Behind all the words, the turning over of facts, the analysis, the frantic speculation (did the wife know?) and the tormented search for meanings (how could this happen?), lies a central image: a woman and her three children buried alive, toothless, hunchbacked, pale-skinned, talking in their own mumbling language, just beneath the surface of everyday life for 24 years.

It is like a Grimm fairytale, like an instant, terrifying myth of monsters and their captives and an image of what humanity is capable of. At the same time it is a tender, damaged little family clinging together, watching TV and drawing pictures of the sun they had never seen in their plumbed and well-lit dungeon in small-town Austria.

The case of Josef Fritzl is beyond the wilder shores of our comprehension and yet it fits with an unsettling neatness to a whole set of domestic stereotypes. It reads both like a work of pornography that brings together every imaginable act of cruelty and degradation, and also like a grotesque cartoon strip of everyday life.

So Fritzl is an electrical engineer with well-polished shoes and a sense of punctiliousness and respectability, who apparently was a 'despot' in his upstairs home, the patriarch with a tyrannical sense of the roles of men and women. Like many men, he had the equivalent of a garden shed where he could retreat and practise his hobby - except in his case the hobby was rape, was imprisonment, was fathering seven children by his daughter, was throwing the child who died into the incinerator like a piece of garbage, was turning his own children into the subjects of some ghastly experiment, was becoming the concentration camp guard of his self-built basement world.

And though his crime was perverse, it was also bizarrely bureaucratic. He applied for, and received, a grant for his underground prison; the building was well planned and expertly carried out; the fridge was stocked with food. Indeed, he is apparently aggrieved that people might think his prisoners could have died when he was on one of his extended sex tourism holidays, insisting that he had always planned ahead. The human mind has an extraordinary ability to defend itself against itself and protect itself from self-knowledge and guilt. Like the camp guards who could go home at night to listen to Mozart, cry at poetry and play with their children, it is perfectly possible that Fritzl feels quite self-justified.

But it's a concentration camp in suburbia, where epic horror meets familiarity. Many women have no idea what their husbands get up to in their garden sheds or down in the cellar with their tools or their developing fluids. Indeed, many marriages are based on respecting the privacy of the spouse and allowing them their own space. Don't ask, don't nag, it's his thing. Fritzl insisted on being undisturbed and apparently his wife obeyed him. The question being asked all round the world is, how could she not have known? But this is the same question asked about wives who don't have any suspicions about their husband's infidelities and about Sonia Sutcliffe of her murderous husband, or Primrose Shipman of hers. They don't know because it's hidden, and they don't know because they don't want to know, and they don't know because they have pushed the knowledge deep inside, and their world is arranged around that denial.

Then there's the stereotype of the father-daughter relationship. Fritzl gives new meaning to the term 'control freak'. Elisabeth was only 11 when he started to abuse her, but it wasn't until she was 18 and an adult that he put her in the literal prison, where he treated her both like his sex-slave and his alternative wife and mother to his second family. He has apparently said that he locked her up because he was worried that she would take drugs and go to the bad: in this crazed version, he becomes the stern but protective father, saving his wayward child from the dangers of life beyond the family. It's a monstrous mirror image of another famous Austrian father, Captain von Trapp, with his seven children and his whistle to keep them in order. And, for many, Fritzl is like a nightmare symbol of Austria itself, a country that has pushed its Nazi past underground and represented itself as a victim rather than a perpetrator.

The fairytale horror is overlaid with provincial order and dull domestic cliché. Home is an ugly grey house with lodgers and an obedient wife; the dungeon guarded by a monster was built on grant money and had running water, tiles and sofas; the pervert maintained his air of respectability throughout and has shown a self-righteous indignation at some of the allegations.

Oscar Wilde's Dorian Gray put the portrait of his moral disintegration into an attic. Perhaps Fritzl's fearsomely efficient compartmentalisation of his life (upstairs and downstairs, open and hidden, propriety and unimaginable depravity) allowed him to avoid looking at the self he has become, a man who will haunt his country for decades. The question of whether Fritzl's wife knew dwindles next to the question of whether Fritzl himself knew what type of loathsome thing he had become.

· Nicci Gerrard is co-author with her husband Sean French of crime novels under the name Nicci French