by an infinite series of influences and lucky breaks we can never fully understand,” she said in a letter supporting the pledge. “In addition to whatever assets life has nurtured in me, I have a disproportionate amount of money to share. My approach to philanthropy will continue to be thoughtful. It will take time and effort and care. But I won’t wait. And I will keep at it until the safe is empty.”

Buffett and Bill Gates started the Giving Pledge in 2010, and each year they invite wealthy individuals to sign on. MacKenzie Bezos is one of 18 new signatories this year that brings the Pledge’s total number of philanthropists to 203.

In covering the news, the Financial Times added this aside: “Mr. Bezos, the world’s richest man, is notably absent from the list.”

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