While the overall reaction to this week’s Giving Pledge has been positive, there have been a few critical voices. Rather than knocking the generosity of the individual billionaires, critics have focused on the wider questions about inequality, taxation and plutocracy. These are important issues that we discuss in detail in the book. As we argue, there are three key tests of a ‘good billionaire’.

First, how did they earn their money? America’s billionaires mostly have a good claim that they have earned their money fairly. “In America, there is equality of opportunity”, argues the Peruvian economist Hernando de Soto. “At the end of the day,” he says “the argument that Bill Gates can use against anybody in the U.S. is ‘you could have done it, too’”. But what about Microsoft’s brushes with anti-trust regulators in the U.S. – surely that means that at least some of his gains are ill-gotten? Well, that’s actually de Soto’s point – in America and other advanced economies the rules of the game mean that corporations are held to account and if they cross the line, as Microsoft was judged to have done, they have to pay the price. In the developing world, however, this may be a different story, where spectacular wealth has been acquired, in some cases, by ripping off the public in dodgy privatisations or through monopoly power. If Gates and Buffett roll the pledge out globally, this is going to be an important question for some billionaire donors. Yet  even in America it is right to question the individual donors about their business practices – making money through shoddy labour practices or pushing sub-prime mortgages on poor people who cannot afford them does deserve criticism – but this should be done on a case by case basis, rather than assuming that all wealth is the product of exploitation.

Second, yes, billionaires need to pay their taxes and billionaires who choose to pay more tax, say by refusing to move to a tax haven, should be given credit for doing so. Some of the critics seem to assume that the wealthy are all a bunch of tax avoiders who would resist paying more tax at all costs. Yet Buffett, along with other billionaire donors like George Soros and Ted Turner, spoke out against President George W. Bush’s abolition of inheritiance tax, observing that he is not a believer in the “lucky sperm club”. Of course there will be anti-tax billionaires on the other side but, again, it’s about judging the super-rich on a case by case basis.

Third, how much do they give? We support the Giving Pledge because it sets a sensible benchmark of half their wealth as a measure of billionaire generosity. As important as how much is given, however, is how the money is used. Some critics rightly point out that American philanthropy enjoys a generous tax subsidy and we would back a British-style public benefit test for charitable donations as a sensible compromise between the rights of the donor and their public responsibilities. We also hope that the pledgers will get on with their giving sooner rather than later – channelling the donor’s own passion and harnessing the donor’s skills is likely to mean that the money is better used rather than just being posthumously dumped into a foundation for some philanthrocrat to give away.

The really important question is how do the rest of us harness this new wave of giving so that it delivers the maximum benefit for our society and our environment?  We believe passionately that for the philanthrocapitalism revolution to achieve its full potential there needs to be transparency from donors – about their failures as well as their successes – and a willingness on all sides to join in the debate. For that reason one of the most important spinoffs of the Giving Pledge is the way that is has, hopefully, started that crucial discussion.