The other day, I learned of some breakthrough psychological research which proves that contributing to good causes stimulates the same parts of the brain as receiving large sums of money - only more so. Giving to others, it turns out, really may be the key to happiness. About 35 minutes later, I ran into a charity mugger, collecting for a human rights organisation, and became consumed with a quasi-homicidal rage that only worsened as he trotted after me down the street, stoking fantasies of breaking his clipboard in two and dropping it in pieces at his feet. There seems to be a contradiction here. Some possible conclusions: a) my brain is hardwired wrongly; b) the psychology researchers screwed up; or c) there are only certain conditions under which giving makes you happy, and being bullied by an out-of-work actor with a goatee isn't one of them.

The researchers, at America's National Institute of Neurological Disorders and Stroke, scanned people's brains while they played a computer game that gave them opportunities to win cash prizes or make donations to charity, sometimes at a cost to their own pocket, sometimes not. All these procedures lit up regions of the brain associated with the release of the "pleasure chemical" dopamine - but giving large sums at a cost to oneself did so the most. (It also triggered the production of oxytocin, the "cuddle hormone", which is associated with forming strong attachments.) Nor was any of this down to the givers thinking they'd get a pat on the back for being so selfless: their donations were anonymous.

Richard Dawkins has explained convincingly why this happens: altruism, he says, is a hangover from an era when we lived in communities so tiny that anyone we ran in to would most likely be genetically related, or, alternatively, in a position to harm our survival if they weren't on our side. (To witness Dawkins's own hyper-evolved capacity for withering put-downs, watch him try to convince a group of creationist college students of this argument here.)

Of course it feels good to do things that benefit our genetic legacy. But that doesn't address the moral quandary. Can it be right to choose who I give to on the basis of how it makes me feel? It's possible, in theory, that giving hundreds of pounds to my goateed haranguer would have been the most efficient way to get money to the people who needed it most, even though I'd have ended the transaction feeling annoyed. Contrastingly, giving to people sleeping rough triggers a warm inner glow - but numerous homelessness organisations advise against it. Then again, it would be nonsensical to give only when it made me feel bad to do so, wouldn't it?

Such are the mental acrobatics, it seems, in trying to make selflessness selfishly rewarding. At least the US researchers were clear on the bigger point: giving makes you happier than getting. So, as a purely philanthropic gesture, I'm willing to receive your cheques care of the Guardian.

· oliver.burkeman@theguardian.com