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.

16th Century French poem
Here lies the daughter,
here lies the father,
here lies the sister,
here lies the brother,
here lie husband and wife,
and yet there are only two bodies in the grave.
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