There is an argument going on in America about comedians stealing jokes from each other. A video clip shows Joe Rogan confronting Carlos Mencia and saying that Mencia pinches gags from other comedians.
The video also shows sections of Mencia's performances, intercut with similar material told by other funnymen. I found it inconclusive. Either the case isn't very strong or it was badly edited. Rogan shows the gig where he interrupted Mencia and made his accusations, and particularly how the audience sided with him. Maybe I lack faith in mob justice, but that didn't clinch the argument for me.
Gag theft is taboo on our circuit. Well-meaning, newish acts can get over-sensitive about it. Some acts have worried that a new joke of theirs is too similar to one of mine. They tell me their joke, and then I normally have to ask which piece of my material they thought it resembled.
I have had a piece of material which is very similar to another act's. It was based on a news item - we must both have been watching the same TV coverage of the Labour Party conference that week, and had the same thought (that Blair reacted to being heckled like a stand-up would). Had all working UK comedians been watching, maybe half of them would have had the same idea. Luckily, it seems, just the two of us tuned in. If the two of us are on the same bill, we just make sure we're not both planning on doing the joke. It's all very civilised.
The "alternative" circuit I work in has its roots in the non-sexist, non-racist movement, opposing the old "mainstream" circuit. But now the difference between these two circuits is in its material morals. The mainstream circuit employs the morals of the real world, that all jokes are public domain (no one has ever got into a fight in a pub for telling a joke they didn't write themselves). It has less concern about who tells them, so long as they tell them well.
On the alternative circuit, acts present themselves as telling some sort of truth, so the material can be more observational, and the performer's persona becomes more important. So gag theft is a more personal, damaging crime. In other words, a mainstream comic will say, "A bloke walks into a pub", whereas an alternative comic will say, "The other day, I went into a pub."
My problem is not with a joke that someone else wrote first. My bugbear is jokes that sound like you've heard them before, even if you haven't. This identikit material gets round the taboo of joke theft on a technicality. But it will never be great comedy because it lacks true originality of spirit.