MONTREAL - Women sex offenders often go unrecognized and unreported, although society is starting to pay closer attention to them, says a university criminologist.

"What we've estimated is that females constitute approximately four to five per cent of all sex offenders," says Franca Cortoni, whose new book pulls together the latest research on the phenomenon.

Female offenders are behind about one to two per cent of all sexual crimes reported to police in Canada in the last 10 years, added Cortoni, who works at Universite de Montreal.

The research is contained in the new book "Female Sexual Offenders: Theory, Assessment, and Treatment."

The book, which was co-edited by Cortoni and University of Kent forensic lecuturer Theresa Gannon, looks at female sex crimes as well as treatment options.

They drew on international research in western democracies.

Cortoni told The Canadian Press that society has had a hard time believing women could be capable of sex crimes and often attributed it to mental illness. Many were institutionalized.

"Traditionally, women are viewed as nurturing, the caretakers, the ones who take care of the children," she said.

"Whenever a woman does something that is outside those norms, it's kind of like, 'How can she be rational?"'

But the psychologist said things have started to change as society becomes more open to discussing sexual abuse, child victims are given more credibility, and the abuse is seen as "not an act committed by a mentally ill person but a criminal act."

A Correctional Service Canada spokeswoman was unable to immediately provide specific data on female sex criminals in the country's prisons.

Canada's most notorious female sex criminal was likely Karla Homolka, who was involved in the sex slayings of two Ontario schoolgirls in the 1990s with her then-husband Paul Bernardo.

Citing previous employment with Correctional Service Canada, Cortoni didn't want to comment on the Homolka case but said, "what people don't realize is that the Bernardo-Homolka duet is not unique in the world."

Cortoni said there are no typical female sex offenders, in the same way there are no typical male sex offenders.

"What we've learned over the years with the males is that they come from all walks of life, from all kinds of education and professional backgrounds," she said.

"Right now we tend to see women who come from lower social economic status, less educated."

However, a common trait among many female sex criminals is severe abuse in their childhood.

In some cases, female sex criminals have been found to be passive and dependent, while in others they see men as threats, thus fuelling the need to seek emotional comfort in children.

A New York study found the average age of their female sex offenders to be 28 although Cortoni noted juveniles also commit sex crimes and all of them had been sexually abused themselves.

Studies indicate that while 13 per cent of men who committed a sex crime are likely to reoffend, only about one per cent of women will reoffend, Cortoni said.