According to this:
http://dare.uva.nl/cgi/arno/show.cgi?fid=152307
A sibling-incest child looses 28 IQ points. The risks of all genetic disorders rises massively, from p^2/4 to about p/8, so if the prevalence of carriers of a recessive disease (p) is 1%, then the disease probability would rise by a factor of 50.
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.
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