April 14, 2007
Brain Genes On X Chromosome Speed Evolution?

An interesting article by Nicholas Wade in the New York Times surveys part of what is known about sexual orientation and other sexual differences in the brain. He discusses potential causes of homosexuality (while notably failing to mention Greg Cochran's germ theory) and quotes sexuality researcher Michael Bailey. All good stuff. But the most interesting part to me: the presence of genes on the X chromosome that get expressed in the brain may allow more rapid selection for favorable genetic variations which enhance cognitive function.

It so happens that an unusually large number of brain-related genes are situated on the X chromosome. The sudden emergence of the X and Y chromosomes in brain function has caught the attention of evolutionary biologists. Since men have only one X chromosome, natural selection can speedily promote any advantageous mutation that arises in one of the X’s genes. So if those picky women should be looking for smartness in prospective male partners, that might explain why so many brain-related genes ended up on the X.

The existence of only one X chromosome allows new genetic mutations to express their effects more drastically in men and therefore to get selected for more rapidly.

Several profound consequences follow from the fact that men have only one copy of the many X-related brain genes and women two. One is that many neurological diseases are more common in men because women are unlikely to suffer mutations in both copies of a gene.

Another is that men, as a group, “will have more variable brain phenotypes,” Dr. Arnold writes, because women’s second copy of every gene dampens the effects of mutations that arise in the other.

This probably at least partially explains why the standard deviation of IQ is higher in men than in women. That higher standard deviation means that compared to women there are more extremely brilliant men and also more very dim bulb men.

An interesting consequence of the higher male standard deviation for IQ is that women above average in IQ generally can find men who are as smart or smarter to pair up with. But in the below average territory the women are going to tend to be smarter than the males of their social class and neighborhoods.

What I've always wondered: When successful men divorce their middle aged wives and marry younger women are the second wives less bright on average than the first wives? In other words, do the men decide to be less choosy on IQ in order to get younger second wives? I'm not looking for anecdotes to the contrary. I want to know about averages. Also, are the children born to second marriages as smart on average as children born to first marriages?

Share |      Randall Parker, 2007 April 14 07:49 PM  Brain Evolution

cathy said at April 15, 2007 4:30 AM:

This means that if a man is extra smart, his 'extra smartness' will be inherited only by his daughters, not his sons.

Randall Parker said at April 15, 2007 8:28 AM:


I'd make a weaker statement in that direction: Since not all the smartness genes are on the X chromosome we'd expect on average more IQ inheritance from father-to-daughter than from father-to-son. I wonder if the IQ literature supports this expectation. I'll ask some psychometricians and see if any of them know.

Randall Parker said at April 15, 2007 9:52 AM:


We should also expect to see smart fathers make IQ contributions to their daughters with less regression toward the mean.

To put it another way: I'd expect to see a higher ratio of daughter/son IQ with smarter fathers than with average IQ fathers.

Larry said at April 16, 2007 8:54 AM:

"When successful men divorce their middle aged wives and marry younger women are the second wives less bright on average than the first wives?"

One of great myths of divorce ("Divorce Dads, Shattering the Myths" by Sanford Braver), is that older men are leaving marriages for younger women in large numbers. Women file for 2/3 of the divorces ... the number one reason: "We've grown apart." ... BTW, adultery and abuse are way down on the list. In fact, adultery by the wife is higher on the list than adultery by the husband.

Loki on the run said at April 16, 2007 10:09 AM:

Cathy says:

This means that if a man is extra smart, his 'extra smartness' will be inherited only by his daughters, not his sons.

And that a mother of medium intelligence can produce one son of below average intelligence and one son of above average intelligence.

It's a crap shoot.

anonymous said at April 23, 2007 12:30 PM:

attractive women who are subprofessional yearn for the good life and are willing to marry an older man. professional track women with more ordinary looks will also sometimes choose an older but attractive, established professional man. I'd say a smart and attractive woman 15 years junior is not going to happen unless the man has some special qualities. Aging undoubtedly subtracts sexual zip from the man. The aging factor for sub-50 men, though, is overrated. Declining partner quality plays a very large role, as a tryst with something younger will prove.

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