In 2007, a German brother and sister took their fight for the right to a sexual relationship to the country's highest court, the BBC News website's Clare Murphy looks at the history of the incest taboo and how it is changing.
Forward thinking? Napoleon dropped incest from France's penal code
When Henry VIII wanted to be rid of Anne Boleyn, he made sure she was accused of one particularly heinous crime: sleeping with her brother.
According to the great modern anthropologist Claude Levi Strauss, the incest taboo has been the driving force of humankind. By forcing man to find a mate outside the home, disparate, warring clans have been brought together and society has flourished.
Others see the abhorrence for sleeping with relatives as having a primarily biological motive - a human instinct to prevent defective genes being passed down.
"Society has long relied on the family unit as its basis," says sociologist Vikki Bell. "That's why it has been so important to keep family roles clear."
It is not hard to see how incest can make family life very complicated, potentially turning brothers into fathers and mothers into sisters.
Yet while most are clear that sexual acts between a related adult and child constitute abuse and as such must be punished, there is no modern consensus on whether society has the right to ban consensual sex between siblings, or indeed parent and adult child.
Too close to attract
If Sigmund Freud is to be believed, everyone would be sleeping with their close relatives given half a chance. Society had to keep these deep-seated desires in check, he argued.
No need, countered Finnish anthropologist Edward Westermarck, who said that if anything, close association in childhood automatically created sexual aversion - in other words, familiarity breeds contempt.
His theory was tested in a study of unrelated children growing up together in an Israeli kibbutz. Despite the parents being keen on their children forming relationships, the children themselves had no sexual interest in one another as they began to mature.
The theory was also backed up by another study in Taiwan by a US academic Arthur Wolf.
He looked at two forms of marriage - one in which the two partners married as adults, and another in which the wife was taken into her future husband's household as a young child, growing up with him.
The latter produced more adultery, more divorces and fewer children than the former. This, he said, indicated closeness as children stifled rather than stimulated sexual feelings.
But these cases, which in any event did not involve actual blood relatives, fitted uncomfortably with the only well documented case of a society which embraced sibling incest outright - that of Roman Egypt.
For about 300 years, a significant proportion of all marriages recorded were between brothers and sisters.
German brother and sister Patrick and Susan did not grow up together
The relationships appear to have been both social and reproductive.
But there might have been years between the siblings given the high rate of infant mortality, so sibling husband and wives may have barely grown up together.
The phenomenon of genetic sexual attraction - where siblings fall for each other on meeting after an estranged childhood - accounts for some of the high-profile incest cases of recent years.
In the German case, Patrick was brought up in a foster home while Susan remained with the biological parents, meeting for the first time when she was 16 and he 23.
And in the US case of Allen and Patricia Muth, which went to the Supreme Court in 2005, the sister was raised in care, not meeting her brother until she was 18.
Both of them have served prison sentences for incest.
Both of these relationships have produced children with special needs, although whether this resulted from their parents' biological proximity is unclear.
Some geneticists put the risk of producing a disabled child as high as 50%, but this is hotly debated. Opponents of the incest ban also argue there are double standards, noting that no-one would ban those with hereditary diseases from reproducing.
In some countries, the law has tried to take into account the risks while legalising incest in certain circumstances. In Brazil, an uncle and niece may have a relationship provided they undergo health checks.
In parts of the US, first cousins may marry if they are beyond reproductive age or ability.
But even in countries where incest between adults is not prosecuted, the rights of both parents and children born of incest are not clear cut.
France dropped incest from the penal code under Napoleon - 200 years ago.
But siblings may not marry, and in 2004, a man who was having a sexual relationship with his half-sister was refused legal paternity of his own child.
In the Netherlands meanwhile, where consensual incest is no longer prosecuted, the legal status of the child born of such a relationship is ambiguous, according to Masha Antokolskaia, an expert in family law at the Free University in Amsterdam.
Sweden is the only country in Europe which allows marriage between siblings who share a parent.
"In many ways society no longer wants the state to intervene in private lives when it doesn't have to," she says. "But it is still not prepared to grant incestuous couples full rights."
There is also debate over how much laws affect behaviour. Some even argue that what is proscribed becomes all the more attractive.
Not according to Joachim Renzikowski, a criminal law professor at Germany's Halle University.
"I don't believe that because incest is banned, there's a certain attraction about doing it," he says.
"But I doubt equally that getting rid of our incest law will result in any measurable increase in cases. Our moral guardians don't need to get too worked up about this."