With Dumb and Dumber and There's Something About Mary, the Farrelly Brothers invented the gross-out comedy genre. But their new movie suggests the bodily-fluid humorists might have turned respectable. Never, Bobby Farrelly tells Donald Clarke

SHOULD we be surprised that Bobby and Peter Farrelly turn out to be scrupulously neat, middle-class boys from Rhode Island? Not really. Films such as Dumb and Dumber, There's Something About Mary and Kingpin may contain moments of unparalleled decadence - reproductive fluid in the hair; pink bits trapped in zippers - but the boys' work has always incorporated a sentimental, conventionally moral undercurrent that speaks of a suburban upbringing.

Sure enough, it transpires that Bobby Farrelly - at 49, a year older than Peter - has lovely teeth, tidy hair and a father who was a successful doctor. One wonders how Dr and Mrs Farrelly reacted when their sons announced their decision to make a living out of filming bodily fluids.

"I think ultimately they were relieved that we were doing anything at all substantial," he laughs. "They wrote us off as ne'er-do-wells at an early age. We had been up in LA, but nothing had gotten made. I think they thought we were bookmakers or whatever. Then they turn up on the first day of shooting Dumb and Dumber and we are talking to Jeff Daniels. He's sitting on the toilet and we're shouting: 'Bigger! Bigger!' They then realised this was a real job."

In the 13 years since Dumb and Dumber first peed all over the box-office, the Farrellys have cemented a reputation as world leaders in the humour of emissions. Yet, in recent years, one begins to sense a leaning towards respectability. The Perfect Catch, their fitful baseball-themed adaptation of Nick Hornby's Fever Pitch, featured little to disgust even the most prim of maiden aunts. The Heartbreak Kid, the boys' updating of an Elaine May 1972 comedy, does include mildly disturbing scenes concerning deviated septa and hirsute pudenda, but nobody is going to mistake it for the terrifically gross Kingpin. I wonder if they feel pressure from Farrelly enthusiasts to include gross-out moments.

"No, not at all," Bobby insists. "We don't play to the Farrelly enthusiasts particularly. We just do what we think is funny. I understand we are making jokes where girls put sperm in their hair and people think that's really gross. But we always show the scene to an audience and if they laugh it stays in. If it just grosses them out, then the scene goes."

Any film academic considering a paper on the Evolution of Attitudes to Sex in Contemporary Comedy may wish to compare the original version of The Heartbreak Kid with the Farrellys' take. Following nervous Ben Stiller as he drifts away from his new wife while still on honeymoon, the new film is, unsurprisingly, more explicit in its depiction of the sexual act, but, unlike the earlier flick, does not have any truck with the anarchic sexual libertarianism that briefly flourished in the early 1970s.

"Yes, that is interesting," he says. "It is one of those movies we both liked, but it is an offbeat movie. He left a rather unattractive girl for a supermodel and the supermodel was really cool about the fact that he was on honeymoon. Nowadays that would make her a sociopath, so we changed it so that he doesn't tell her he's on honeymoon. Also, if he left the wife just because she was less attractive that would make him a cad. We changed it so that the supermodel figure is the wife."

You can see what I mean about the Farrellys being traditional moralists at heart. The best of their humour remains, nonetheless, furiously lowbrow and purposefully appalling. Nobody should be surprised to learn that they are currently working on a feature version of The Three Stooges.

Twenty years ago, when Bobby and Peter started out as comedy writers, the school of disgusting slapstick they so adore was, however, terminally unfashionable. Though they did receive a credit on the well-remembered Virgin episode of Seinfeld, producers and financiers simply refused to accept there was a market for the team's brilliant idiocy.

"What changed it all was our saying we were going to direct Dumb and Dumber ourselves," Bobby explains. "It is quite hard to find someone to direct a film when you have no money and no backing. We thought: we can figure out how to do this ourselves. Suddenly people said oh, this script has a director. It must be a real movie."

Following Jeff Daniels and Jim Carrey as they barfed and farted their way across the US, Dumb and Dumber proved to be an entirely unexpected smash and, to the disgust of many, managed to accidentally invent a whole new genre. Without the Farrellys there would be no American Pie, no Freddie Got Fingered. The Gross-Out Comedy is their unlovely creation. When, in There's Something About Mary, the boys put something unpleasant in Cameron Diaz's fringe, the movement had its iconic image.

"We never imagined we had created a genre," Bobby says. "One out of four people might be grossed out by this stuff, but the rest find it funny. And we just want to do what's funny."

This is the second time he has put forward this slightly disingenuous argument. Surely the two reactions are inextricably intertwined. Those who find the sperm-in-the-hair scene amusing do so precisely because it's disgusting. "Maybe, but we never set out deliberately to break these taboos. We are never looking around for new barriers to break down."

Fair enough. But as they grow older they may, perhaps, encounter the desire to be taken a little more seriously. (Peter has, it should be noted, written a couple of novels.) Am I right?

"Oh no, not at all," Bobby says. "We have no desire whatsoever in that direction. The world is in a tough state right now. If you can get people to laugh their heads off that's good enough. We don't want to make some great statement."

The Heartbreak Kid opens on October 5th