You're 40, happily married - and then you meet your long-lost brother and fall passionately in love. This isn't fiction; in the age of the sperm donor, it's a growing reality: 50% of reunions between siblings, or parents and offspring, separated at birth result in obsessive emotions. Last month, a former police officer was convicted of incest with his half-sister - but should we criminalise a bond hardwired into our psychology? Alix Kirsta talks to those who have suffered the torment of 'genetic sexual attraction'

At first, Ivor Lytton's emotional predicament seems unremarkable, no different from the woes that make up any agony aunt's weekly column. On Sunday October 4 1998, Lytton, an Edinburgh public relations consultant, met the love of his life. The meeting took place at a dinner party at a fashionable country inn. Rita Meadows, who lives in South Africa, was on holiday in Scotland. Describing their meeting, Lytton's words overflow with sentiment. "From the moment we met, I was smitten, and continued to be drawn to her like a magnet. As I got to know her, I felt she had given me a life transmission. She put a smile in my heart and a spring in my step." Each October for the past four years, he has sent her a card to commemorate the date of their meeting.

What Lytton didn't know was that the consequences of that love would plunge him into the most devastating crisis of his life. "To say that I have been to hell and back wouldn't be accurate. The situation is far worse than that." On the surface, his ordeal seems a classic case of unrequited love. Despite forming a warm friendship with Lytton, and meeting him several times a year either abroad or in Britain, Meadows, a divorcee, has never shared his feelings. Irritated and, at times, angry with his outpourings of affection, she recently emailed him curtly to think of her as "just a casual friend". Unable to reconcile himself to her indifference, Lytton's subsequent depression and sense of rejection, and the continuing compulsion to declare his feelings, are no different from the irrational fixation and emotions that mark any young adult's first major infatuation. But that is as far as the Mills & Boon scenario goes.

In the circumstances, Lytton's new-found love was doomed to be a nonstarter. He is a married man of 66, semi-retired, with grown-up children and grandchildren. The story might be relatively straightforward, and there may even have been a happy ending of sorts had he merely fallen for a woman half his age or abandoned his family to start a new life in another country. Instead, his love for Meadows represents society's most abhorrent taboo. She is his younger sister, adopted as a baby shortly after the death of their mother from an illness contracted after the birth, when Lytton was two. Although Meadows had no idea she had a brother, Lytton, who was raised by his stepmother after his father's death during the second world war, had always known of her existence, but waited until 1995 to begin searching for her. The 1998 party at which they met had been organised by Lytton, with the help of his wife and children, as a celebratory reunion after he had tracked down his 60-year-old sister in Durban.

"I knew Rita was my sister," he says now. "I didn't choose to fall in love with her, or expect to feel sexual desire. It just happened. Even in front of my wife, I made no attempt to hide my adoration, I just buzzed whenever she was around. It was as if no one else existed. The two biggest mistakes I made were deluding myself that I could become all-important in her life, a brother and a surrogate lover, even though she didn't desire me, and then believing I could control and resolve the problem by myself."

When we met, Lytton brought with him several large files bulging with four years' correspondence, mainly email printouts, to and from Rita. A tall, white-haired and articulate man, he has recorded every emotion, thought and incident involving his sister since their reunion. Photographs taken on his trips to South Africa, and on her visits to Scotland, show a vivacious and elegant redhead, seemingly little older than 40. From the sharp intake of breath as he begins reading aloud from his correspondence, it is clear that his feelings are still raw. Letters written shortly after their reunion begin with such endearments as "my special girl", "goddess", "darling miracle", "my princess". Declaring that she "walks on water", Lytton confesses how much he misses and thinks of her, miserable at the distance between Edinburgh and Durban. The places they have visited together are described as "sacred shrines".

But a persistent undercurrent of uncertainty and despair runs through almost every message, as he urges her to write more frequently and to reveal her own feelings. "I have found it easy to love you since we met, and am totally committed to our relationship. You illuminate my life... let this be our secret. But how important am I to you? Do you feel affection for me?" Breaking off, Lytton's voice cracks. Far worse than the pain, he says, are the shame and guilt. "You see how besotted I was? Every line oozing with obsession. I mean, what normal brother ever spoke to a sister in this way? How can a man approaching 70 experience emotions usually attributed to a screwed-up adolescent? It's sick."

He then produces a diary, one of several in his briefcase, labelled The Journal Of An Emotional Junkie, and offers to lend it to me. He started to keep the journal eight months ago, after discovering that his sister had begun a relationship with a 40-year-old South African banker. He became intensely jealous - an emotion, he stresses, that is virtually alien, and therefore deeply shameful, to him. In one revealing passage he fantasises about his sister having sex with her new lover. "On a visit, she'd shown me some sexy underwear she'd bought in London, including a thong. Once she left England, I visualised her gyrating around a pole, in a G-string, her boyfriend watching lustfully on the bed."

Although this is the first time Lytton has told any of this to a stranger, he feels that, by doing so, he is beginning to control and resolve the situation. "I'm letting you inside my head. Perhaps my experiences can help anyone else in a similar predicament, let them realise they are not alone, that they aren't going mad and haven't turned into some sick, perverted individual - all of which I thought until very recently."

In the past year, Lytton says, he came close to wrecking his marriage, having a complete nervous collapse, even committing suicide. What saved him was his sister's emotional detachment, his wife's extraordinary patience and understanding - and, most crucially, learning about a little-known phenomenon called genetic sexual attraction (GSA), increasingly acknowledged by post-adoption agencies to be a common feature of reunions between blood relatives who have never before met. "I seem to have contracted this condition, GSA, in its severest form," he declares, as if describing a virus. "Now that I know there is a condition, and why it occurs, I feel I have reached a turning point and will be able to work towards building a normal, balanced relationship with my sister." If, as seems possible, he comes through the crisis with his marriage, mental stability and relationship with his sister intact, Lytton will be in a fortunate minority.

The term GSA was first coined in the US in the late 1980s by Barbara Gonyo, the founder of Truth Seekers In Adoption, a Chicago-based support group for adoptees and their new-found relatives. The emergence of GSA both in the US and the UK coincided with the relaxation of adoption laws in the mid-1970s, which gave adopted children easier access to their records and led to an increase in the number of reunions between adoptees and their blood relatives.

The unexpectedly high number of reported cases of men and women struggling with sudden and terrifying emotions after a reunion has surprised and perplexed most post-adoption agencies. So far, because of the taboos surrounding GSA and its variable and complex nature, the frequency of these cases is almost impossible to quantify, although some agencies estimate that elements of GSA occur in 50% of reunions. Growing awareness of its potentially devastating implications, especially in cases where relatives embark on a sexual relationship, has prompted some organisations to warn all clients attempting to trace a relative about the phenomenon, while also training counsellors to recognise the warning signs and to help adoptees and their families cope with the damage.

These may sound like important and timely advances but they don't, in fact, add up to much. Because of the revulsion aroused by incest, and the stigma attached to anyone who admits experiencing GSA - let alone those who embark on sexual relations with a parent or sibling - the condition remains obscured by myth, tainted by smutty innuendo, under-reported by sufferers and, worse, virtually ignored in academic circles. Although, occasionally, a story involving GSA is given predictably lurid tabloid coverage, ignorance prevails. Why GSA occurs only in some reunions, whether certain people are more predisposed to GSA than others, or whether it manifests itself differently between parents and children or siblings, is simply unknown. Above all, GSA raises serious questions about what factors influence sexual attraction: are the origins of GSA social, environmental or biological?

The lack of any serious scientific research is especially disturbing in view of the growing number of reunions between adoptees and their birth parents, and the prospect of many future reunions between children born through IVF involving sperm and egg donors. In the view of Sue Cowling, deputy director of the Post-Adoption Centre, "Genetic sexual attraction associated with IVF births is a time bomb waiting to go off." Cowling, like many professionals, suspects that the subject has remained a no-go area, even for psychologists, because even in a society wide awake to the spectre of paedophilia and sexual abuse in families, GSA - which falls into neither category - threatens to explode too many cosy assumptions about "normal" and aberrant sexual instincts.

Gonyo, the non-academic who originally "outed" GSA in the 1980s, has written the only book on the subject. In it, she suggests that romantic love and erotic arousal may be the delayed by-product of "missed bonding" that would have normally taken place between a mother and her newborn infant, or between siblings had they not been separated by adoption. "Many such people, as adults, need to go through that early missed closeness. It may become sexual, or it may not."

Gonyo's reputation as the world's leading GSA "expert" came about largely as a result of her own experience of strong sexual attraction, when, in 1979 and aged 42, she was reunited with her adult son 26 years after she had given him up for adoption. Now a 65-year-old grandmother, she admits, like Lytton (whom she has been counselling by email since he contacted her via the Truth Seekers website), that what saved her marriage and allowed her eventually to build a healthy relationship with her birth son Mitch was that she did not have sex with him, due to his unresponsiveness.

An energetic, cheery and straight-talking woman, Gonyo estimates that it took her a dozen years to overcome the desire to sleep with Mitch. "Believe me, the state of arousal, which grew as I got to know him, was as erotic as anything I felt for my husband. I wanted to get naked with Mitch, feel his flesh against mine. The first time I hugged him, it beat any feeling I've experienced in my life. If he had felt the same way, I don't know if I could have stopped myself. But Mitch was very afraid of my feelings, and wouldn't ever talk about any of this, or how he felt."

At that time, Mitch, an art teacher, had various girlfriends. "Despite this, my behaviour around him was atrocious. I was flirtatious, coquettish and playful. When getting ready to see him, I primped and primed, becoming like a 16-year-old in mind and body. I was trying to win him over, like someone I wanted to date or marry." Gonyo recalls feeling ashamed and dirty. "At the beginning, the urge was less erotic, more like bonding with a newborn child. As with all my subsequent children, I wanted to smell him, stroke and run my fingers through his hair. I saw so much of myself in him, and he also reminded me strongly of his father, my first teenage love." But having experienced that primary stage of "delayed bonding", Gonyo wanted more. "I was no longer looking for the baby, I wanted a relationship with the adult - the man." What frightened her was that these emotions did not fit into any appropriate context. "I wasn't Mitch's lover or girlfriend, and I couldn't be his mother, because he had one, although he never allowed me to meet her. I felt like an intruder, unimportant and humiliated."

When Mitch got married 12 years ago, Gonyo finally established a relaxed friendship with him. "It's as if I've turned him over to his wife, so now we can be friends. It took me until then to be able to say honestly that I don't have those sexual feelings any more. What meeting Mitch taught me was self-control." It also led to her passionate "mission" to encourage widespread understanding of GSA.

Twenty-five years ago, that would have seemed an absurdly unrealistic goal given that this realm of human desire was guaranteed to repel most people, including Gonyo's clients. Since then, not much has changed. "GSA becomes an incest issue, whether or not it is carried out in a sexual act," she says, pinpointing the most likely explanation for the paucity of research. "Most people will only reveal their own situation once someone else breaks the ice." Gonyon recalls that, when she told a support group for adoptees and birth mothers about her own feelings on meeting Mitch, her disclosure was met with repugnance. "Some openly dismissed such feelings as 'sick'." But, a few days later, she was phoned by the wife of one of the group's male participants, telling her that she was convinced her husband was similarly infatuated with his birth mother, whom he had recently met, and asking Gonyo if she could help him.

It wasn't until almost a decade later, when Gonyo became the director of Truth Seekers In Adoption and raised the issue of GSA, that others began plucking up the courage to confess their own "forbidden" attraction to a parent, adult child or sibling with whom they had been reunited. She vividly remembers the first time someone raised their hand in one workshop. A man in his 30s, he was the first person she saw stand up in a room full of people and speak the unspeakable. "He simply said, 'I slept with my mother. I was 21 when I found her. We were very much in love. After several years, it stopped.' His mother had ended the relationship because it was too painful for her; she felt guilty and was afraid of being discovered. That was more than 10 years ago, and he said he'd not only lost his lover but what was even more important: his mother. He said he had never regretted having sex with his mother, only that losing her was a high price to pay."

Many clients consult Gonyo privately, even anonymously, by phone or email. "Often, the attraction isn't sexual, but it's still frightening and alien, and therefore perceived as abnormal and sinful. One woman told me that she and her birth mother, soon after they met, slept together in the nude: there was no sex, only a strong need to be close as parent and child. Grown men tell me they've sat in their mother's lap, just being rocked and held. One man talked about his need to be sexual with his newly found brother, but not being homosexual they shared a woman instead."

Sometimes, she warns, there may be an underlying element of revenge: "One man admitted openly, 'My real mother fucked me over. Now I'm going to fuck her.'" In contrast, many others experience an almost primordial sense of having "belonged" to the other person all their life. For Gonyo, the recognition that she shared her son's sarcastic humour and artistic talents, and saw the "male side" of herself in him, were especially powerful. (This sudden "shock of familiarity" is often also commented on by twins separated at birth.)

Gonyo is not surprised that attraction between fathers and daughters should be the least reported variant of GSA. "That group tends to stay very silent. It's still regarded as dangerously close to abuse, even though it is no different from other forms of GSA." But it does happen: a woman who recently traced her non-identical twin daughters, and included the birth father at the reunion, was horrified when one of the daughters and the father became instantly attracted to one another. As is common in cases of parent-child GSA, the attraction involved the girl who most resembled her father. "It's like my meeting Mitch: for a woman, meeting your father is to meet the male aspects of yourself for the first time."

Sometimes, that recognition may act as a powerful aphrodisiac. A young woman in one of Gonyo's workshops confessed that she had been in love with her father, and he with her, since their reunion. Only after they both suffered mental breakdowns did they have sex. "Sadly, although they imagined it would be cathartic, after sleeping together they felt no happier."

Almost invariably, the outcome of sexual relations between reunited relatives is that any subsequent lasting relationship, platonic or otherwise, is doomed - a dilemma illustrated in Kathryn Harrison's 1997 memoir The Kiss, a frank, sometimes shocking, account of her affair with her father.

That awareness has led New York psychotherapist Joe Soll to adopt the term "genetic attraction", believing the word "sexual" is in many cases inaccurate and also responsible for the underlying shame and fear that make the condition so distressing. He has noticed that the "romance" that develops, especially when mothers meet their adult children, mirrors the sensuous bonding between a new mother and her baby. "These people regress to a very early stage of development. The relationship is sensual, but we don't call it 'romance' or being 'in love' when it's breastfeeding, cradling and stroking, or when it's a mother and baby gazing into one another's eyes. Often, people tell me all they want to do is snuggle up together. A woman reunited with her adult son felt an overwhelming urge to suckle. There's an urge for intimacy, which they were previously denied."

Where that urge leads to sex, which appears to be more common between brothers and sisters, Soll and Gonyo encounter the greatest desperation and refusal to exert self-control. "I'll get calls from clients asking me in which of the American states they would be allowed to set up home together, or even get married," says Gonyo. "It doesn't matter what age they are - when your hormones are raging, you don't think about the consequences."

GSA rarely features at conferences on adoption, however, because the big US institutions, such as the National Council For Adoption, are reluctant to recognise the phenomenon. On the rare occasions that workshops do take place, they are usually packed. Unlike the UK, most US states are opposed to "open" adoption, so adoptees' records are sealed, which presents a formidable obstacle to family research. Gonyo, meanwhile, is convinced that the more an adopted child is told about their original family, and sees photographs of parents and siblings indicating shared characteristics, the less likely this will come as an intense shock if they meet. "Although it is better to play in the sandbox as kids than in bed as adults," she says, "the authorities prefer to ignore GSA, so it remains confused with real incest, rape and child abuse."

But in the light of the confessions and desperate actions of some "victims" of GSA, isn't it understandable that such confusion exists? Understandable, perhaps, but not inevitable. In contrast to America's squeamishness in addressing the issue, by the early 1990s British post-adoption agencies such as Norcap, the Child Migrants Trust and the Post-Adoption Centre were already admitting that, far from being either unique or bizarre, or a sign of deviance or emotional disorder, GSA was an all too normal reaction to an extreme emotional situation - and more commonplace than supposed.

Not that this makes it any easier to understand. Today, the Post-Adoption Centre, which offers practical information and counselling at any stage before, during and after adoptee reunions, and sees 3,000 new clients a year, estimates that up to half of reunions are accompanied by anything from temporary attraction to obsessive sexual obsession - and, very occasionally, even to the birth of a child.

At their most extreme, such relationships can have dangerous and potentially tragic consequences for families, especially spouses. In a recent, well-publicised case, a mother of two, Jennifer Grant, and her adopted half-brother, John Shannon, a former mayor of Pickering, North Yorkshire, left their respective spouses and children and set up home together after being united for the first time in 46 years. Interviewed by a Sunday newspaper in 2001, Jennifer's husband Graham, whose physical resemblance to John is striking, talked about his ordeal, typical of casualties floundering in the riptide of such obsessions. "I asked her if she had gone to bed with him and she said they had. I just left the house, got in my van and drove. I wanted to do myself in. Then I thought of the boys and what it would do to them. When I got back, she had gone."

Graham Grant reportedly cannot come to terms with what has happened: "It fills me with shock and dread," he said. "Members of our family have found it hard to discuss the matter with me. There's a sense of shame and disgust. It's left me feeling like a leper." Although, reportedly, there was a police inquiry into his wife's relationship with Shannon, with which they co-operated fully, the investigation ended due to lack of evidence that any crime had been committed.

Under the Sexual Offences Act of 1956, sexual intercourse between a brother or even a half-brother and sister is an offence that carries a maximum prison sentence of seven years. Although he escaped imprisonment, Scarborough police officer Tony Smedley's nine-month affair with his half-sister Janet Paveling tore apart his life. When Smedley's colleagues accidentally came across love letters that clearly referred to their sexual relationship, he and Paveling were arrested and committed for trial on charges of incest. He pleaded guilty when the case came to York crown court last month, and received a conditional discharge; the charges against Paveling were not pursued. Even so, Smedley lost his job, and must now try to rebuild a future with his wife and children, and with his sister's family. Another British brother and sister, Kim Straker and Terri O'Neill, who lived together as a couple and eventually had a child, were taken to court in the early 1990s. They were given suspended sentences and allowed to keep their daughter; they have since parted.

Even where such relationships do not end in turmoil and trauma, the effects of the taboo itself remain inescapably powerful. One of the strangest cases in recent years is that of Gary Klahr and Micka Zeman, who met in 1979 in their Connecticut hometown and enjoyed a casual six-month affair. In 1998, by which time both were in their 50s and married, Micka, knowing she was adopted, had traced her biological parents and found that she was one of 13 children born to the same couple, nine of whom were given up for adoption to couples in the area - one of them was Gary Klahr. On realising that she had had sex with her brother, Micka was physically sick. "Although it was brief and we were not that involved or serious, it was a shocking revelation and I was filled with tremendous guilt and sadness. I have since forgiven myself and realise it was foolish to feel guilty: after all, we did not know we were brother and sister, and when we re-met in 1998 the chemistry had long disappeared."

Although the news also shocked Gary, he maintains that someone less emotionally well-adjusted might have suffered greater damage. "A person with a different personality might have jumped out of the window, or at least had profound long-term feelings of guilt. But if you understand that nine out of 13 children from the biological family were adopted out to different families, with different names and different religions, within a 15-mile radius of the hospital where we were born, then something like this was bound to happen. I never had an idea, until 1998, that I was adopted: how could we have known that we were brother and sister?"

Cowling says that neither the threat of prosecution nor the suffering of families are a deterrent to those caught up in and determined to pursue such relationships. "I've heard women, including mothers sleeping with their sons, tell me, 'It's the most amazing sex I've ever had. Don't ask me to give it up - I can't.'"

When the relationship becomes obsessive and violent, especially between mothers and sons, the danger seems only to heighten the sexual chemistry and magnetic bond. "I've worked with cases where the violence has been terrifying because one person becomes fixated, phoning their relative 10, 12 times a day, demanding to know their movements, stalking them like a jealous lover," says Cowling. "But the other person still can't pull away. It's like an addiction." She cites a man who was imprisoned for violence against his mother: "The woman went into hiding, but he found her. We were getting panic-stricken calls from her at all hours, saying, 'He's coming for me, what shall I do?' Yet she, too, was obsessed with him. In another instance, a woman referred to her son as 'my lover' and talked of her body 'aching' for him. Unfortunately, for some men, the sex and violence is a way of punishing the birth mother for abandoning them, and for mothers the sex is a guilt trip: they feel they owe it to their son after giving him up for adoption."

The dilemma originally faced by professionals such as Cowling was that everyone wanted to help, but no one knew how. In 1992, Dr Maurice Greenberg, a consultant psychiatrist, head of student counselling services at University College London and former adviser to the Post-Adoption Centre, conducted what, incredibly, remains the only academic study into GSA. He interviewed eight male and female adoptees and analysed another 40 cases, including birth parents, reported by the Post-Adoption Centre; the objective was largely to gather information to help guide counsellors. Greenberg, who has the gentle, amiably absent-minded manner that instantly makes you want to tell him your troubles, admits he knew he was entering an unusual and special area and asked the Post-Adoption Centre why it did not simply acknowledge that these people were having incestuous relationships, rather than use the euphemism genetic sexual attraction. But was it really such a euphemism? What Greenberg couldn't foresee was how promptly he would do a u-turn, concluding that the consummation of GSA was "incest" only in the strictest biological sense. Today, he insists that it is essential to distinguish GSA from incest, and especially from child abuse. "There is no force, coercion, usually no betrayal of trust. And no victim. If sex occurs, it involves consenting adults."

He stresses that none of the interviewees who were sexually aroused by or had sex with a parent or sibling considered this incestuous, or that their behaviour was wrong, "But when I asked them if they might ever have similar feelings about members of their adoptive family, they shuddered at the suggestion".

Most interviewees described the period before a reunion as already exceptionally emotionally charged, filled with excitement and fantasies about meeting their relative. Reunions were characterised by so-called "mirroring" - the shock of familiarity and self-recognition on first meeting. Even where there is little physical resemblance, the emergence of shared interests, similar traits, mannerisms and instincts, often subtly transmitted through sense rather than verbal communication, tended to have a profound impact on one or both relatives. Greenberg says that many used the terms "finding a soulmate" and "like looking in the mirror for the first time".

Body odour, too, held an especially powerful attraction: there was, says Greenberg, frequent fascination with a relative's characteristic smell - acknowledged to be a potent factor in both human and animal attraction - as well as the feel of their skin and the sound of their voice. "The sudden, overwhelming sense of falling in love, a profound need for unusual closeness and intimacy, was almost universal. As adults, we have very limited abilities for communicating such intense feelings, and sometimes sex becomes the only familiar means."

The intriguing paradox that Greenberg appears to have uncovered is that, no matter how shocking it appears, GSA is a largely normal response to an extremely unusual situation: blood relatives meeting as strangers. More crucially, the existence of GSA, as distinct from habitual incest and child abuse within families, raises fundamental issues concerning sexual attraction, as well as with the origins of the "incest taboo" - areas that have only recently been the subject of serious research.

No analysis of incest and sexual desire is possible without the shadow of Freud looming over the debate. A new study by psychologists at the University of St Andrews shows that men and women are more likely to choose a spouse whose eye, skin and hair colour resembles that of their opposite-sex parent. Last year, a study by the same team revealed that women with older fathers, and men with older mothers, are usually attracted to older-looking partners. The same principle applies to racial characteristics, and to the smell of an opposite-sex parent. Although the reasons are unclear, one theory is that we are "imprinted" from birth with certain familiar characteristics with which we feel comfortable and to which we are eventually attracted.

However, Freud would have had an altogether different take on it, believing that the Oedipus complex was paramount in determining all sexual behaviour. Freud's theory, propounded in 1897, that every male infant has an overwhelming sexual desire for his mother, and every female for her father, is the cornerstone of psychoanalytic theory. He maintained that these incestuous drives were so powerful that they had to be suppressed. Our transition, between the ages of two and five, from the incestuous Oedipal phase to the post-Oedipal phase, resolves these impulses and, according to Freudian theory, is crucial to healthy human development. By the time we reach the post-Oedipal stage, the incest taboo, Freud reasoned, is indelibly imprinted on the psyche, governing future sexual behaviour.

But how persuasive is this Oedipal theory nowadays? Because Freudian ideas dominated much of the 20th century, what is less well known is that, at the turn of the 19th century, a contemporary of Freud's, the Finnish social anthropologist Edward Westermarck, put forward the opposite view, based not on the theory of natural attraction but of natural aversion. According to Westermarck, children growing up in close proximity are not sexually attracted to each other as adults. Quite the contrary: the "Westermarck effect" meant that overfamiliarity and boredom automatically caused siblings and other close relatives raised together to go out of their way to avoid sexual contact. Westermarck also reasoned that, since we find the idea of sex with our relatives so distasteful, we developed moral codes and laws to ensure that society conformed to this "norm" to avoid any social disruption, shame or discrimination.

Although these ideas were rubbished by Freud for their lack of supportive evidence - despite his own inability to provide a scientific rationale for the Oedipus complex - in recent years evidence confirming the Westermarck effect among humans and other species continues to grow. By revealing more about what lies behind our choice of sexual partners, these findings may hold clues to the "mystery" of GSA.

In one ongoing study of children raised on Kiryat Yedidim, an Israeli kibbutz, between the 1950s and late 1960s, US and Israeli anthropologists were amazed to discover that the sabras - boys and girls of almost identical ages from different families - did not, as their parents hoped and anticipated, marry each other. As one of the first researchers, Melford Spiro, observed in 1958, the intimacy between these children, especially between the ages of seven and 12, could not have been greater. Not only did they shower, sleep and run around naked together and explore each other's bodies, as they approached puberty they began openly to play sex games, including intimate kissing, fondling and simulated, or attempted, sexual intercourse. Despite this climate of sexual freedom, by their mid-teens the girls, especially, displayed signs of shame and became hostile towards the boys, to the point of insisting on having unisex showers. At around 15, the girls became attracted to older students and young unmarried men in the kibbutz, admitting that they saw their peers as "brothers".

In a second phase of the study, when these children had grown up, it emerged that not only had no marriages taken place between any of the sabras from Kiryat Yedidim, and three other kibbutzim, but neither was there a single reported incident of sexual intercourse. Eventually, another team of sociologists analysed the records of almost all known kibbutz marriages, totalling nearly 3,000: in only 16 cases did members of the same peer group marry - and in these cases the couple had met only after the age of six.

In the 1960s, about the same time as the kibbutz studies were being concluded, Professor Arthur Wolf, an American anthropologist from Stanford University in California, travelled to Taiwan to study the effects of child-training methods on child behaviour. He ended up living for long periods in Chinese communities after discovering, by chance, that these had a high incidence of a certain type of arranged marriage - known as the sim-pua, or "minor form" - in which the bride was sent away as a young child by her parents to be brought up alongside her future husband as an adopted "daughter-in-law" of the family.

Wolf, now 70, has spent the past four decades examining the effects of this now almost extinct practice, and revealing its previously unforeseen consequences. "Although the age at which the girl went to the future husband's family was between three and five, in some areas of Taiwan they were under two. Many who entered these marriages were, in fact, nursed by their future mothers-in-law." When Wolf asked some of these surviving mothers-in-law why they did this, he was taken aback by their candour. "They explained that the children weren't treated as daughters: they were referred to as 'little daughter-in-law'. They'd say, 'It's better to raise your son's wife, because she will listen to what you tell her and won't always be talking about your son behind your back.' It was the classic mother-in-law strategy!"

A shortage of suitable brides in these developing communities in the late 19th and early 20th century made this "trade" in girl children an attractive proposition. Wolf discovered that the mothers of infant boys whose next child was a girl preferred to give her away and then adopt someone else's infant daughter as a future daughter-in-law. As in the kibbutzim, the future couple, very close in age, were effectively raised as siblings. Unlike the children from the kibbutz, however, they had to marry - and, as grown-ups, many refused to go through with the marriage, or did so only under threat of severe punishment. Some women, says Wolf, became prostitutes rather than marry their fiancée. And in marriage adultery was rife: "One man promised he would marry any other woman as long as it wasn't his fiancée, although she was very attractive. This was more than lack of sexual interest - it was a complete sexual indifference towards their intended partner, which, as Westermarck claimed, led to disgust and aversion when the act was merely thought of or became a possibility."

Wolf's studies of government records of marriages, divorces and births of everyone in Taiwan prove that, compared with other arranged marriages in which couples were introduced as adolescents, such "minor marriages" were a disaster. His analysis of 25,000 minor marriages found that many ended in divorce and few offspring. Most significant of all is Wolf's discovery that these marriages were spectacularly unsuccessful if the girl had been adopted into her husband's family when aged five or younger. "If she came at three or younger, the sexual aversion and rate of marriage breakdown was very high. After five or six, there wasn't much difference between married couples who met at 16. There is some factor in developmental psychology during the first three years of life that seems critical in determining sexual attraction, but we don't know yet what this factor is."

It may be a long time, if ever, before we can identify the complex interplay of nature and nurture behind the dramas of love and loathing played out in the kibbutz and in Taiwan. Or know precisely at what stage in the sweaty rough and tumble of shared childhood that the potential for mutual sexual desire is switched off. But one thing seems clear: GSA is neither a horror, an illness, nor a perversion. Indeed, given what we already know, might it eventually prove to be not that much of a mystery?

Some names have been changed.