Pas de Deux of Sexuality Is Written in the Genes - New York TimesAgain one wonders how insect-like humans are. Does a mother's womb select for homosexual behavior in younger sons because they will do a better job defending her as she and her children age? In addition to the obvious example of the "queen bee" there's the precedent of the convict fish, who's young may forage for the adult.
April 10, 2007
By NICHOLAS WADE
... In the womb, the body of a developing fetus is female by default and becomes male if the male-determining gene known as SRY is present. This dominant gene, the Y chromosome’s proudest and almost only possession...
In puberty, the reproductive systems are primed for action by the brain... probably the brain monitors internal signals as to whether the body is ready to reproduce and external cues as to whether circumstances are propitious for yielding to desire.
... It is a misconception that the differences between men’s and women’s brains are small or erratic or found only in a few extreme cases, Dr. Larry Cahill of the University of California, Irvine, wrote last year in Nature Reviews Neuroscience. Widespread regions of the cortex, the brain’s outer layer that performs much of its higher-level processing, are thicker in women. The hippocampus, where initial memories are formed, occupies a larger fraction of the female brain.
Techniques for imaging the brain have begun to show that men and women use their brains in different ways even when doing the same thing. In the case of the amygdala, a pair of organs that helps prioritize memories according to their emotional strength, women use the left amygdala for this purpose but men tend to use the right...
... Dr. Bailey said. “I’m not even sure females have a sexual orientation. But they have sexual preferences. Women are very picky, and most choose to have sex with men.”
...Helen Fisher, an anthropologist at Rutgers University, argues that three primary brain systems have evolved to direct reproductive behavior. One is the sex drive that motivates people to seek partners. A second is a program for romantic attraction that makes people fixate on specific partners. Third is a mechanism for long-term attachment that induces people to stay together long enough to complete their parental duties.
Romantic love, which in its intense early stage “can last 12-18 months,” is a universal human phenomenon, Dr. Fisher wrote last year in The Proceedings of the Royal Society, and is likely to be a built-in feature of the brain...
... The best evidence for a long-term attachment process in mammals comes from studies of voles, a small mouselike rodent. A hormone called vasopressin, which is active in the brain, leads some voles to stay pair-bonded for life. People possess the same hormone, suggesting a similar mechanism could be at work in humans, though this has yet to be proved.
... since gay men have about one-fifth as many children as straight men, any gene favoring homosexuality should quickly disappear from the population.
Such genes could be retained if gay men were unusually effective protectors of their nephews and nieces, helping genes just like theirs get into future generations. But gay men make no better uncles than straight men, according to a study by Dr. Bailey. So that leaves the possibility that being gay is a byproduct of a gene that persists because it enhances fertility in other family members. Some studies have found that gay men have more relatives than straight men, particularly on their mother’s side...
... A somewhat more straightforward clue to the origin of homosexuality is the fraternal birth order effect. Two Canadian researchers, Ray Blanchard and Anthony F. Bogaert, have shown that having older brothers substantially increases the chances that a man will be gay. Older sisters don’t count, nor does it matter whether the brothers are in the house when the boy is reared.
The finding suggests that male homosexuality in these cases is caused by some event in the womb, such as “a maternal immune response to succeeding male pregnancies,” Dr. Bogaert wrote last year in the Proceedings of the National Academy of Sciences...
The fraternal birth order effect is quite substantial. Some 15 percent of gay men can attribute their homosexuality to it, based on the assumption that 1 percent to 4 percent of men are gay, and each additional older brother increases the odds of same-sex attraction by 33 percent...
...A significant recent advance in understanding the basis of sexuality and desire has been the discovery that genes may have a direct effect on the sexual differentiation of the brain... Arthur Arnold of the University of California, Los Angeles, has found that male and female neurons behave somewhat differently when kept in laboratory glassware. And last year Eric Vilain, also of U.C.L.A., made the surprising finding that the SRY gene is active in certain cells of the brain, at least in mice. Its brain role is quite different from its testosterone-related activities, and women’s neurons presumably perform that role by other means.
... 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.
“It’s popular among male academics to say that females preferred smarter guys,” Dr. Arnold said. “Such genes will be quickly selected in males because new beneficial mutations will be quickly apparent.”
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.
Greater male variance means that although average IQ is identical in men and women, there are fewer average men and more at both extremes. Women’s care in selecting mates, combined with the fast selection made possible by men’s lack of backup copies of X-related genes, may have driven the divergence between male and female brains. The same factors could explain, some researchers believe, why the human brain has tripled in volume over just the last 2.5 million years...
On the subject of male/female differences, I recall that when I did my women's studies courses in the 1970s, there was a powerful meme that the differences between male and female were culturally conditioned and relatively small. That idea has been in decline ever since, and reached its nadir a few years back when it seemed that male humans were more like male chimps than like female humans (since then we've decided that chimps and humans are less alike than once thought). It is remarkable that males and females can communicate at all!
The rapidity of brain volume increases is presumably similar to the florid changes seen in animal phenotyptes that play a role in mating. As a male non-academic, however, I've not noticed displays of academic expertise attracting as many females as, say, wealth, good looks, or charm...
The author doesn't mention that the average male brain is substantially larger than the female brain, so it's been a puzzle that women, on the whole, are smarter than men. (The gap among children is breathtaking.) A thicker cortex for women helps explain that. Studies of the effects of menopause on cortical thickness must be coming soon ...
As for the autism connection? I tend to lean towards the thinking that the recent prevalence increase (past 10 years) is mostly reclassification of what was once called 'mental retardation' but that that there's also a component of what, over a period of a thousand or so years, might turn out, in retrospect, to be another instance of the ongoing and difficult adaptation of the human brain ...
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