CLAD in an orange jumpsuit in court, Nancy Garrido looks distraught, dishevelled and utterly confused. In prison, she cries for her "children", the two young girls her husband Philip fathered after years of systematic rape of Jaycee Lee Dugard, abducted by the couple 18 years ago. Make no mistake, Nancy Garrido is the new Rosemary West – a woman destined to become as infamous as her husband for her heinous crime, regardless that he was the driving force behind it.

A woman of Nancy Garrido's description snatched 11-year-old Jaycee from outside her home in California 18 years ago. The child's stepfather Carl Probyn watched in horror as a dark-haired woman grabbed Jaycee as she left for school and pulled her into a car, which then sped away. He gave chase but it was too late – the child had gone.

Police didn't believe his story that a woman had been involved in the abduction. Their suspect was male – it's almost always men who kidnap, rape and then often murder children. Probyn's story was so implausible to the authorities that he himself became a suspect in Jaycee's disappearance. His relationship with Jaycee's mother Terry Probyn deteriorated and he has spoken since of how the allegation defined and destroyed his life.

Meanwhile, Nancy and Philip drove the 11-year-old 200 miles to their home in Antioch, near San Francisco. And there she remained for the next 18 years. Kept sheltered from the world in a secret, leafy backyard, she was raped by at least one of her abductors and gave birth to two children, both girls now 11 and 15.

Nancy and Philip Garrido have both been charged with 29 counts relating to the abduction of Jaycee in 1991, including rape, kidnapping and false imprisonment. The world is shocked and fascinated in equal measure.

"As a society, we have a presumption that a woman could never be as deviant as a man. When a woman is involved in violence towards a child, particularly a sexual attack, these women are regarded as much more deviant than men," says Jonathan Culleton, a lecturer in sociology and criminal justice at Waterford Institute of Technology (WIT).

"When a woman is involved, it's seen as a crime against nature almost – it's such an unnatural thing for a woman to do. What she did is far beyond our idea of what is socially acceptable for a woman. The accusation that Nancy Garrido acted in such a predatory manner is still shocking enough to cause a stir. What she did is seen as so unfeminine. She may well be judged more harshly in court if convicted because her crimes are seen as abhorrent to nature."

Kidnapping and rape

Nancy met Philip while visiting her uncle in Leavenworth prison. Garrido was serving time for kidnapping and rape. Despite this, she agreed to marry him in 1981, while he was still in jail. She moved into their Antioch house before him, caring for his ailing mother until he was released from prison eight years later.

"She was probably a virgin. She was profoundly naïve, coming from an isolated family background that we still know little about. I think she saw Philip as glamorous, worldly and, most importantly, highly sexualised," according to Geoffrey Wansell, biographer of the UK murderer Fred West, who identifies many parallels between Nancy Garrido and Rosemary West.

It's unknown if Nancy shared her husband's sexually deviant leanings or if, over time, she complied with his wishes after sustained pressure and brainwashing – which will be her defence in court.

"Folie à deux is a rare disorder where two people share the same psychosis. That would mean both of them wanted to abduct and rape this child and keep her hostage, that neither was influencing the other to do it," says Wansell. "The other possibility is that she wasn't as deviant as Philip, that he groomed her to be the sorcerer's apprentice. Then, the more she participated, the more she enjoyed it until she became a partner."

Claims that Nancy was brainwashed by her husband are unlikely to hold up in court. Philip violated his parole in 1993 and was sent back to jail for four months. Police believe that Nancy then took over the role of captor until her husband returned.

What drives a woman to abduct a child, keep her captive for almost two decades and be complicit in her repeated rape? "It's simple," says Wansell. "Because she liked the power and it turned her on. I suspect she was involved in the initial sexual assaults."

There are three types of female sexual abuser, according to clinical psychologist Rhonda Turner, who works with the National Organisation for the Treatment of Abusers (NOTA) in Donegal: women teachers who abuse male adolescent students after falling in love with them; women who abuse children after being coerced by a male offender; and women who sexually abuse children entirely of their own volition.

Very few cases of women raping and sexually assaulting children on their own have ever come to light. "Sexual abuse by females we believe is under-reported. Sexual abuse is still predominantly a male offence," adds Turner.

It is likely that Nancy suffered abuse or maltreatment herself as a child, Turner continues. "This could have made her more vulnerable to having a relationship with someone who could control her. Society still sees women as nurturers and carers. We really do not want to believe that some women could be the opposite of that. We have to believe in humanity to get on with our lives. When a woman violates that, sometimes there is a backlash against that woman."

Bizarre

Last month, Melissa Huckaby (28), a former Sunday school teacher in California, was indicted on charges of kidnapping, raping and murdering her eight-year-old neighbour, Sandra Cantu. The child's body was found stuffed in a suitcase 10 days after she disappeared in April. Post-mortem tests found the child, a playmate of Huckaby's daughter, was raped with a foreign object.

"That case is at the top of the bizarre scale. It is the only case I know of where a woman acting alone is accused of raping and murdering a child," says Dr Gary Mears, director of neuropsychology at the University of the Rockies in Colorado.

Mears has evaluated hundreds of sex offenders and is often called to give expert evidence in criminal trials about a person's state of mind. "I've spent a large proportion of my life evaluating serial killers and abusers and their spouses. The overall astonishing principle is that all these women seem to be very different from one another. From a look at Nancy Garrido's photo in court, she could possibly have below average intellectual functioning and suffer from periods of psychosis. How could a smart woman want to be with a man who she met in prison while he was in jail for rape?"

It is also possible that Nancy Garrido, in an attempt to rebuff the sexual advances of her husband, was prepared to allow him to keep a child captive in the back garden to be his sex slave, he adds.

Wansell also wonders about the sexual relationship within the marriage – Nancy never had any children while Jaycee bore Philip two girls – and whether there was competitiveness between Nancy and Jaycee as the girl developed into adolescence.

It's also possible that Nancy couldn't give her husband children and this was the warped justification for their kidnapping the child. "It could be a case of substitution, that she could not conceive and the child was brought in to fill that gap," says Culleton.

Police have searched the couple's home for evidence linking them to the slayings of several prostitutes but nothing concrete has emerged. When their separate trials proceed, Nancy's lawyer has indicated she will claim she was brainwashed and Philip's lawyers are expected to cite a mental disorder.

The maximum prison sentence – which Mears believes both will receive – is 800 years. "What's worse, to kidnap a child and then murder them? Or abduct a child, keep it in captivity, and use the child as your sex slave but let them live?"