It's a Saturday night in north London, and a group of people are listening to one white man speak. First he suggests that all Muslim men are secretly gay. Next, he's using the n-word. Then he draws his eyes into slits to mock the Chinese. One woman in the crowd has had enough. "You're awful," she says, leaving the room. "You're a disgrace." Soon, others join her; the man abuses them as they leave. The atmosphere is sour.This is not an unruly seminar on racism, but comedy, 2009-style. It's a world where all the bigotries and the misogyny you thought had been banished forever from mainstream entertainment have made a startling comeback. Tonight's comic is San Francisco comedian Scott Capurro, and his routine is not unusual in the taboo-teasing world of 21st-century standup. Before the gig, I ask Capurro how he feels about routinely offending his audience. "It's great," he says. "I'm not friends with my audience. I'll never see them again. If they want to fight, they can have one with me. How often does an audience get the chance to stand up and say, 'You are fucked up'? It's so exciting – it's a conversation."Is Capurro probing the boundaries of what is sayable or not? Or is he just smuggling out bigotry under a veil of irony? It's a question that will be asked at the Edinburgh Fringe next month, which in recent years has resembled less a comedy festival than a sounding board for racial and sexist provocation. Notorious examples range from this charming Jimmy Carr quip – "the male Gypsy moth can smell the female Gypsy moth up to seven miles away. And that fact also works if you remove the word moth" – to the serial political incorrectness of Ricky Gervais. "One false move," as Gervais likes to say in his live act, "and I'm Jim Davidson."