That's just not funny - or is it?

`If you look into the darkest corner and pluck out humour, it can be da ngerous'

I approach the subject of comedy with caution. The last time I wrote about it provoked an angry response I shall treasure for ever. After I had questioned whether there was sufficient variety among comediennes at the Edinburgh Festival, an angry r ant inthe London magazine Time Out urged with apparent sincerity: "David Lister should expose himself to more women comics."

I have passed many an idle moment since, speculating on which comedienne's day I could ruin by following this advice.

But despite the dangers, I return to the subject to wonder if it is justifiable to make jokes about incurable diseases? Jasper Carrott has been the subject of numerous complaints to the BBC for making a joke about Alzheimer's disease in his show and following it up by saying he knew that people with the illness might complain, but he didn't mind because they would forget to post the letters.

Another joke I have heard innumerable times in the past few weeks concerns the man who went to a doctor to be told he had both cancer and Alzheimer's disease. "Oh, well," he replies, "at least I haven't got cancer."

Are such jokes acceptable? My gut feeling is yes, because I think they are not nasty or malicious, even though I recognise that no one with a friend or relative suffering from that particularly awful illness will find them amusing. Concern over jokes about illness seems to be something new. Concern over racial jokes is, of course, much older. The Jewish playwright Arnold Wesker addresses this in his new autobiography. Wesker cites the David Hare film Wetherby, in which the character played by Vanessa Redgrave ruminates: "Why is it that when a Jew tells a Jewish joke it is considered Jewish humour but when anyone else tells it it's anti-Semitic?" The answer, according to Wesker, is "because when a Jew tells a Jewish joke it's Jewish humour but when anyone else tells it it's anti-Semitic."

Wesker overstates the case, but there's a lot in what he says. An ethnic minority joke has a quite different texture when told by a member of that minority. There is an affection, knowledge and, not least, self-knowledge that give the joke a gentle, self-mocking quality usually absent in the hands of an outsider. It is also true of gay jokes, funny when delivered by gay comics, considerably less so when delivered by Jim Davidson or Bernard Manning.

Does this mean that a joke about Alzheimer's disease is perfectly acceptable when told by someone unfortunate enough to be suffering from the disease, but not otherwise? Perhaps so. But I suspect it is more complicated. First, there is almost certainly apecking order of distress where jokes are concerned. A joke about heart failure is pretty unlikely to cause offence, but one about a disease that carries with it untold suffering for the victims and their relatives does. Put more bluntly, a mental condition might be a no-go area, a physical condition is not.

The goalposts are always moving, occasionally because of fads in political correctness but more often moving rightly as people become more aware of where offence can be caused. When I was at school it was not altogether uncommon to hear jokes about rape.Such a thing would be near unthinkable now, and I would give my own children a sharp ticking off if they ever considered it a matter for humour.

Yet they do bring home from school the odd racially incorrect joke, and I'm unsure how to react. To lecture them about racism seems inappropriate, as they have friends from all cultures and are decidedly non-racist. Perhaps it is hard for those who have grown up during a period of transition in society, when jokes about blacks or Asians were always suspect, to accept that now such jokes might be delivered with friendliness and a complete lack of malice.

Paul Blackman, who runs the BAC drama and comedy centre in Battersea, south London, sees a difference between comedy and drama as far as no-go areas are concerned. "You can dramatise anything," he says, "you can look inside the darkest corner of a man's soul and the result can be cathartic. But if you look into the darkest corner and pluck out humour, it can be dangerous.

"I think there are very few no-go areas for humour. Race certainly isn't one any more, though racism is. For me the one no-go area would be cases such as James Bulger or the Moors Murders. But maybe that's because I've got children. If I had a relation with Alzheimer's, I might feel differently about that."

Precisely. We all have our no-go areas, and more often than not they will be areas that touch our own experience or our own fears. The joke teller can't be expected to know this private experience. And so I feel that with the exception of real horrors such as violent crime, there should not be no-go areas, provided the jokes do not suggest malice or stir up hatred. It's an issue that PG Wodehouse pondered on many years ago, and concluded that even if he made a joke about a porcupine, he would receive a letter on his mat the next day starting: "Dear Sir, with regard to your thoughtless and hurtful remarks about the porcupine ..."

Jasper Carrott was not being malicious about Alzheimer's disease. He was making light about one of its more distressing aspects. For some that is beyond the pale, but I believe that humour, like drama, can be cathartic, and even help one to live with illness and its effects.