>>12521510Natural selection. The risk of a severe birth defect in a child produced by parent-child incest or sibling-sibling incest is about 50%, but of course that means half of them are okay, lacking the bad recessive genes which killed their siblings or made them retards, and you can breed the viable progeny with eachother continually until all the recessive genes have been plucked out. This is, basically, how we got lab rat breeds. Lab rat breeds are the product of extreme incest for hundreds of generations, but are relatively healthy and stable because the bad genes were bred out decades ago by scientists. They're also extremely homogenous, essentially being clones of eachother. This is desirable for experiments where you want to reduce genetic differences as a factor to nil.
