A sibling-incest child looses 28 IQ points.
That's almost two standard deviations and looks iffy to me. Yes, I followed the link, the main study resulting in this number is behind the paywall, but I suspect that the sample wasn't very representative.
Basically, for a sub-population with a recessive trait that leads to mental retardation the outcomes are going to be massively different from the outcomes for a sub-population without such a recessive trait.
Available the usual places: https://www.dropbox.com/s/8gv0el0anfyymed/1971-seemanova.pdf / http://moscow.sci-hub.bz/092ee3d082e9cffd04b7064c36ba808a/10.1159%40000152391.pdf (Personally, I'm impressed she managed to get that big a sample in just Czechoslovakia. Creepy. Also, note that's only one of the studies being meta-analyzed.)
It's very clear that inbreeding is really bad: to give some examples, it drives species extinct within generations, its effects are long-term and underestimated, inbreeding can result in 20+ IQ points loss in other populations suc...
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