The incest trap

It is hard to think of a code of behaviour which is common to all societies on earth, let alone to most other species too — except, that is, for the avoidance of incest. Even cockroaches have developed a breeding strategy that prevents them mating with their own siblings. And yet as we understand more about the genetic dangers of inbreeding, so the social infrastructure that guards against it is being dismantled.

In the 40 years since the birth of Louise Brown, the world’s first test tube baby, births by IVF have become routine — almost 2,500 a year using donated eggs, sperm or both. And yet there is virtually no guard against the children growing up and accidentally breeding with half-brothers and sisters of whose existence they are unaware.

The failure to develop such safeguards is serious because the risks of genetic abnormalities from incestuous conceptions are so high. In 1971 the Czechoslovakian Academy of Sciences studied 141 children born in such relationships and found that more than half had some kind of health problem, while 42 per cent had severe birth defects and 11 per cent were mentally impaired.