Joan Smith: We stereotype sexual predators, and we get it wrong

Vanessa, Colin and Angela: they sound like inoffensive neighbours whom you might invite round for a drink. In fact, they're three of the worst paedophiles ever convicted in this country.

Sickening details of their assaults on babies and toddlers emerged at Bristol Crown Court last week, when Vanessa George, Colin Blanchard and Angela Allen admitted 38 charges of child abuse. They appear to have made contact through the internet, and engaged in a competitive form of exhibitionism which involved exchanging sadistic fantasies and pictures of children and babies being abused.

Such cases inspire revulsion and agonised soul-searching about what kind of society we have become. As in the case of the murder of the toddler James Bulger, aspects of human nature are revealed that seem scarcely credible, reinforcing anxieties about a "broken" society which is, on occasion, incapable of protecting its most vulnerable members.

When one of the perpetrators turns out to be a 39-year-old woman, the mother of two daughters who passed a criminal records bureau check to get her job at Little Ted's nursery in Plymouth, the public reaction is disbelieving as well as outraged; on Thursday the judge explicitly rejected the suggestion that Blanchard was the prime mover in the assaults, although George and Allen seem to have regarded each other as rivals for his attention. They all objectified their victims to an extraordinary degree; the infants in the pictures taken on George's mobile phone haven't been identified, leaving parents who used the nursery in the agonising position of not knowing if their children were abused.

Women don't often feature as perpetrators in cases of rape and sexual abuse involving adult victims, although it isn't unknown for them to be charged as accomplices. (Who could forget the US army reservist Lynndie England posing next to naked Iraqis at Abu Ghraib prison?) But it's known that a proportion of adults with a sexual interest in children is female.

Last week's events – not just this horrific case, but the furore following the arrest of the director Roman Polanski, who jumped bail more than 30 years ago after pleading guilty to statutory rape – highlight something that is persistently overlooked to the detriment of victims: the extent to which sexual predators of both sexes depend on cultural myths to get away with their crimes.

Every parent is on the alert these days for unshaven middle-aged men who hang around outside schools, but George doesn't look like a paedophile; she has blond hair, and unsuspecting parents described her as big and bubbly. Polanski's friends rushed to defend him after his arrest, demanding his release on a variety of spurious grounds. But what they were really saying was that an Oscar-winning director couldn't be guilty of abusing a 13-year-old girl. George's respectable facade has produced a very different result, increasing public fury about what was undoubtedly a sickening breach of trust.

The underlying problem is the same, however, and it has huge implications for the safety of vulnerable adults, children and babies. Public notions about sex offenders haven't caught up with reality: it is still wrongly assumed that "ordinary" men can't possibly be sex-attackers – hence the shamefully low rate of convictions for rape – and that almost all paedophiles are gay men.

Sexual predators may share common factors, such as having been abused themselves when they were children, but making unjustified assumptions based on gender, class, sexual orientation or public achievements allows abusers to remain free.