The cartoon isn't as amusing as it once was. "On the Internet, nobody knows you're a dog," one Web-surfing canine barked to another in that 1993 classic from The New Yorker. Back then, of course, at the innocent dawn of the Internet Age, the idea that we might all be anonymous on the Web promised infinite intellectual freedom. Unfortunately, however, that promise hasn't been realized. Today, too many anonymous Internet users are posting hateful content about their neighbors, classmates and co-workers; today, online media is an increasingly shadowy, vertiginous environment in which it is becoming harder and harder to know other people's real identities.

Those of us who have been flamed by faceless critics in online discussion groups are intimately familiar with the problem. This isn't illegal, of course, because online speech — anonymous or otherwise — is protected by both the First Amendment and by the Supreme Court's much-cited 1995 McIntyre vs. Ohio Elections Commission ruling protecting anonymous speech. But is today's law adequately protecting us? What happens, for example, when anonymous Internet critics go beyond rude and irremediably blacken the reputations of innocent citizens or cause them harm? Should there be legal consequences?

The most notorious case is certainly the cyber-bullying of Megan Meier, a 13-year-old girl from a St. Louis suburb. In 2006, Meier, a troubled, overweight adolescent, became embroiled in an intense, six-week online friendship with "Josh Evans" on MySpace. After "Josh" turned against Megan and posted a comment that, "The world would be better place without you," the girl hanged herself. Later, when it became clear that the fictitious Josh Evans was Lori Drew, a 47-year-old neighbor and mother of a girl with whom Megan Meier had argued, there were calls for a criminal prosecution. But the St. Charles County Sheriff's Department didn't charge Drew; its spokesman said that what she did "might've been rude, it might've been immature, but it wasn't illegal."

Fortunately, the Meier suicide is making officials get more serious about holding anonymous Internet users accountable. In Los Angeles, federal prosecutors reportedly were exploring whether they could charge Drew with defrauding Beverly Hills-based MySpace. In Missouri, the St. Charles County Board of Aldermen passed a law making Internet harassment a misdemeanor punishable by a fine as much as $500 and 90 days in jail. And a Missouri state representative introduced legislation that could criminalize online harassment and fraud.

Online free speech fundamentalists, no doubt, would cite the McIntyre ruling in any defense. Yet that was a ruling focusing on anonymous "political speech"; Justice John Paul Stevens' opinion for the court cited the example of the Federalist Papers, originally published under pseudonyms, as proof that anonymity represents a "shield from the tyranny of the majority" and is, therefore, vital to a free society. But such a defense doesn't work for cases like the Meier suicide, in which the anonymous speech was anything but political.

The Web 2.0 revolution in self-published content is making the already tangled legal debate around anonymity even harder to unravel. Take, for example, the case of Dr. Lisa Krinsky, president of SFBC International, a Miami-based drug development company. In 2005, Krinsky's professional and personal reputation was so vilified by anonymous critics on Yahoo message boards that she pursued a lawsuit (Krinsky vs. Doe) to subpoena the real names of 10 of her online tormentors.

Or take the case of a couple of female Yale Law School students whose reputations have been eternally sullied on an online bulletin board called AutoAdmit by "Sleazy Z," "hitlerhitlerhitler," "The Ayatollah of Rock-n-Rollah" and others. Having been publicly accused of lesbianism with the dean of admissions at Yale Law School, possessing "large false breasts" and indulging in exhibitionistic group sex, the two women filed an amended complaint (Doe vs. Ciolli) in U.S. District Court in Connecticut against the operator of AutoAdmit to reveal the identities of the anonymous critics and take down their libelous posts.

It is troubling that judges in both cases have failed to rule in favor of these victims of anonymous defamation. In the Krinsky case, a California appeals court ruled last month that her accusers had a First Amendment right to speak their minds. Although Doe vs. Ciolli (filed in June 2007) has yet to be ruled on, the plaintiffs had to drop Anthony Ciolli, the law student in charge of AutoAdmit, from the suit. This is because the law treats Web sites differently from traditional publishers in terms of their liability for libelous content. In Section 230 of the 1996 Communications Decency Act, Congress granted Web sites and Internet service providers immunity from liability for content posted by third parties. So a paper-and-ink newspaper can be sued for publishing a libelous letter from a reader, but, under Section 230, Web bulletin boards like AutoAdmit have no legal responsibility for the published content of their users. Thus the students are now pursuing the identities of their defamers independently of AutoAdmit — a near impossible task given the sophistication of today's software for disguising online identity.

All three of these cases indicate that the U.S. Supreme Court soon might need to rethink the civic value of anonymous speech in the digital age. Today, when cowardly anonymity is souring Internet discourse, it really is hard to understand how anonymous speech is vital to a free society. That New Yorker cartoon remains true: On the Internet, nobody knows you're a dog. But it is the responsibility of all of us — parents, citizens and lawmakers — to ensure that contemporary Web users don't behave like antisocial canines. And one way to achieve this is by introducing more legislation to punish anonymous sadists whose online lies are intended to wreck the reputations and mental health of innocent Americans.

Andrew Keen is the author of "The Cult of the Amateur." He wrote this commentary for the Los Angeles Times.