Joan Smith: Women are abused because men like Fritzl are believed

Even after the singular horrors of the 20th century, the case of Josef Fritzl has the power to shock. It seems unbelievable that an apparently ordinary man could imprison his own daughter in a home-made dungeon for 24 years, raping her thousands of times and forcing her to give birth to seven children, one of whom died as a result of his neglect.

But it's too easy to draw parallels between Fritzl's crimes and the Holocaust, despite the existence of pictures of women from his home town in Austria welcoming the Nazis in the 1930s. But the fact that such atrocities took place during the Second World War should have shattered our illusions about what some human beings are capable of doing.

Fritzl was convicted of rape in 1967 and served a short prison sentence. He'd already come to the attention of the authorities for indecent exposure, and the rape should have flagged up the fact that he was a serial sex offender, placing him on an offenders' register for life. Instead, his conviction for rape was wiped from the record and the right questions weren't asked when one of his daughters disappeared without trace in 1984.

Even worse, his explanation was believed when babies started turning up on his doorstep, left there by the missing daughter – so he said – who had supposedly joined a cult.

This transparent fairy story should have launched a major inquiry into the young woman's whereabouts and the paternity of the babies, with Fritzl as the chief suspect. But police in too many countries – the UK isn't any better, as the case of the London taxi rapist John Worboys showed 10 days ago – are staggeringly credulous when faced with a confident serial sex attacker. Most of their scepticism is directed towards victims, and time and again rape complaints don't get to court because of minor discrepancies in a woman's version of events.

Fourteen of Worboys's victims went to the police, saying they'd been assaulted or thought they'd been drugged after getting into a black cab, but the cops didn't even notice a pattern of offences. When a young woman identified him, they believed his story that she'd kissed and cuddled him of her own free will. Despite her denials, he was released to carry out more attacks.

Poor Elisabeth Fritzl wasn't able to tell her story to the police for 24 years, much of it endured in total darkness while she waited for her torturer to return. She finally had her day in court last week, overcoming what must have been scarcely imaginable feelings of revulsion to confront her father. After seeing her and hearing her harrowing videotaped testimony, Fritzl changed his plea to guilty on all counts, including murder and enslavement.

I'm unconvinced by claims that he suddenly realised what he had done; faced with such damning evidence, it seems much more likely that he knew the game was up.

There has been endless agonised discussion in recent years about sex crimes, and the difficulty of getting convictions. All sorts of explanations have been advanced, from basic mistakes in policing to the nature of rape trials (one person's word against another) and the reluctance of juries to convict.

I've never believed that all men are rapists, and I'm sure as many men as women have been sickened by the details of Ms Fritzl's ordeal. But this horrible case shows once again that some men can dehumanise women, turning them into objects for their depraved pleasure. The police should know this by now, and stop behaving like big softies towards men whose lies are as flimsy as their moral sense.