Anthropologists occasionally like to trumpet the glories of some culture in which everyone pretty much screws as the mood takes them, but, to the extent that those cultures actually exist, they have some common characteristics: they involve a few hundred to a few thousand people living in primitive squalor in environments nobody else can be bothered to take from them.
They would eventually be doomed, genetically, as their marriage pool would be far, far too small.
Remember Zuben Mehta? He was a famous conductor and was a Farsi and said that though the intermarriage allowed by the Farsis was generally very bad, it sometimes DID produce a GENIUS, he said, pointing to himself, "like me."
I do remember Mr. Mehta, although I didn’t know he was Persian. Consanguineous marriage does concentrate traits, including, sometimes, extremes of talent or intelligence.
When you have a culture that values academic achievement, such as that of some Jewish communities, you get something like a eugenics process orientated toward high intelligence, because demonstrably smart men are the most desirable marriage partners. Unfortunately, you can also concentrate hereditary physical defects, such as Tay-Sachs disease.
The more endogamous the community is, the more both the advantages and the defects proliferate in the group.