Sometimes new scientific knowledge means that the case against a practice which has previously been regarded as acceptable, perhaps even normal, reaches the point where society is justified in discouraging or banning that practice. This can be hugely controversial, particularly in communities where the practice concerned is more common and who may feel that they are being targetted. Any change in legislation has to be sold and enforced in a way which ensures that such a charge neither is, nor is seen to be, justified. But sometimes nettles need to be grasped.
I am now convinced that Britain has reached that point in terms of new marriages between first cousins because of the increased risk that children of such unions may experience crippling or lethal defects due to doubled recessive genes.
Almost all societies have some form of taboo or rule against incest, but there are variations in how close a relationship has to be before it is banned. Almost all societies ban brother-sister marriages, for instance: some allow marriage between first cousins while others do not.
Due to improving understanding of genetics, we now understand why the children of closely related parents are much more likely to suffer from certain crippling or fatal defects.
Every cell in the human organism contains not just one, but two complete sets of blueprints for making a human being. We have a full set from each parent. One set of these genes - 50% of those from each parent - is actually used to build us, and are known as the dominant genes. We also have a backup set known as the recessive genes, which do not impact on our own bodies but can be passed on to our children. Half the genes we get from each parent are dominant in that parent, the other half are not. That's why two parents with brown eyes can have a child with blue eyes.
Some genes - those for brown eyes are an example - are always dominant if present. If you and your wife have brown eyes and she gives birth to a child with blue eyes, you have nothing to worry about - this just means that both of you have recessive genes for blue eyes. However, if you and your husband both have blue eyes, and you give birth to a child with brown eyes, you may have a great deal of explaining to do.
The mechanisms in our bodies which convert these blueprints into actual human beings appear to include effective means of screening out damaged or harmful genes or combinations of genes when selecting the dominant ones. Many people appear to have damage to their genetic code which does not cause them any problems because the genes concerned are recessive, and their dominant genes are fine. When they in turn reproduce, their children are likely to be healthy provided they don't inherit the same damaged gene from both parents.
However, when closely related individuals have children together, those children are vastly more likely to get the same damaged gene from both father and mother. The consequences can range from stillbirth or miscarriage, to the live birth of a child with expectations only of a few years of pain or disability. This can be agonising both to that child and to loving parents.
The average risk of a serious genetic disorder is about 2%. But the conditional probability of such a disorder in a child whose parents are first cousins rises by a factor of five to about 10%. According to a BBC report, a Primary Health Trust study in Birmingham found in 2005 that 10% of children born to first cousins in the city in previous years had either died or developed a serious disability.
The human and emotional cost of trying to forcibly separate existing marriages between first cousins would be enormous, and attempting to do so would almost certainly undermine any legislation on this subject. Particularly as it's going to be hugely controversial anyway in some communities.
But we can, and in my opinion should, legislate to set a date in the near future beyond which contracting a new marriage between first cousins in this country will be illegal, and marriages between first cousins contracted abroad and taking place after that date will not be recognised.
Every effort should be made to emphasise that this legislation will apply to every ethnic and religious group, and does not represent any criticism of those who have supported cousin marriage in the past.
But if we do not grasp this nettle, thousands of children every year will be born with avoidable crippling or lethal disabilities.