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IT IS an oft-told story, but it does not get any less horrific on repetition. Fifteen years ago, a paedophile enticed seven-year-old Megan Kanka into his home in New Jersey by offering to show her a puppy. He then raped her, killed her and dumped her body in a nearby park. The murderer, who had recently moved into the house across the street from his victim, had twice before been convicted of sexually assaulting a child. Yet Megan's parents had no idea of this. Had they known he was a sex offender, they would have told their daughter to stay away from him.

In their grief, the parents started a petition, demanding that families should be told if a sexual predator moves nearby. Hundreds of thousands signed it. In no time at all, lawmakers in New Jersey granted their wish. And before long, “Megan's laws” had spread to every American state.

America's sex-offender laws are the strictest of any rich democracy. Convicted rapists and child-molesters are given long prison sentences. When released, they are put on sex-offender registries. In most states this means that their names, photographs and addresses are published online, so that fearful parents can check whether a child-molester lives nearby. Under the Adam Walsh Act of 2006, another law named after a murdered child, all states will soon be obliged to make their sex-offender registries public. Such rules are extremely popular. Most parents will support any law that promises to keep their children safe. Other countries are following America's example, either importing Megan's laws or increasing penalties: after two little girls were murdered by a school caretaker, Britain has imposed multiple conditions on who can visit schools.

Which makes it all the more important to ask whether America's approach is the right one. In fact its sex-offender laws have grown self-defeatingly harsh (see article). They have been driven by a ratchet effect. Individual American politicians have great latitude to propose new laws. Stricter curbs on paedophiles win votes. And to sound severe, such curbs must be stronger than the laws in place, which in turn were proposed by politicians who wished to appear tough themselves. Few politicians dare to vote against such laws, because if they do, the attack ads practically write themselves.

A whole Wyoming of offenders

In all, 674,000 Americans are on sex-offender registries—more than the population of Vermont, North Dakota or Wyoming. The number keeps growing partly because in several states registration is for life and partly because registries are not confined to the sort of murderer who ensnared Megan Kanka. According to Human Rights Watch, at least five states require registration for people who visit prostitutes, 29 require it for consensual sex between young teenagers and 32 require it for indecent exposure. Some prosecutors are now stretching the definition of “distributing child pornography” to include teens who text half-naked photos of themselves to their friends.

How dangerous are the people on the registries? A state review of one sample in Georgia found that two-thirds of them posed little risk. For example, Janet Allison was found guilty of being “party to the crime of child molestation” because she let her 15-year-old daughter have sex with a boyfriend. The young couple later married. But Ms Allison will spend the rest of her life publicly branded as a sex offender.

Several other countries have sex-offender registries, but these are typically held by the police and are hard to view. In America it takes only seconds to find out about a sex offender: some states have a “click to print” icon on their websites so that concerned citizens can put up posters with the offender's mugshot on trees near his home. Small wonder most sex offenders report being harassed. A few have been murdered. Many are fired because someone at work has Googled them.

Registration is often just the start. Sometimes sex offenders are barred from living near places where children congregate. In Georgia no sex offender may live or work within 1,000 feet (300 metres) of a school, church, park, skating rink or swimming pool. In Miami an exclusion zone of 2,500 feet has helped create a camp of homeless offenders under a bridge.

Make the punishment fit the crime

There are three main arguments for reform. First, it is unfair to impose harsh penalties for small offences. Perhaps a third of American teenagers have sex before they are legally allowed to, and a staggering number have shared revealing photographs with each other. This is unwise, but hardly a reason for the law to ruin their lives. Second, America's sex laws often punish not only the offender, but also his family. If a man who once slept with his 15-year-old girlfriend is barred for ever from taking his own children to a playground, those children suffer.

Third, harsh laws often do little to protect the innocent. The police complain that having so many petty sex offenders on registries makes it hard to keep track of the truly dangerous ones. Cash that might be spent on treating sex offenders—which sometimes works—is spent on huge indiscriminate registries. Public registers drive serious offenders underground, which makes them harder to track and more likely to reoffend. And registers give parents a false sense of security: most sex offenders are never even reported, let alone convicted.

It would not be hard to redesign America's sex laws. Instead of lumping all sex offenders together on the same list for life, states should assess each person individually and include only real threats. Instead of posting everything on the internet, names could be held by the police, who would share them only with those, such as a school, who need to know. Laws that bar sex offenders from living in so many places should be repealed, because there is no evidence that they protect anyone: a predator can always travel. The money that a repeal saves could help pay for monitoring compulsive molesters more intrusively—through ankle bracelets and the like.

In America it may take years to unpick this. However practical and just the case for reform, it must overcome political cowardice, the tabloid media and parents' understandable fears. Other countries, though, have no excuse for committing the same error. Sensible sex laws are better than vengeful ones.