The new film "Ted," starring Mark Wahlberg, is about a teddy bear who is magically animated through a Christmas wish. The idea of a foul-mouthed children's toy come to life is of course nothing new, and been fodder for many TV shows and films, from horror (Chucky in the "Child's Play" franchise) to comedy (TV's short-lived "Greg the Bunny").

But "Ted" is from "Family Guy" creator Seth MacFarlane, who is known for his edgy humor, and some claim he went too far when one character says to another, "From one man to another, I hope you get Lou Gehrig's disease."

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The joke, weak as it is, recently prompted protests from victims and advocates for victims of Lou Gehrig's disease, or amyotrophic lateral sclerosis (ALS). An online petition urges MacFarlane and the film studio to apologize and donate $200,000 to ALS research.

Is this political correctness gone awry, or valid criticism of insensitivity?

The line between edgy comedy and offense is blurry; Gilbert Gottfried was fired from his job voicing the AFLAC insurance duck for tweeting jokes about the Japanese tsunami while Lady Gaga Tweeted a joke about anorexia in pop stars and registered barely a blip.

In a world where prominent comedians like Sarah Silverman make jokes about rape, where do we draw the line? Are some topics off-limits?

Wishing death or disease upon fellow humans is not nice, as film director Vincent Gallo found while feuding with critic Roger Ebert over his film "The Brown Bunny." Ebert panned an early cut of the film, and in response Gallo said he hoped Ebert got cancer — which he was soon diagnosed with (though it was cancer of the salivary gland and not the prostate or colon that Gallo had cursed).

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National tragedies and disasters have always been the subject of humor; after the Space Shuttle Challenger exploded on national television in the sky above Florida in 1986, one popular joke asked, "What does NASA stand for? Need Another Seven Astronauts." And most of us have heard jokes about the September 11, 2001, attacks.

But unlike jokes about real diseases and real tragedies from people like Gaga, Gottfried and Gallo, "Ted" is different; the target of the curse is not a real person but instead a fictional character.

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In a statement, MacFarlane responded to the criticism: "ALS is a horrific tragedy for those who suffer from it, and by no means do I or anyone associated with this film have anything but compassion for the individuals afflicted. However, the joke in the film is made at the expense of our villain, Rex, and not at the expense of those suffering from the disease."

This distinction — who the humor is at the expense of — is key to understanding the controversy. Some critics thought that real-life sufferers of the disease were being mocked and ridiculed. For example according to an ABC News story an ALS sufferer named Randy Pipkin said "I didn't expect to go to a movie and sit with an audience laughing at the expense of people with ALS… I think the message this film sends out is a huge slap in the face to people dying from this horrific disease."

Traci Bisson, spokeswoman for a group called the ALS Therapy Alliance, said she was concerned that the joke in Ted has created an "alarming trend" of people who believe that Lou Gehrig's disease is humorous.

Note that "Ted" does not have a character with Lou Gehrig's disease who was made fun of in the film; instead an insult between two fictional characters was applied outside the context of the film to real-life situations — and deemed offensive.

Pipkin, Bisson and others are concerned that film audiences will confuse fiction for reality, and think that what happens in a fantasy-based comedy film is appropriate behavior in real life. (To put this in context, "Ted's" main characters engage in many socially inappropriate behaviors, including profanity, reckless driving, prostitution, cocaine use and bar fights — none of which audiences will presumably think are okay in real life.)

Dark humor is nothing new. When I was a teenager in the 1980s, Blanche Knott's series of "Truly Tasteless Jokes" books was immensely popular, and it contained all manner of vile (and sometimes hilarious) humor about race, sex, excretions and dead babies.

There will always be those who find humor in tragedy; some will find it offensive, while others will find it cathartic. Short of censorship or boycotts there's not much that critics can do, since artists won't stop pushing boundaries in search of humor.