Toby Ord has been giving away a third of his income for quite some time. A 33 year old academic at Oxford, he allows himself £18,000 a year, and donates the rest to charity.
"I’d always been idealistic about helping others", he says. "People would say “if you think that why don’t you give all your money to people in poor countries?" I think this was meant to make me shut up."
Instead he started giving away a large sum each year, and in the process set up a campaign - Giving What We Can  - to encourage others to give away at least 10 per cent of their earnings.
Ord's subject is philosophy, and he came to the decision in a philosophical way. "I was writing on the subject of luxuries, and I came across the question of whether we should forgo a luxury if it would save a life. I realised this decision happens in our lives all the time."
"I was reading [the philosophers] Peter Singer and Thomas Pogge. The two of them think furiously about the problems of the world today. They both took their ideas very seriously and gave away a large proportion of their incomes. I decided I was going to make a commitment to give money to poor people."
To his surprise, people soon contacted him to see if they could do the same thing - and Giving What We Can was born.
"I'm not a natural leader for such an organisation", Ord says, describing himself as "more of a theoretician". But he has a very clear vision for the project.
"For me it [giving away a large percentage of income] doesn’t seem too odd. We literally have a choice to save hundreds of lives."
Yes, but giving money to charity never seems quite as straightforward as that. Can we ever be sure that what we give is really saving a life?
He agrees that would be more motivational if there was a "clear line of operation", and that the path from your wallet to saving someone's life can sometimes look "messy". "But if your money goes on, say, 200 mosquito nets in a malarial area there is no question. You will save a life."
What about the horror stories, the charity money that never makes it, the corrupt organisations that divert aid to their own ends?
"There are lots of rather garbled stories about problems with aid. These are bit stupid. The whole of the world is paying billions of dollars in aid, and people point to one or two things that go wrong. It is quite easy to find these problems which then act as excuses for not giving", he says.
"There is a perfect storm of excuses for forgetting about charities."
He admits that some charities are more effective than others, though.
"Charities can often benefit from thinking of effectiveness more. One of the targets for criticising them is the amount of money that goes into administrative costs – but this is a bad measure of ineffectiveness - an organisation might need that level of spending on admin. I do think organisations should think about how they can help people more. There is a big academic literature on this. They should focus on outputs, and not get hung up on brand – for example they should be prepared to change the disease they are working on, if they could help more people that way."