[FITZ]

But breeding humans --- how are you going to cull? It's easier with animals --- you just neuter or spay the less than perfect when you're inbreeding --- but even with that inbred animals are not the best, even if they have some traits you wanted, they're usually mentally not the best.

A one time breeding of cousins isn't so much the problem because it bring together the same genes for one generation --- but when it's done for generation after generation, after a while all the genes are the same.

[Rifleman]

Oh, sure. You can't just inbreed to maintain a healthy population. You must also outbreed. But the inbreeding lets you select and maintain characteristics. As to culling people, that is the problem and the reason that marryin' inside the family (very often) is a bad idea.