Warning: Do not read on if easily offended! You have been warned!
You might like to think of comedy as being a spectrum. Ranging from subtle shades of light pink humour at one end through to gentle irony, satire, parody, saracasm, 'alternative' humour, blue humour and on to physical comedy and broad slap stick at the other.
Sick jokes sit right in the middle of this spectrum. They are indeed a classic form of humour. But just not to everyones taste.
At their basic level many forms of humour aim to surprise. The punchline. The prestige. But a sick joke goes further, it aims to shock. Say the unsayable. Think the unthinkable. "Oh My God, did you really say that? (that was just what I was thinking but didn't dare say it)". These jokes can say things we just can't. They can confirm (or indeed puncture) prejudices and stereotypes. They can provide a touchstone to modern thinking. They can attack it and they can nuture it. The sick joke is not constrained like other things. It is free. Free to tell it like it is. Or isn't.
Although the humour behind a sick joke comes mainly from the shock value, a good sick joke goes one stage further and is shocking and funny. Sick jones tend to be short too. The conciseness often adds to the impact. In addition the sick jokes needs a strong and often plain ridiculousness mental image. These can frankly be as bizarre as you like and are often utterly surreal.
People's responses to sick jokes however are rather more predictable. They laugh, gasp and reply with "Nooooooo! That's terrible!" - but secretly want to know if you know any more.
I will give just two examples of sick jokes to illustrate my point. Both cruel, unnecessary. But very, very funny.
Q: What do you do when a dead baby washes up on shore?
A: Try to hide your erection.
Q: What was Princess Diana's favourite brand of shampoo?
A: Head & Shoulders. They know because they found hers on the dashboard.
For further reading the wonderful Rob Manuel of b3ta has compiled a sick jokes web site and a bumper book of sick jokes book. A must for any Christmas stocking I'd have thought.