Intercourse between siblings over the age of 16 (whether half or full) is a criminal act punishable by a prison term of up to seven years. The law labels the love between once-estranged, consenting adult siblings as incest and says it is wrong, even perverse. But is it? Such cases can no longer be written off as freak occurrences.

With almost half the children of divorced parents losing touch with their fathers within two years of separation, and with the possibility that there will be more stepfamilies than nuclear families by the year 2010, the possibility of siblings meeting as sexually active adults is increasingly likely. If they then experience carnal desires and act on them, can they be regarded as two strangers having sex, or is the blood tie so profound that it should never be tampered with?

'The incest taboo, as regards siblings, appears to have two sources,' says Paul Brown, a sexual and marital therapist. 'The first is genetic and relates to the degeneration of the species that results from in-breeding. The second relates to the psychological health of the individual, and concerns people separating from their families in order to become independent adults. But if siblings didn't grow up together, there is no precedent for boundaries within their relationship, and you can't talk about them being brother and sister in a real sense. And if the couple agree not to have children, then one could argue that the incest taboo is inappropriate.'

Richard Beckett, an expert on sexual deviance, cautions that there is a fine line between consent and coercion. 'The adult sibling may consent, but the context in which sexual contact first developed is crucial in determining whether real consent has taken place. If the seeds of sexual contact are sown as teenagers but come to fruition only in their twenties, you could say that they haven't escaped an abusive relationship.'

Willa Woolston, director of Child Abuse Survivor Network, puts it differently. 'An adult man who has sex with his sister must ask himself, 'Why do I look at my sister and want to have sex with her? If society recognises her as my sister, why don't I?' ' This raises the question as to why brothers and sisters may feel sexually attracted. Is there such a primordial urge? Sigmund Freud thought so. He said that the first love object of the male is 'the image of his mother and perhaps of his sister'. He also argued that normal people outgrow such attachments and find the image of 'these two beloved persons' outside the family.

When siblings meet for the first time as adults, they will not have experienced the Freudian rite of passage, and their shared genes may cause an unusual coincidence of interests that electrify the relationship. Jane Hawksley, a Relate counsellor, says that 'we are instinctively drawn to people that fit in with our lives in ways we're not even conscious of, and in that sense, the pull between estranged siblings is understandable'.

Despite numerous biblical instances of sibling incest, most religions take the view that there is no overwhelming incestuous urge. Naftali Loewenthal, a Hassidic Jew and honorary research fellow in London University's Department of Jewish Studies, argues that to rationalise the incest taboo is to miss the point. 'In Jewish thought, sex is not just a physical experience, but an intensely spiritual one. If you do it with the wrong person, the ramifications are infinitely negative. In Jewish law, offspring of incestuous relationships are prohibited from getting married.'

A less proscriptive view is taken by Anthony Harvey, canon and sub-dean of Westminster Abbey. 'The ban on incest in the Old Testament has been taken up by the Church without revision because the consequences of incest are clear. However, if the siblings have not grown up together and will not procreate, it may be said that their relationship should not be defined as incestuous. Nevertheless, Christian understanding is that procreation is potentially present in all sexual activity, so it's hard to conclude that this exception is so different that previous rules don't apply.'

Although the power of the taboo means that incest is rarely discussed in public, it has been a subject of enduring fascination within the world of literature and film. Stephen Poliakoff, writer and director of the film Close My Eyes, says, 'it is haunting to the public when someone falls in love with their estranged sibling, because people secretly find such things understandable. Many people recall sexual feelings for their siblings during adolescence, and so are not revolted. It is almost as if there should be another word for brother-sister incest because, unlike sex between parent and child, it isn't necessarily abusive.'

Despite the fact that the letter of the law makes no distinction between types of incest, the courts have at times found it difficult to sustain the argument that incest between once-estranged, consenting adult siblings is a criminal offence. In 1988, for example, a judge granted an absolute discharge to a brother and sister who met for the first time when she was 25 and who had broken off their respective marriages after falling in love and committing incest. The judge said that he had taken into account that they did not intend to have children.

Such cases have prompted proposals from the legal profession to remove incest between consenting adult siblings from the Sexual Offences Act. In France, for example, incest is not a crime and, although marriage between incestuous couples is illegal, nothing prevents them from living together.

Would liberalisation be a good thing? Rationally yes, intuitively no. That is the opinion of every secular professional here quoted, and half a dozen more that aren't. Perhaps we are a more spiritual nation than we care to admit.