[extropy-chat] heterosis, consanguinity and IQ

[Rafal Smigrodzki]

Damien wrote:
> Subject: Re: [extropy-chat] heterosis, consanguinity and IQ
>
>
>
>> [po:] inbreeding, should register as lower IQ on a
>> society-wide scale.
>
> Is this right, though? Two recessive deleterious genes will be
> expressed more often, munging the offspringen (who might die as a
> result, often in utero), while two recessive advantageous genes will
> pop up more often as well. I had the impression that this is a
> partial explanation for the elevated IQs of Orthodox Jews, e.g., even
> though they pay the price in certain rare horrid afflictions. (I seem
> to recall a post from Eliezer implying that this might account in
> part for his own unusual brain.)

### The mechanism you describe was indeed proposed as explanation for why
consanguinity in societies where it is uncommon (Europe) produces a
significant increase in birth defects and mental retardation but in
societies where it predominates, there is no clear effect on this frequency.
In effect, consanguinity may over centuries cleanse the genetic pool of
deleterious recessive alleles. Specifically I heard it as explanation for
the lack of significant increase in birth defects in Kerala. On the other
hand, the issue is made less clear by other differences - such as
consanguinity in Europe occurs most frequently among marginal, low-income,
relatively stupid persons to begin with, while in Kerala it is accepted
practice among the population at large. There are probably too many
variables involved to predict from first principles the effects of
inbreeding on IQ, although we can with more confidence predict that
inbreeding will reduce the ability to survive infections. Most likely,
however, the net effect of inbreeding is still a lowered average IQ, even if
recessive alleles are removed more efficiently.

Regarding Jews and IQ, however, the explanation is probably somewhat
different - it involves centuries of eugenic practices which favored most
intellectually gifted boys. The increased frequency of some genetic
afflictions among Jews is unrelated to the IQ increase and is caused by the
founder effect (in a way, a mild form of inbreeding) which is also present
in many other ethnic groups derived from a small founder population, like
the French Canadians, or the Afrikaaners. However, without eugenic practices
or other forms of selection favoring high IQ the founder effect alone is not
sufficient to increase IQ.

Rafal