Is this the worst taste film ever?

To their critics, the Farrelly brothers embody everything that is wrong with Hollywood today. With films such as Dumb and Dumber, There's Something About Mary, Me, Myself and Irene and Shallow Hal, the brothers stand accused of cynically (though wittily) exploiting the misfortunes of others - stupid people, overweight women, schizophrenics - to get rich. Very rich they are too; their films have grossed - gross is the appropriate word - more than $1 billion.

But no project has outraged as much as Stuck on You, their film about conjoined twins, which opens this weekend in the US. "I cannot believe these directors have stooped so low," says one person on a movie website. "They are making fun of a tragic condition that has attacked its victims and their families all over the world."

Indeed, the premise of the film was so troubling that it took the Farrellys - Peter, 46, and Bob, 45 - 13 years to get anyone to finance Stuck on You. Finally, Twentieth Century Fox stepped in.

"No one wanted to make it," says Peter Farrelly. "But I'd argue, 'Is it mean to make a movie about people in wheelchairs or who have Aids or heart disease? Are you saying you can't make a movie about conjoined twins just because you aren't one?'"

While the film was in production, the Farrellys faced a barrage of calls from the families of conjoined twins. One father of two conjoined daughters was particularly disturbed by a scene in which one twin is having sex while his brother lies in bed next to him trying to study.

"This isn't about making fun of conjoined twins," adds Peter Farrelly. "I'm sure there's stuff you won't agree with, but these guys in the movie are winners. I believe our hearts are in the right place."

Whatever the merits of Farrelly's argument, the row over Stuck on You highlights a critical point about comedy. To be really funny and relevant, comedians must continually test and step over the boundaries of taste and taboos.

There's no doubt that the Farrellys and other younger American filmmakers have embarked on a deliberate coarsening of the comedic palette that would make even Sid James blush. The famous scene in There's Something About Mary, for example, involves the innocent Mary (Cameron Diaz) using sperm as hair gel.

The Farrellys have thus become the modern masters of making the unacceptable mainstream. Even John Waters, director of such cult classics as Pink Flamingoes, says: "What used to be called sick humour is now called American humour. The biggest movies now contain sick humour." But now the Farrellys may have out-grossed even themselves.

The rot seemed to set in with Me, Myself and Irene, about a schizophrenic. One critic complained that "the really outrageous sexual humour is present, but it has a certain dragged-in quality, as if the Farrelly brothers had a gross-out quota to fill and inserted the penile/rectal/vaginal gags, so to speak, wherever they could."