In Britain last week, the sad case of a brother and sister who had inadvertently fallen in love and married was revealed by Lord Alton.
The marriage was annulled since such unions are banned in all European countries. The couple had been separated at birth and adopted by different parents.
Growing up, unknown to each other, they subsequently met and only after their marriage did they realise that they were not just distant blood relations, but siblings.
Other similar cases have been recorded round the world, such as that between a Cambodian man and his sister and a German couple, Patrick and Susan Stubing, who have four children, two of whom have developmental problems and all are in care. Patrick has already served a prison sentence for incest and has challenged Germany's law on incest.
This phenomenon has been called genetic sexual attraction (GSA), not because the attraction is genetically determined, but because the people are genetically connected.
The term is reserved for those who meet as adults. GSA can affect parents reunited with children but most commonly siblings, resembling each other, who meet after years of separation, usually since babyhood.
The emotions are described variously as intense, terrifying, electric and are often accompanied by a primordial sense of having been together all their lives.
The term appears to have been coined by Barbara Gonyo, who was unprepared for the attraction she felt on being reunited with her 26-year-old son, whom she had given for adoption as a baby. The relationship was never consummated and the feelings dissipated after he married. She speculated that it resulted from a disruption to the normal bonding process and wrote movingly about it in her book The Forbidden Love.
But why should this attraction affect only those who have been separated at birth and not children who have lived together all their lives?
Freud's theory was that members of families lusted after one another and that this necessitated the creation of an incest taboo.
However, the Westermarck effects, so called because of the studies of an anthropologist of that name, found that when two people live in close proximity in the early years of life, both are desensitised to later sexual attraction.
He disputed Freud's hypothesis and argued that this was the result of an innate taboo that evolved so as to suppress inbreeding with all the genetic complication that this produces.
Interestingly, studies carried out on children brought up in Israeli kibbutzim found that of 3,000 marriages only 14 were between those brought up in these communes and of these none had lived together for the first six years of life, lending further support to the Westermarck theory and to Gonyo's hunch.
Sexual imprinting refers to identification of the characteristics of a desirable mate and is part of normal sexual development. This is believed to be reversed in those brought up together, occurring especially in the critical period between one and six years of age.
So, children reared apart from their family members do not achieve this reverse imprinting. Meeting later in life, the couple may be attracted to each other because of physical, or personality similarities resulting from a common genetic inheritance.
If they are unaware of their blood relationship, the scene is set for a Greek tragedy, since neither the incest taboo nor any personal moral objections to sibling relationships can be activated.
While GSA is reported by some agencies as occurring in 50pc of adoptee reunions, and can lead to both heterosexual and homosexual relationships, the number resulting in marriage has been rare.
This may change in the future, as IVF by donor becomes more common. In particular, the failure to provide the names of donors to their offspring could result in unchecked GSA with all the complications, legal, moral, emotional and medical, that this entails. Inevitably this will lead to a debate about the t remaining sexual taboo.