Thomas Sutcliffe: You couldn't make it up... could you?

On the face of it, there isn't much connection between 50 Cent - the American rapper whose posters have just been censured for glamorising gun crime - and J T Leroy, an American writer who may, or may not, have transformed his hardscrabble street upbringing into voguish literary art. Their recent appearance in newspapers represent two quite different archetypal media stories.

In 50 Cent's case, it's that old warhorse, the Corruption of Youth. Posters for his film, Get Rich or Die Tryin', show the heavily tattooed Mr Cent with his rap credentials - a large handgun - protruding from his underpants. "At the end of the day, what will you hang on to?" asks the tag line, a reference to the fact that the rapper is also cradling a baby, its head just visible over his heavily inked shoulder. Responding to complaints that the poster glamorises gun crime, the agency responsible argued that it actually represented the moral dilemma facing those who grow up in the ghetto. Cling to human values or get yourself a Glock?

If so, it seems to be a dilemma that 50 Cent has resolved in favour of firepower. Just the other day, he was reported to have said of a rival bigmouth, currently polishing up his street cred in some high-security institution, that "they should let him out so he can die in the streets like he's supposed to". It doesn't sound as if he wants to meet him down at the soft play centre to discuss teething gels.

The J T Leroy story, on the other hand, is that hardy perennial, the literary hoax, in which it is revealed that an author's circumstances are not quite as advertised. This has happened before, with men writing novels under female pen names, and whites writing as members of an ethnic minority. In Leroy's case, the doubt concerns his purported upbringing as a "lot lizard" - a truck-stop whore who was pimped by his mother until his talent was recognised by a San Francisco outreach worker. Leroy's writings have excited admiration among the Hollywood set - with Courtney Love and Winona Ryder, among others, turning up at readings to give the author a voice (he's prone to bursting into tears when placed before a microphone). But he has been championed by writers, too, including Dave Eggars and Zadie Smith. So the suspicion that his vibrant jacket biography might be as fictional as his short stories obviously caused a certain amount of anxiety.

What unites both of these stories is an anxiety about authenticity. This is a commonplace paranoia in rap circles, where establishing that your street origins are genuine can be a fatally exigent business. And since the art itself is childishly obsessed with the assertion of ego, that's hardly surprising. So many rap artists wave their CV in your face that it's understandable if people occasionally want to check the details. Do the boasted prison sentences match the public records? If the music was seen as an exercise in style, it would hardly matter, but there's a naive insistence that nothing is made up. First you pay your dues - by acquiring scars and serving time - then you boast about how the dues were paid.

It is perhaps a little more surprising that writers should be subject to the same kind of credential checking. We've had several thousand years, after all, to accommodate the idea that fiction is not subject to the Trade Descriptions Act. And yet it is clearly the case, with a certain type of writing, that past experience is regarded as an indispensable part of the package. Would Norman Mailer have been so quick to champion the writings of Jack Abbott if their creator had not been serving time? (Or, indeed, if he'd been serving time for a crime less existentially chic than murder). The recent vogue for victim literature has only increased the sense that the true key to a literary career isn't a childhood dodging PE so you can finish Anna Karenina, but to be manacled inside a bin bag while your crack-whore mother flicks lit cigarettes at your head. Experience is deemed to deliver something that mere imagination cannot - in the case of both writer and rapper.

The true childishness of this position lies in its antithesis. If violent, difficult lives are seen as truer and more real, then comfortable ones must presumably be inherently inauthentic, which is bad news for most of us. In the case of rap, there may be no way round this - the form is too wedded to a sense of grievance and exclusion - but in the case of literature, it simply needs contradiction. Anyone worried about Leroy's biography should take comfort in the fact that all the best authors are hoaxers anyway.