They've got to be joking: Audiences are being tested to the limit by a rash of politically incorrect plays

There are two kinds of laughter to be heard during performances of Clybourne Park, a play that busts a lot of the taboos that surround the way we talk about racism and, in relation to the way a deaf character is treated, disability too.

The two types of laughter are subtly different from each other and each is prompted by some of the sharpest and funniest dialogue about racism ever written for the stage. But one is the right kind of laughter, and one is the wrong kind.

"It cuts me," says Lorna Brown of the wrong kind. "When I hear it, it hurts." We are talking an hour before Brown steps onto the Wyndham's Theatre's stage as the temperamentally cool and watchful Francine, a black maid who works for a malfunctioning white Chicago family in 1959.

In the second half of Bruce Norris's provocative play, which has transferred from the Royal Court to the West End, Brown takes those same qualities, adds some assertiveness and becomes Lena, a resident of the eponymous Chicago neighbourhood in which the first half of Norris's play is set.

We are in the same sitting room, in the same house, only instead of overtly racist 1959 it is now supposedly enlightened 2009. A young white couple have moved into the place whose location is now predominantly black. Negotiations about their building plans are underway with their new black neighbours and it is during this electrifying section of the play, during a series of racially charged exchanges that lead to a series of offensive and often very funny jokes, that the wrong kind of laughter is most likely to be heard.

"I don't want to waste time interpreting other people's laughter," says Brown, "but there are two kinds." She imitates both. The first is stifled, the kind of laugh, she says, where the audience can't believe that they are laughing at something they know they should not be laughing at. And she's right. One of the joys of Norris's play is the cathartic licence it gives its audience to laugh at subjects as dangerously politically incorrect as race and disability. "And then," continues Brown, "there is this..." Here, Brown lets out a series of hard, pitiless, hoots.

Now, all the jokes feature racial stereotypes. But there is one that would go down well during an after-dinner speech at a BNP fundraiser. Without giving the game away, it suggests that black men are stupid. And some people in the audience either don't spot or don't care about its cruelty. It is as if Norris has set a brilliant trap for his mostly white, liberal, audience. In that sense, the play keeps its audience on its toes as much as it does the cast.

Clybourne Park is the most conspicuous in a wave of daring, politically incorrect plays whose characters say the unsayable. In Gina Gionfriddo's Becky Shaw at the Almeida, the lovelorn title character confesses to having fantasies about torturing black men after a series of disastrous relationships with black lovers. In Nina Raine's NHS play Tiger Country, recently at the Hampstead Theatre, a woman Indian registrar not only has to deal with the chauvinism of colleagues, but the defiance of "vacuous fat, black nurses".

So is it any coincidence that these plays, which revel in a straight-talking boldness, arrive at the same time as David Cameron's speech on the failure of multiculturalism? And nearly two decades after David Mamet's Oleanna (1993) warned that political correctness distorts professional and personal relationships to the point of destruction, do these plays signify that political correctness is in retreat?

Raine says that during the run of her play at the Hampstead Theatre she overheard people in the foyer talk about the "black nurse" line during the interval. There was no outrage. Many members of the audience were medics who recognised the truth of Raine's observation that attitudes about race – even between ethnic minorities – inform working relationships in the NHS. Potentially more explosive was a line from Raine's other recent play, Tribes in which the patriarch of an intellectual family disapproves of his deaf son joining the deaf community.

"The deaf!" he declares. "The fucking Muslims of the handicapped world." The production's first captioned performance for the deaf was particularly nerve-wracking for the author. "They all sat up in the circle because that was the best vantage point for the captions," remembers Raine. "I thought, 'I don't know if I've got the guts to go up there. What if I get recognised and lynched? No. I have to sit up there and see what their reaction is.' I've never been more nervous."

It is not known if any of the deaf people watching the captioned performance were Muslim. But no one heckled. No one walked out. They all came back for the second half.

"I think the thing about political incorrectness is that you're not setting out to be inflammatory," says Raine, who spent three months in NHS hospitals researching Tiger Country. "In my case I was trying to observe the truth. And sometimes the truth is a bit angular and a bit ugly."

Sometimes there is resistance to passages of dialogue reaching the stage. American writer Gionfriddo encountered an artistic director in America who wanted to cut Becky Shaw's monologue about taking revenge on black men. Never mind that the character crucially makes clear that she hates herself for having such fantasies.

"My response was to say, 'Would you have said that to David Mamet about Oleanna?'" says Gionfriddo speaking from her home in New York. "Does anyone say to Neil LaBute that you can't put that kind of cruelty onstage? I mean, as a friend of mine pointed out, Becky Shaw can't have this little moment about black men ruining her life, and Neil LaBute can have a whole play [about an overweight woman] called Fat Pig? I wonder if people just want women writers to be more sensitive."

There have been those who have wished that British playwright Richard Bean was more sensitive, or to put it another way, kowtowed to political correctness. There is no chance of that. If the plays by Norris, Gionfriddo and Raine can be described as a new wave, they arrive in the wake of Bean's England People Very Nice, a comic history of English immigration that opened at the National Theatre in 2009. It drew flak for its flagrantly provocative portrayal of racial stereotypes.

"I remember Dominic Cooke [director of the Royal Court and of Clybourne Park] once said to me that the most dangerous thing in theatre after a character who says offensive things is to make him or her likeable," says Bean. And I suppose that was one way in which England People Very Nice got people's hackles up. To make the racist people likeable was dangerous."

For Bean, in Raine's play Tribes it was not the father's line about deaf people and Muslims that hit home, but his attack on people from the North who wallow in "fake joviality" and have "a construct personality".

"I'm from the North and I thought, 'Good on you Nina. Go for it.' But then I'm in the same business," says Bean, whose latest play, The Heretic, about a climate-change scientist, is running at the Royal Court.

For Bean, a former occupational psychologist, Clybourne Park is not just gripping piece of theatre, it works on a Freudian level too, illustrating how each individual's ego manages the instinctive id. "Ego manages the way we behave in society," he explains. "But the id is operating at all times. I thought it was terrific to watch characters working from their ego and then suddenly the id of both [white and black] sides takes over, and that is almost the definition of what it is to be human in a pluralistic, multicultural society."

There is, says Bean, a world of difference between criticising culture and criticising race, something none of us has any choice in being part of. And playwrights must never be outlawed from criticising culture.

Perhaps this is why the laughter at that joke in Clybourne Park cuts Lorna Brown so deeply. Her character takes revenge with a joke about whites that almost starves the auditorium of oxygen, so breathtaking is Lena's riposte. But still, that first laugh usually rings out hard and clear. Because of the glare of the stage lights, Brown refuses to make any assumptions about whether the wrong kind of laughter comes from white or black people, or both. "There may be black people who find it funny," she says. "Maybe they are caught up in something too. Everybody brings their own issues to the play."