Kathryn Harrison's account of an affair with her father is being called tasteless, cynical and irresponsible. But there's one thing her tale is not: unique.
Reviewers have reacted with shock and outrage to novelist Kathryn Harrison's The Kiss, an unnervingly frank account of her love affair with her father, published this week. In the book, she tells the story of her reunion, at the age of 20, with the father she had not seen since childhood, and how their relationship became a physical one. Scenes such as their first kiss - his tongue was "wet, insistent and exploring" - and later, when she lies beneath his substantial flesh as he penetrates her, have provoked charges of cynicism, queasy bad taste and an irresponsible exposure of her husband and children to the glare of the media.

Yesterday in the US, the New York Observer published what it stated was the first interview with Kathryn Harrison's father, a retired Protestant minister whose anonymity the author has protected. He said that, living in a small town in the South, he had seen nothing of the book and was "pretty shaken" to learn its contents - though, according to the newspaper, he did not directly deny that an affair took place.

What has been missed in this furore, however, is that Kathryn Harrison's story is by no means unusual - and not because it is yet another account of one man abusing his young daughter. She was an adult when they met, albeit a vulnerable and emotionally damaged one, and her story is an illustration of one of the best-kept secrets of adoptees who later meet their parents. There is even a clinical name for it: genetic sexual attraction (GSA).

It is 22 years since the Access to Birth Records Act gave adoptees the right to know the identity of their real parents. Since then there have been thousands of reunions - some of them highly publicised, such as Clare Short and her son, or Joni Mitchell and her daughter. What is not well known is that in an estimated 50 per cent of cases the meeting is accompanied by strong feelings of sexual attraction.

It is a sensitive issue, and one that post-adoption counsellors find hard to deal with. How can they maintain a supportive and non-judgemental attitude when dealing with a case such as this?

A 22-year-old civil servant in a long-standing relationship with his girlfriend traces his mother. "It made me feel good. It built up my expectations. I kind of knew I would be attracted to her." They speak on the phone and "love each other's voices". They have lunch. "I've got to touch you," she says. "And she touched my face with her hand. I felt really odd, buzzing, over the top." He has an erection. They walk in the park, hold hands. They finish up in Camden in a dark passage. "We kissed, tinkering [sic] on the edge." The counselling agency suggests he introduce her to his adoptive parents. He cannot decide what to do.

This account comes from one of the few pieces of research into the topic, by Roland Littlewood, professor of anthropology and psychiatry at University College London, and published in the British Journal of Medical Psychology. "What makes these cases so hard to deal with," he says, "is that on the one hand they are breaking a powerful and fundamental taboo, but on the other they are the result of a natural and biological urge."

Littlewood conducted in-depth interviews with nine adoptees - no parents were prepared to be questioned - and uncovered a wide range of behaviours. One married professional woman of 40, for example, was reunited with her sister and at first denied any sexual feeling. "All we want is a cuddle." But then she described her feelings as "just like falling in love".

Eventually they had sex, and now say that although they don't see themselves as lesbians, they would like to live together, and only don't because they both have families.

While Kathryn Harrison still feels bitter about her father, it doesn't always end like that. Another of Prof Littlewood's cases involved a 35- year-old nurse who began kissing with her biological father the first weekend they met. "His skin felt like mine and he smelled like me." They had intercourse frequently over the next year, but the sexual side gradually faded into a "parent/child relationship". On balance, she says, she would rather they had not had sex, "but I got it out of my system".

Although most people who go through an agency to find their parents - and many don't - are shocked to be warned about the possibility of sexual attraction, no one has any idea about how many consummate it. "Usually it happens when something else is wrong in their life," says Sue Greenwood, counsel manager at After Adoption in Manchester. "If they've just broken up with a partner, or if they are particularly needy for some reason, then it's more likely. However, once you've crossed that boundary it can be hard to go back, and the result can be that you lose the relationship all over again."

And that is quite apart from the devastation that an affair with a parent or sibling can wreak in existing families - partners feel wounded, the adoptive family feels betrayed.

Conventional incest, parents with young children, is analysed in terms of an abuse of power. But this isn't the case with GSA. The term was coined by an American, Barbara Gonyo, who launched a support group, Truth Seekers in Adoption, as a way of dealing with a powerful but unreciprocated attraction for her reunited son. Sometimes having sex with a blood relative may be enough to satisfy the urge, she says, "but mostly it is a problem and it hurts like hell".

So given the destruction they can cause, and the revulsion they inspire, why are these doomed liaisons so common?

To begin with, a reunion is a highly charged situation. Expectations, anger, guilt and regret all mingle with echoes of the sheer physicality of childhood. Sometimes a mother's last memory of her child is a brief hug and a touch of skin. "Adults often find it hard to separate the purely sensuous feelings that are normal around small children from the sexual urge towards a grown-up," says Greenwood.

On top of this comes physical similarity, and we know that people are drawn to others who look like them. Reunited parents and children are almost obsessively interested in each other's bodies. They talk about recognising one another's smell, having the same skin or hands or eyes.

"So there's all this intimacy and closeness and powerful emotion around a stranger," says Greenwood. "And what do you usually want to do when you feel very intimate with a stranger? You want to have sex with them."

But while counsellors are concerned about helping adoptees to understand and manage this potent brew of emotions - calling attention to boundaries by referring to "mum" or "dad" is one way - Prof Littlewood is fascinated by these cases because of the light they throw on our sexual mechanisms.

"Freud had us believe that humans are innately incestuous and that the dire act is held in check only by the fear of castration, which in turn leads to the Oedipus complex, the cornerstone of psychoanalysis," says Prof Littlewood.

It turns out that Freud was wrong about this, although the original Oedipus myth points in the right direction. Like modern-day adoptees, it is precisely because Oedipus was separated from his parents at birth that he could find Jocasta attractive. Freud believed that incest-avoidance was uniquely human and the key to our sense of morality, but animal studies indicate that it is rare among mammals in general.

The very process of being brought up together kills off sexual attraction. And we know that the same thing happens with humans. A marriage system in Taiwan is a good test case. The children are betrothed at infancy and live together from the age of one. Recently a large-scale study revealed that such couples regularly fail even to consummate their marriage, and that they have a far higher divorce rate and a far lower birth rate than other couples.

The whole phenomenon of GSA has a number of useful and depressing lessons. The shame and secrecy that surrounds it suggests that we really haven't come far in sexual openness since Freud formulated his theories almost 100 years ago. Second, that the best way to avoid incest in the home is to have a close and physical relationship with children from infancy onwards and to ignore hysterical claims that this can be dangerous or abusive. Finally, it is clear that cases like Harrison's need to be treated with understanding and sympathy rather than pious rejection. She was able to write a book to heal her wounds - few adoptees have that luxuryn