The law is based on old [read: uninformed] ideas about morality, and so we have criminalized something that is allowed in countries like Holland, France, and Spain.People will argue that incest should not take place because of the elevated risk of having children with genetic disorders. However, there is some serious disagreement about how much higher the risk is. For example, in the Danish article in Urban, Karen Brøndum-Nielsen, president of a research center in genetics, says that her best bet is that siblings has a risk of 10 to 30 percent for getting a "handicapped" child, while Vagn Greve contends that the risk increases from three percent among non-related parents to five percent among siblings (i.e. an irrelevant increase). Where do those numbers come from?
Alan Bittles, Australian geneticist, has collected data to show that the risk of birth defects increases from 2% in the general population to 4% when the parents are closely related (source). It would appear that Brøndum-Nielsen's numbers are pure fiction, while Greve actually based his assertion on real data.
Additionally, a comparison with the increased risk of birth defects in mother of high age reveals that the risk of birth defects has nothing at all to do with why people are universally against incest:
A woman over 40 has an elevated risk of giving birth to a child with defects. People who with autosomal dominant disorders have a 50% chance of having children with the same disorder. If we allow these people to give birth, why not allow the same for first-cousins (or, indeed, for them to marry).This quote is from an article I mentioned earlier which argues that there is no sense in having laws against first-cousin marriage. Similarly, we also wouldn't dream of forbidding someone with cystic fibrosis, a terrible hereditary disorder, from choosing to have a child. Rather, we would all insist that it was their right to have one. And the baby's risk of getting the same disease is 50% (autosomal recessive disorder)
Our revulsion against incest can also be shown to be emotional and not based on hard facts by considering a hypothetical example: A brother and a sister are camping in a tent, and one day decide that they will try to have sex. They agree to use a condom, and after they're done, they agree that it was nice but that they won't do it again. Do you think what they did was wrong? Why? With no chance of becoming pregnant (you can even imagine that one of them are infertile), no other change in their personal lives, and no effect on any other person, because no one else will ever know, on what grounds is it that many of us think of the act as repugnant?
In a previous post I argued that science does indeed have something to say about morality. Not directly, but only inasmuch as our moral values are based on what we know about the real world. So, for example, if we think that incest leads to many children being born with defects, then we will think sex between siblings is wrong. And when we discover that what we thought we knew was based on anecdotal evidence, and has no statistical merit, then we should reconsider what we think about the matter. I am not saying that our emotions have nothing good to say about what we should do and think, but that when barring others from doing so, we should have better reasons than that.
And, please, for heaven's sake, stop saying things you know absolutely nothing about. Danish member of parliament Karen Hækkerup says "There will always be psychological damage when family members sleep with each other - even if they can't see it themselves right away." Such a statement is blatantly outrageous and paternalistic in the extreme. She knows nothing of the kind. Show me the numbers, or shut up!
Oh, and there is no evidence presented that the girl the couple had as a result of their affair has any defects whatsoever. Imagine a law saying that it was illegal if was born baby with any defects, and otherwise lawful. Imagine the same law applied to anyone else. Jail time for having a baby with a genetic disorder has got a certain ring to it, don't you think?