@Hangerbout: in Germany, first cousin marriage is illegal, too.
Agree with @matresense
I see the fear about it being a smaller step from banning cousin marriage to banning people with disabilities to have children, but I don’t think it’s the same.
For one, the cousin marriage tradition in certain communities is a structural / belief-system habit; while disabilities are far more complex and random. The people who know that their disability could be inherited often have access to information and testing (in most western / westernized countries at least). However, not every disability can be tested for. But these cases are far more distributed throughout a population and far-less concentrated in one geographical area or a specific population (unless the disability stems from a local environmental source, like radiation).
As PP said, the difference is the repetition of this pattern throughout generations plus a genetic predisposition of certain populations for specific diseases, thus the probability of a child with disabilities becomes much higher.
Usually you would think that immigrant populations would with time assimilate to the habits of their new home country, which would widen the group of potential partners and weaken traditional rules. Some PP here have shared their own experiences of this change, so it does happen.
In that case, better education and support by medical and social professionals would make a difference and the negative effects of cousin marriage would start to fizzle out. Some people mentioned that the rates go down, according to some research, but they do so only very slowly.
(to continue from what @Pigriver said)
But when there are groups where the tradition is kept alive by marriage not only to cousins but always cousins from a specific region in their former home country, there is not a big chance to counteract the effects.
New incomers might be beholden to their relatives, as they are their first contact in a new country, and they seem to get married at the same time as immigrating, thus there is not much chance to make own and independent experiences and get to know different values. Then the cycle keeps on repeating without changes.
For other groups of people who practice this tradition, like the aforementioned Travellers or some Jewish people, the background is different, as they’ve lived in Europe often for centuries, so probably the suggestions on how to avoid children with disabilities due to closely related parents need to be different, too.
But to say: “things have always been that way, it’s a tradition”, is not enough when the outcomes are so tragic and difficult. It must be possible to include newer information into decision-making. The decisions that families made hundreds of years ago or even hundred years ago were on a completely different landscape of information.
And yes, eventually the children of children with disabilities might not be able to naturally bear offspring anymore, and at some point the line with stop, but not immediately. These are still some generations where both the parents and the kids will suffer needlessly.
In a wider sense, when everyone groans about the aging population and workforce, wouldn’t it be more sensible to support communities to shed the tradition and ensure that healthy babies are born? I am not advocating for some “ideal human”, but at least trying to avoid the most severe physical disabilities and children dying young.
In fact, the advantages of cousin-marriage and a small gene pool in general (keeping wealth together, ensuring partners are from the same background, keeping separate from the majority population etc), which are meant to ensure survival of a group by not “diluting” it are in the end those which will lead to the opposite outcome. How can a society survive when so many of their children die early or are disabled?
However, Darwin’s own situation of a cousin-wife did inspire him to start research on the matter and he did advise against cousin-marriage later, too.
There are also communities in which it is a rule to marry outside of your group, for example all women must marry outside of the group, but women from elsewhere marry in (i.e. the men stay, the women move to other families often a fair distance away) and vice versa. So clearly there can be beneficial traditions, too.
To say “it’s tradition, it must be morally good” is not complex enough. A tradition needs to be reassessed with different circumstances and information.
Some articles I found interesting:
https://thecritic.co.uk/issues/november-2025/dont-keep-it-in-the-family/ (written by a biologist)
https://unherd.com/newsroom/why-is-the-nhs-still-defending-cousin-marriage/
https://thecritic.co.uk/why-cousin-marriage-endures/