I read in another study that 3rd cousins are the sweet spot for variance and to avoid outbreeding depression too. Think it was this one http://isteve.blogspot.co.uk/2008/02/darwinian-sweet-spot-3rd-cousin.html
It has long been known that inbreeding is bad for you. A new paper in Nature (Directional dominance on stature and cognition in diverse human populations) finally gives us a good quantitative estimate of just how bad it is. They find that the offspring of first cousins suffer an average reduction of 1.2 cm in height and 0.3 sd in g ( ~4.5 IQ points) . They directly measured runs of homozygosity – more accurate than estimating from genealogy, and better in other ways as well. Children of first cousin marriages also suffer an elevated incidence of significant genetic disease, roughly 1.5-2 times the non-inbred risk.
That’s not all that far from my previous horseback guess. The field was full of old and messy studies, so horseback guesses were the best you could do, until now.
The article mentions that Darwin recognized negative effects of inbreeding (a bit too late):…
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