About 20 years ago, I gave a talk about sexual abuse to the RAF. At the end, a young airman came up to me and said, "It's not just men, you know," before hurriedly walking away. That pulled me up sharp. Up till then, like most people working in the area of sexual abuse, I'd always assumed the abusers were men.

This just isn't so. We can't be sure of the precise prevalence of sexual abuse by women, as there hasn't been enough research into the subject. Academics have just assumed it doesn't happened. But conservative estimates suggest that 5% of girls and 20% of boys who have reported being abused have been abused by women. From my own research – I have had 800 cases reported to me – I believe the more likely figure is that it is 20% of all sexual abuse that is done by women.

It is women themselves who have done most to propagate this conspiracy of silence. It has almost become a feminist axiom that it is men who are to blame for abuse and that if women are in some way implicated, it is only because they have somehow been forced or controlled into doing so against their will. Again, this turns out to be completely incorrect: 75% of the cases reported to me involved women acting on their own.

But the stereotype is still perpetuated. Partly, this is because most people have little understanding of what abuse is. They look at women and think, "How could they abuse someone if they don't have a penis?" Yet most abuse on small children is done using fingers. There was one case in Cornwall where a man reported his suspicions that his female partner was abusing their child – yet it was the man who was investigated by social services.

Like most male abusers, female abusers tend to have been abused themselves as kids. However, that does not mean abusers have no choice in their behaviour: everyone who is abused has a choice of either taking it out on someone else, taking it out on themselves or trying to make sense of the experience and working though it. Paedophiles often try to claim their attraction to children is a sexual orientation, like homosexuality or bisexuality; it isn't – it is a learned deviant behaviour.

Women abusers are also treated very differently by the media. If, as in the current Plymouth case, a woman is accused of abusing very young children, then she is likely to be far more vilified than if she were a male. It is as though we don't really expect any better from men, but from a woman, it is the ultimate taboo.

Yet, when women are accused of abusing teenagers, a very different picture emerges. Women become Mrs Robinson characters – temptresses – and there's an unspoken assumption that the child somehow "got lucky". You can see that in newspaper headlines, such as "Lessons in Lust", that was used recently in the Sunday Times. There's a collusion that implies that it's somehow not as serious if there is a woman involved. People are more inclined to look for excuses – "the teenager came on to her and she couldn't help herself" – and don't apply the same rules as they would for men.

We need to rethink how we understand sexual abuse. We've managed to drive most men away from primary education by treating them as if they were potential paedophiles, comforting ourselves with the knowledge that our children were being looked after by natural carers. But we have to ask ourselves if the children in Plymouth who, it is alleged, have been abused by a female nursery worker might not have been better-off if they had been looked after by a man.