Background
Right now, as we are all aware, there is a debate in the UK as to whether cousin marriage should be illegal. I should imagine that we all believe that it should be, but I don’t want to stop there.
Roman law, as I understand it, was simply that sex between people within four degrees of consanguinity was illegal. How they calculated this originally was to count generational links to the nearest common ancestor and back down again to the proposed spouse. This meant that marrying your cousin was illegal, but your cousin’s child was not, for example.
Later, this was changed to seven degrees of consanguinity, which is your third cousin, and the method was changed to just count from the most distantly related spouse from the common ancestor (which would make marrying any of your Great-great-great-great-great-grandparents’ Great-great-great-great-great-grandchildren illegal).
These were the rules of the Catholic church for a while, but unfortunately, the laws got messed with a lot due to nobles wanting to marry other nobles. The protestant split also caused cousin marriages to become more common due to them being acceptable in the Old Testament, and protestants rejecting the traditions of the Roman Empire for the textual interpretation of the bible.
Consanguinity Proposal
In the UK right now, marrying your cousin is legal despite the many problems that it causes, both socially and genetically. From the discussions we have been having on here, I think that forcing people to marry outside of their family is also generally a good thing. It creates bonds of kinship between families and acts as the social glue for communities; in this way, marriage and family connect us as the British people, and we should encourage people not to marry (or have sex with) the people who are related to them. As such, I propose introducing into law a requirement that no marriage or sex between people who are within three degrees of consanguinity be legal, using the new consanguinity method. This would be alongside a law prohibiting sex between direct line descendants (you are highly unlikely to ever be in a position to have sex with your great-great-grandparent consentually, but just to cover that basis).
Above is a chart I’ve put together to demonstrate these levels. This proposal is that anyone yellow or above be restricted, but from blue is fine. Old Roman law would have it that only green is fine.
It is worth noting that, from the law of genealogy, we are all some degree of cousin to each other, so the question is “where is the line?” and not just “why can’t we ban it entirely?”
Affinity
Affinity is a weirder concept, but no less important. It is the concept that by marrying someone, their relatives become your relatives and that that creates an impediment to marriage (and sex). For example, if you marry a 48-year-old woman, you should not have sex with her 22-year-old daughter, even if both consent, because you are now the 22-year-old’s father.
We can think of affinity in the same way that we think of consanguinity. If you marry a person, you take their place in the consanguinity tree, and your spouse’s degree of consanguinity becomes your degree of affinity. Traditionally, affinity hasn’t been that enforced (although it was the reason that Henry VIII argued that she should get an annulment from Catherine of Aragon!), and I am not as worried about it, but I think that we should have the discussion.
To me, setting a prohibition of two degrees of affinity seems reasonable. That would mean that once you have married someone, it is illegal for you to ever marry or have sex with their first cousin or their grandmother.
Collateral Affinity
The last topic I want to discuss is something which I am calling collateral affinity. Generally, affinity only attaches to the married person, not their family, but I want to introduce the concept of collateral affinity to address this (sorry gooners, we are coming for your step-siblings).
My final proposal in this topic is to set a prohibition against relations between people who have collateral affinity of three or less. How I suggest calculating collateral affinity is as follows:
For a person, X, who wishes to marry a person, Y, let X be consanguineous to Z and let Y be affined to Z. Add together the degree of consanguinity and the degree of affinity to find the collateral affinity.
This rule would, for example, make it so that one can not marry their cousin’s step-daughter. Necessarily in this rule, we require both affinity and consanguinity to be less than 3, because otherwise it would entirely replace the affinity rule (maybe it should?).
Conclusion
Sadly, by nature, this is line drawing. We could try to make genetic arguments, but we would end up in places where people start quibbling over half-relatives vs. full-relatives or “what if we do a DNA test and find that we are less related than average?”, so as much as we can point to the genetic ills, the line has to come from social policy to me. I think that I have presented a reasonable case, but I appreciate that a fair amount of this will be vibe-based.
