The cartoon's gags include an emaciated Ethiopian boy named Starvin' Marvin, recurrent jokes about animal sexuality and flippant depictions of child abuse.

A recent episode brutally mocked Scientology, basically implying that every believer is a naive idiot. Jesus Christ makes frequent appearances on the show, most memorably when he defecated on President Bush.

A sensitive soul watching "South Park" since its 1997 debut would've been offended several thousand times by now, a fact that doesn't seem to bother Comedy Central much, although the network has recently declared one topic taboo.

It might be OK to show Jesus defecating, but network censors refused to depict the Islamic prophet Muhammad "just standing there looking normal," as an Internet commentator put it.

In the episode in question, Americans fearful that Muhammad would be depicted by the Fox network ran around searching for enough sand to bury their heads in, hoping that if they did so the problem of Islamic radicalism would go away. At the moment in the episode when Muhammad was to be shown, viewers saw the following message: "Comedy Central has refused to broadcast an image of Muhammad on their network."

They refer to recent riots prompted by a Danish newspaper's decision to depict Muhammad in political cartoons and the subsequent decision of radical Danish imams to use the episode to stoke violence.

In a quickly globalizing world where Western countries have taken in hundreds of thousands of Muslim immigrants, religious taboos we've never had to worry about before suddenly have the potential to stoke offense in immigrant communities, trigger domestic disturbances and exacerbate international tensions.

These facts necessitate that we consider the cultural norms of Muslim immigrants as we decide how to behave, whatever our ultimate conclusions. In the Danish cartoon controversy, for example, most American newspapers refused to reprint the cartoons even after they became newsworthy, arguing that doing so would needlessly offend the sensibilities of Muslim readers. It's a defensible position.

But the "South Park" episode's censorship is different. It is significant because Comedy Central clearly hasn't any qualms about offending religious groups. Rather, is has resorted to censorship out of fear that Islamic radicals will kill people over the episode.

"The lesson is that if you want your religion not to be mocked, it helps to have a reputation for senseless violence," he wrote on his blog InstaPundit. "Is this the incentive structure we want?"

It sure isn't, but it's increasingly the incentive structure we've got, and that's a dangerous thing if we plan to keep welcoming newcomers with different cultural norms from around the globe.

At New York University, law students sponsoring an event to discuss the cartoon controversy were told that if they displayed images of Muhammad, the university would bar the public from attending.

"If you don't like ideas, don't bother arguing with them. Just threaten to kill people," he writes. "They'll back down. Or at least their booksellers, universities and governments will. How long before other groups take this lesson to heart?"

After all, Islamic radicals don't have a patent on terrorism. If their tactics work, it's only a matter of time before neo-Nazis or environmental radicals or Reconquista groups or anti-abortion radicals or Chinese-immigrant separatists step up their own campaigns of intimidation.

Even if the trend is isolated to Islamic radicals, these events are troubling because Western society needs frank discussion about the threat posed by Islamic radicalism now more than ever. It's surely true that one need not display cartoon images of Muhammad to conduct that discussion. Indeed, I think it's generally a good policy to refrain from needlessly offending religious taboos.

But the "South Park" episode, the Borders and Waldenbooks ban and the NYU event represent self-censorship motivated by fear, censorship that treats groups threatening violence more respectfully than other groups, and censorship that impedes serious discussion about a defining issue of our time.

If Islamic radicals (or any other faction of immigrants) threaten to kill anyone who flies the American flag outside their house, the best response would be for everyone in America to raise the Stars and Stripes. When they threaten violence over an obscure magazine, we shouldn't remove it from bookstore shelves. We should make sure every supermarket and convenience store is stocked with it, too.

Terrorism is a tactic employed only so long as it's effective. If we stand together, refusing to be cowed by fear, it is no longer effective. Let the networks you watch, the businesses you shop at and the universities you attend know that any time they seem to forget, particularly if you value a nation that can welcome immigrants without being intimidated by the most violent among them.

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