Why do people tell sick jokes about tragedies?
Tasteless tweets about the Japanese tsunami have landed celebrities in trouble. So what makes people tell sick jokes about terrible disasters?
Have you heard the one about the tsunami that killed thousands of people?
If not, someone might have told you a gag about the threat of nuclear meltdown. Or any other horrific event that happens to make the headlines.
Death, destruction and widespread devastation may be the unfunniest subject matters imaginable. Yet for some people they make up a comedy sub-genre.
Sick jokes have a habit of springing up in the immediate aftermath of any catastrophe, and modern communications mean they are heard by more people and closer to the event than ever.
The rapper 50 Cent and the US comedian Gilbert Gottfried have faced an onslaught of criticism after joking on Twitter about the tsunami that has caused devastation in Japan.
The hip hop star was upfront about setting out to offend. "Some of my tweets are ignorant," he wrote. "I do it for shock value. Hate it or love it. I'm cool either way."
Gottfried - who had previously come under fire for joking about 9/11 shortly after the attacks - could not afford to be so sanguine after he was fired by an insurance company who used his voice in adverts.
Nor are they the first public figures to face such opprobrium. Football pundit Rodney Marsh was sacked by Sky Sports in 2005 for making wisecracks about an earlier Asian tsunami.
Billy Connolly was roundly condemned for joking onstage about the death of Ken Bigley, the British hostage killed by his captors in Iraq, Jimmy Carr was attacked for material involving amputee British service personnel and Frankie Boyle faced widespread criticism after a routine about the Cumbria shootings.
Of course, it is not only professional comedians who are responsible for this type of humour.
Following any disaster, deeply offensive gags swiftly proliferate around playgrounds, workplaces, pubs and, of course, the internet.
The website Sickipedia, which prides itself as "the world's best collection of sick jokes", proudly displays dozens of user-generated contributions about Japan.
Text messaging, too, means that some people can now expect the first off-colour SMS to arrive within hours of any disaster.
Veteran comedian Barry Cryer says that he has long been "fascinated" by sick humour.
He insists that, although those cracking such jokes may be children in the playground or saloon-bar braggarts advertising their cynicism, making light of terrible events can be an entirely understandable coping strategy.
Observing that medical professionals and the police have always been known for their gallows humour, he believes black comedy helps us make sense of occurrences that would otherwise be painful and upsetting.
Indeed, Cryer recalls being approached by one young man who had recently lost his mother to cancer and asked the comedian if he knew any good jokes about the disease.
"It's a natural reaction," he argues. "It's entirely normal that people want to laugh at times of tragedy.
"All that's new is that in the past you'd have to wait until you got to the pub to hear these jokes. Now they're on your phone as soon as the disaster happens."
Psychologist Dr Linda Papadopoulos worries that sick humour's popularity is symptomatic of an unhealthy culture which has been desensitised to the suffering of others.
"One of the reasons we laugh at tragedy is that it makes the enormity of the issue easier to deal with," she concedes.
"But we do live in a society where tragedy has become something that we've become conditioned to laugh at."
Any fan of Peter Cook or Bill Hicks will attest that dark humour predated the internet, however, and none other than Sigmund Freud addressed the topic in his 1927 essay Humour (Der Humor).
In it, the father of psychoanalysis argued that sick jokes were the mechanism by which the ego "insists that it cannot be affected by the traumas of the external world".
His analysis is shared by Dr Oliver Double, an expert in comedy at the University of Kent who believes that tackling offensive subjects can be a very effective tool of satire as well as a form of therapy.
For instance, Dr Double was in the audience for Connolly's Ken Bigley routine and argues it was a carefully-argued attack on media prurience rather than the opportunistic swipe at a family's tragedy it was portrayed as in the press.
But he has little time for performers who set out to do no more than shock - and worries that the internet makes it harder to distinguish well-intentioned satire from cheap nihilism.
"A comedian like Stewart Lee is fantastic because he takes on difficult subjects in a way that is very challenging but is also, ultimately, extremely principled," Dr Double says.
"When you think about someone like Frankie Boyle or Jimmy Carr, the appeal is basically: 'I'm going to say the worst thing I possibly can.' I find that a bit tiresome, to be honest."
And when it comes to subtlety and nuance, Dr Double notes, 140 characters makes life difficult.