MacKenzie Bezos, who recently became the ex-wife of Amazon founder Jeff Bezos, has pledged to give away half of her nearly $37 billion fortune to charitable causes (Bezos’s divorce settlement made her the world’s fourth-richest woman). She has signed the Giving Pledge, an initiative–launched by Warren Buffett and Bill and Melinda Gates–which encourages the world’s wealthiest people to donate at least half of their fortunes for philanthropic purposes, either during or after their lifetimes. The “after” seems noteworthy.

Since launching in 2010, the Giving Pledge has earned the support of 204 individual billionaires and families. In addition to Bezos, newly announced signatories include Brian Acton, the co-founder of WhatsApp, Paul Sciarra, the co-founder of Pinterest, and Brian Armstrong, the CEO of Coinbase.

In a letter released Tuesday, May 28th, Bezos shared why she chose to sign the pledge. “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 wrote. “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.”

Jeff Bezos has a net worth of roughly $114 billion, making him not just the richest man in the world, but the “richest man in modern history.” He tweeted in praise of his ex-wife’s choice, writing that “MacKenzie is going to be amazing and thoughtful and effective at philanthropy, and I’m proud of her. Her letter is so beautiful. Go get ‘em MacKenzie.”

He has not signed the pledge.

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