Philosopher Toby Ord and his partner Bernadette Young give at least 10% of their income to charity. Can they persuade more of us to follow suit?

"How Rich Am I?" asks a panel on Giving What We Can's website. Fill in your household income, after tax, and the calculator gives you an answer.

If your family of four has the 2011 median UK income of £26,000, you learn you are in the richest 13% of the world's population. Fill in some more blanks and you find out how many lives you would save, based on the cost of treating TB, if you took the organisation's pledge to donate 10% of your earnings until retirement.

If you earn more than the average, say £60,000 after tax as part of a couple without children, it turns out you are in the richest 1% of the world's population – and would stay there even after giving 10% away. Which is why philosopher Toby Ord, Giving What We Can's Australian founder, thinks the Occupy movement's claim to speak for the 99% and against the 1% is a "bit strange". Millions of people in the west, and not just investment bankers, are in the 1% if wealth is reckoned globally.

"For a lot of us, what really defines us in the world is having so much money. We're exceptionally wealthy compared to the average," he says in the precise, logical voice of the studious, overgrown schoolboy he resembles. "Once you get used to the idea, I think you realise, wow, this is a really nice position to be in, to be able to help people."

Persuading people in rich, developed countries to look at their income and assets on a global scale, rather than comparing themselves to those who are even richer, is a big part of what Giving What We Can is about. Founded two years ago in Oxford by Ord, supported by his partner Bernadette Young, a junior doctor who took the pledge at the same time, the organisation has grown to 177 members in five chapters including Princeton, Cambridge and Harvard. Early pledgers included development economists and the philosopher Peter Singer, whose 1971 essay Famine, Affluence and Morality was a guiding influence. These grandees, and the couple's friends and family, have been followed by students, doctors, teachers, an interior designer, a video game developer and a management consultant, all listed on the website.

Ord and Young, sitting across from me in their favourite Oxford cafe – where the money for the homemade mince pies on the counter goes straight into a collection for the homeless – say their "kitchen-table operation" has already ballooned beyond expectations. Currently administered by around two dozen volunteers, next year it will turn itself from a society into a charity, so it can employ staff and apply for grants from foundations.

"It's about putting on a positive spin," Young says. "I think a lot of people feel, 'Well if everyone else in the world did this, no problem, but everyone else isn't pulling their weight, so why should I?' What we're trying to say is that a small number of people can still make a difference."

Young is 31, Ord 32, and together they live on £43,000 after tax, from which they still manage to save for pensions. They have recently got a mortgage – the bank was initially nonplussed by the line in their accounts devoted to giving – and moved from their rented flat into a small house, where this weekend they host their first Christmas, with an aunt of Young's who lives in Brighton. Any treats planned? "No," they chime, but stress that they are not particularly frugal. One reason they believe the whole experiment has been so painless is that, unlike someone suddenly deciding in later life to sacrifice 10% of their income, they have never seen their incomes drop. Both spent most of their 20s as students, he in Oxford and she in Melbourne, living on grants worth a few thousand pounds. "And we were very happy," says Young emphatically – just as happy, they couldn't help noticing, as friends with bigger incomes. The only issue was saving for a couple of plane tickets each year so they could see each other.

Currently Ord gives away around 40% of what he earns. "If anything my quality of life has probably improved," he says. "It's not particularly heroic or anything. At least half the people in Britain could probably think much more seriously about how much they could give."

Aren't some people just more selfish than they are? They both laugh loudly. "We're not being drawn into that one," Young replies. "We've made definite efforts not to have that kind of discussion. We know everyone has their own circumstances, but this is something we do and we're really happy doing it."

"It's not that different from being a vegetarian," is Ord's more cerebral answer. "If people are vegetarians for ethical reasons, they think there's something pretty wrong about eating meat, but they can deal with friends who eat meat, most of them. This is a similar kind of thing. We're not saying you have to give away 10% of your income to be a decent human being."

It's hard not to feel that this is exactly what they are saying – it is pretty much what Singer said in the essay that inspired them – but they are very nice about it. This is tactical: part of their philosophy is that making people feel guilty and depressed is not effective. Young refers to "bad reactions" when people get defensive, and cites contrary examples of people influenced to be generous by their upbeat example: a colleague who decided to donate a fee to one of their charities, a hedge fund manager who gave half a million pounds.

Such ideas about positivity and empowerment come from psychologists and economists who promote happiness as a measurable goal. Ord refers to the "hedonic treadmill" whereby people expend more and more resources to maintain the same level of satisfaction, and to evidence that people get more happiness from spending money on other people than buying things for themselves.

Presumably they also didn't want to alienate friends who were unwilling to sign up. Ord admits they can make others feel a bit uncomfortable, but his face breaks into a grin when I suggest he must get worked up sometimes about how little many people do: "I don't feel all that concerned. Maybe that's partly a deliberate choice."

There are glimpses of the more hardline underpinnings to the couple's way of life. Ord briskly dismisses the "feelgood activity" often associated with volunteering, and makes the more impulsive, less carefully researched charitable giving many people engage in sound like a kind of egomania. For rhetorical purposes, he dreams up a person who believes they can't make a difference on their own, or can't be bothered, only to shoot them down abruptly: "They're factually incorrect, they could definitely save hundreds of people's lives, and it's actually quite immature to say that … that's a stupid comment."

Having spent five years researching poverty and development before launching Giving What We Can in 2009, Ord has absolutely no time for critics of aid, calling them "silly", rattling through some figures and referring me to his website for more. Young describes how they turned one would-be pledger away because they wanted to give half their 10% to animal charities. Later Ord has a pop at academic philosophy, suggesting that as a discipline "we mainly end up talking about things that are contentious rather than things that are important".

For Giving What We Can, philanthropy should be strictly evidence-based, scientific. The charities it recommends have been chosen on the basis of statistical analysis of data, and focus on such mundane but life-saving interventions as supplying mosquito nets. But while Ord and Young say they are uninterested in being directly involved in aid projects – despite the millions pledged, Ord has only once been to Tanzania for a fortnight – they recognise that people don't want to be philanthropists in isolation, and the point of their society, apart from persuading people to give to the right causes, is to provide a sense of companionship.

Young, whose mother died when she was 10, says they both came from "fairly lefty" families. She went to a Catholic school, which she says encouraged an interest in social justice, and seems more engaged in practical political considerations than her partner. When I arrive she is talking about bed-blocking in NHS hospitals. Later she brings up the issue of very expensive, life-prolonging drugs. Didn't this month's Social Attitudes survey reveal that Britain is getting meaner, with hardening attitudes, in particular, towards welfare claimants?

"I think the NHS operates on an enormous amount of goodwill," she says. "I wouldn't say you see people being mean, I think you see people worried."

"One nice thing about setting up Giving What We Can," Ord adds, "is that we meet a lot of people who are pretty nice" – they both laugh – "and willing to sacrifice a large amount of income in order to help other people."

On the whole, taking a longer view, both are convinced the world is getting better. "Something Peter Singer talked about, which I totally agree with, is that over hundreds of years our pattern as humans is to widen our circles," Young says, "so we've gone from tiny kin groups to the community being larger, and I don't see there being a rational boundary as to why someone in the country I live in is part of my community and someone in another part of the world is not, in the sense that they are still someone I have a duty to help if I can."

Perhaps this sense of a globalised community is enhanced if you have moved across the world from Australia. Neither Ord nor Young seems to have much comprehension of those for whom charity or activism is strongly linked to the place they come from, and more about repairing the social fabric or building relationships than their fiercely utilitarian accounting of lives saved thousands of miles away. When I ask whether they've ever been tempted to get involved in something closer to home, Ord says, "This is just our adopted home to start with," before pointing out how much further their money goes in sub-Saharan Africa than it ever could in Oxford.

About increasing levels of inequality and the redistribution of wealth within developed countries they don't say much, though Ord suggests some members may be involved in the Occupy camps. But he insists GWWC's rejection of consumerism, while radical, is not leftwing, and believes their highly individualised model of voluntary tithing is likely to be more effective than increased taxation.

If Young is the warmer and more natural of the two, perhaps Ord's dryness and empirical rigour are what is required if people are to be persuaded to part with more of their money, via either increased foreign aid budgets or donations. After all, it does take intellectual effort to imagine what £50 or £100, so easily spent in the supermarket, could mean in a country where children routinely die of treatable infections.

He suggests giving away 10% of your income might, like vegetarianism, one day become a social norm. And he suggests philosophers should pay more attention: "Quite a few people think obviously it's good to donate to charity, so we're not going to talk about it. I think that's a mistake, because you can go further and say, do we have an obligation to do it? Is it not merely something that's nice, but something we really have to do? And I think the answer is yes."