I’ve been reading about leptin receptor deficiency recently, which is mostly (only?) observed in children of cousin marriages. Stephen Guyenet describes affected children as follows:

Usually they are of normal birth weight and then they’re very, very hungry from the first weeks and months of life. By age one, they have obesity. By age two, they weigh 55-65 pounds, and their obesity only accelerates from there. While a normal child may be about 25% fat, and a typical child with obesity may be 40% fat, leptin-deficient children are up to 60% fat. Farooqi explains that the primary reason letpin-deficient children develop obesity is that they have “an incredible drive to eat”…leptin-deficient children are nearly always hungry, and they almost always want to eat, even shortly after meals. Their appetite is so exaggerated that it’s almost impossible to put them on a diet: if their food is restricted, they find some way to eat, including retrieving stale morsels from the trash can and gnawing on fish sticks directly from the freezer. This is the desperation of starvation […]

Unlike normal teenagers, those with leptin deficiency don’t have much interest in films, dating, or other teenage pursuits. They want to talk about food, about recipes. “Everything they do, think about, talk about, has to do with food” says Farooqi. This shows that the [leptin system] does much more than simply regulate appetite - it’s so deeply rooted in the brain that it has the ability to hijack a broad swath of brain functions, including emotions and cognition.

Marrying your cousin is like winning access to a whole new, much more interesting tier of genetic diseases.

Cousin marriage, according to wikipedia, has about the same risk of congenital disability as giving birth to a child over age 40. If lots of people marry their cousins, the risk gets worse.

I suspect that even if cousin marriage is legalized and destigmatized most people in the US aren’t going to want to do it, because family really isn’t that important in our culture. So banning cousin marriage implies that one should also ban giving birth over the age of 40. While that might be intractable, banning assistive reproductive technology to mothers over forty (or even forbidding it to be covered by insurance) would be more doable.

So there’s a rather large jump here which is always disturbing.

“Cousins reproducing leads to some avoidable painful illnesses” becomes “cousins engaging in any romantic behavior with eachother is evil, and they should feel shame and anyone who even depicts that in fiction should also feel shame.”

When the logic is “there’s a long term material effect AND we feel really icky about it”, then you are usually engaging in ideological hounding. You’ve cut around all the logical discussions regarding proportionality, humility, actual cause and effect, and let emotive response takeover combined with some fairly far-off consequence horror stories. This leads to dehumanization and cruelty for no particularly useful reason.

(Which, as Ozy points out, are questionable horror stories at that.)

Geez, let them cuddle whoever they want and give them a really strict lecture about safe sex.

(In the modern age, the real risk might be that in absence of a strong anti-incest taboo, the social effects of in-group sex are fairly pernicious. Your drama is terrible, power dynamics get involved, and you have even less incentive to ever talk to anyone outside your hive. But by that logic you might as well ban sex in group houses and subcultures too.)