Re: Intelligence on the X chromosome

Matt Beckwith (
19 Aug 1996 02:44:10 GMT

catherine yronwode <> wrote:
>Matt Beckwith wrote:
>> So, if I understand correctly, the fact that women have twice as many
>> alleles would tend to increase the probability of their distribution
>> being a normal one. Yeah, that makes sense. So a woman is less likely
>> to have all high-intelligence genes or all low-intelligence genes.
>> This pretty well implies that these genes are not recessive, but
>> co-dominant.
>> I wonder what the evolutionary advantage is in having these genes on the
>> X chromosome.
>I believe it would only be of advantage to a matrilinear culture that
>was patriarchal. In such a set-up, if a son "married out," his offspring
>would be discarded by the tribe, thus losing some intelligent but
>non-ruling females, but also removing from the tribe's gene pool all
>potentially-ruling males of unknown intelliegence. If a daughter
>"married out," her children would be accepted into the tribe and all the
>potentially-ruling men would be of a known intelligence level while the
>daughters would be of unknown intelligence but would have no rulership
>potential. There is only one culture quite like this that i know of at
>the present time, and that is traditional, matrilinear, patriarchal
>Judaism. I am unwilling to discuss the relative intelligence of Jews vis
>a vis other cultures, merely noting that this is the only culture that
>(unconsciously, no doubt) conforms to what i see as the one breeding
>plan that would maximize intelligence over the long haul if there were
>multiple alleles for intelligence and if they only occured on the X

I think you got more of the right kind of X chromosome genes than I did.

After reading your post about fifty times, I'm beginning to get the

The X chromosomes of a daughter come from both parents. The X chromosome
of a son, however, comes from the mother (only). So a son's intelligence
comes entirely from his mother.

In a patriarchal society, intelligence only has value as a trait in the
males of the tribe (because they're the leaders). The function of the
females is to pass that intelligence genetically on to male inheritors.
But it has to go from the intelligent male through his daughter on to the
intelligent grandson.

So we don't want the leader's daughters leaving the tribe. It doesn't
matter whether he marries within the tribe. It only matters that his
daughters remain within the tribe (whether his wife is of the tribe or

Now, what about the mating of his daughter? She now has the intelligent
gene. It doesn't matter whether she marries within or without the tribe.
But her child--if male--must stay within the tribe. Otherwise the tribe
is giving away a potentially ruling (and therefore competing) male.

So it would seem to me that, in order to keep the intelligent ruling
genes in the tribe, daughters of rulers should stay within the tribe, and
grandsons of rulers should stay within the tribe. Sons of rulers could
do anything they wanted, as could granddaughters of rulers.

In a patriarchal matrilineal society, daughters of rulers who marry out
are lost. So this is not the optimal social structure for optimizing
intelligence when the intelligence genes are on the X chromosome.

On the other hand, there probably would never be a society structured in
this particular way.

The sons of an intelligent man are not getting his intelligence, so we'd
like at least some of those to leave the tribe. If he married outside
the tribe (in a matrilineal society) they would leave the tribe.

In a matrilineal society, all sons of women within the tribe would be
kept within the tribe, whether the women married out or not. These are
the important sons to keep.

Please forgive me for re-iterating what you already said quite
eloquently, but I needed to restate it a bit more simply in order to
understand it.

Getting back to my hypothetical optimal social structure, it would seem
to be best to have the leadership itself transmitted from father to
daughter to grandson. A truly great male leader would want to have
daughters (not sons). Isn't that ironic, considering how many men prefer
sons to daughters. A truly great female leader would want to have sons,
not daughters.

We could extrapolate this to our present society. If you as an
individual want your intelligence to live on, then you should prefer to
have children opposite in gender to your own.

If you're a man, only your daughters will get your intelligence genes.
(Intelligent sons would get their intelligence entirely from their
mother.) Your daughters' sons would get either your intelligence genes
or those of your wife. So there's a fifty fifty chance (ignoring
chromosomal crossovers) that your grandsons through your daughters will
have your intelligence. (Your grandsons through your son won't have
any of it.) Since your intelligence will only be transmitted purely into
male descendents (they're the ones that get the single X chromosome, and
aren't "polluted" by some X chromosome inherited from someone else), you
should really be looking at male grandsons as your ultimate goal for
preserving your own individual intelligence. If you want to not only
preserve your intelligence but enhance it, you should marry an
intelligent woman. That way, if there are crossovers at the X
chromosomes, they'll not detract from your grandsons' intelligence. If
you're not concerned about your great grandsons (perhaps you don't expect
to live that long, for example), then you shouldn't care how dumb your
daughter's husband is. (Isn't that interesting!) In any case, the
intelligence of your daughter's husband will have no bearing on the
intelligence of those male descendents who receive your particular
intelligence genes.

If you're a woman who wants to pass on her intelligence, then first of
all you should realize that your particular mix of intelligence is
probably not going to get passed on as is. Your descendents will none of
them be you. You can't pass both of your X chromosomes down the tree to
the same person without some inbreeding somewhere, and you wouldn't want
that. But let's say you want your X chromosomes to show up in
descendents maximally. They will maximally show up in males (since
they're unpolluted by someone else's X chromosome). So you should also
have as a goal an intelligent male descendent. But in your case, it may
as well be a son, because each of your sons will get one or the other of
your X chromosomes. So you want sons. And when you have sons, you can
take all the credit for their intelligence (or lack of it).

All of this assumes that all of the genes for intelligence are on the X
chromosome, which of course hasn't been established.

I'm intelligent. My mother was intelligent but not as intelligent as I
am. My father was extremely intelligent. I always thought I got my
intelligence from both parents, but mostly from my father. If this
theory about the X chromosome having all of the intelligence genes is
accurate, then my father's intelligence was irrelevant. I'm more
intelligent than my mother because I only got one of her X chromosomes,
and the one I got had the preponderance of the intelligent intelligence
genes. Or there was an advantageous crossover at my mother. Let's say
there were no crossovers anywhere. In that case, my intelligence could
be my maternal grandfather's, or my maternal grandmother's father's, or
my maternal grandmother's mother's father's, etc. In any case, barring
crossovers, there was some male on up the maternal branches of my
ancestral tree who was as intelligent as I. Believing as I do that
intelligence is the most valuable and personality-determining feature of
a human being, that male ancestor was in a sense I.

Hm, I wonder how often X chromosome crossovers occur. Naturally, they'd
only occur in women, so their incidence would be half that of crossovers
in any autosomal chromosome.


Matt Beckwith