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The Aristocrats is a joke that has been around since vaudeville days, a kind of secret handshake for comedians that's rarely told in public because it's just too filthy. The very definition of a one-joke movie, Paul Provenza and Penn Jillette's documentary asks dozens of comics for their version - the enduring appeal of the gag is that there are countless ways of telling it - and gains some fascinating insights into the craft of comedy along the way.

The central irony of The Aristocrats is that the joke itself isn't especially funny. It goes like this: a family walks into a talent agent's office to pitch their new act. They perform various acts of unspeakable depravity, and when they're done, the agent says, "That’s incredible! What do you call it?" And the family say, "The Aristocrats." Not exactly side-splitting stuff, but the humour comes from the middle section, a parade of incest, scatology, violence, rape and eye-watering sexual perversion that varies with the teller.

"A WHO'S WHO OF COMIC TALENT"

Shooting on low-grade digital cameras, Provenza and Jillette (one half of alternative magicians Penn and Teller) have assembled a who's who of comic talent from Billy Connolly to Robin Williams to talk and tell jokes. Some of the wilder variations include a mime artist, a card sharp and an animated version (one of the dirtiest) from the South Park crew. The film has the feel of a long and fascinating chat rather than a formal thesis. But if it feels a little self-congratulatory at times, that's a small price to pay.