Brit Richard Branson is worth $3.9 billion (£3.9bn) according to Forbes. Branson earned his money through his Virgin Group of companies. He signed up to the Pledge in 2013 with his wife Joan. In their Pledge note they wrote: "We are honoured to be joining this great group of leaders as part of a movement where all sectors are working together towards radical new approaches to solving issues that will help to truly change the world for the better."

From Elon Musk to Larry Page: billionaires using tech to live forever

Bill Gates’s Giving Pledge and who has signed it