Brain is a Sex Organ; How it Drives Sexual Desire (science article)

note: this piece is long but worth it. Excellent summary of sex – brain – hormone research, with the usual boring assumption that evolution is about producing more babies (rather than evolving the entire field of consciousness), but filled with fascinating details that enrichen our grasp of the differences between men and women.
It supports the Taoist yin-yang sexual thesis, that the sexual impuilse is built into every cell of nature. So if you read the reserach as describing oen layer of our reality, the surface physical reality and ignoring the underlying Energy Body that drives the process, the scientific research is very useful. It is like describing the workings of a car under its hood, while pretending that the driver (soul) and the designer of the car (Tao/Life Force) are taboo subjects.
-Michael

PAS DE DEUX OF SEXUALITY IS WRITTEN IN THE GENES
By Nicholas Wade
New York Times
April 10, 2007

http://www.nytimes.com/2007/04/10/health/10gene.html

When it comes to the matter of desire, evolution leaves little to chance.
Human sexual behavior is not a free-form performance, biologists are
finding, but is guided at every turn by genetic programs.

Desire between the sexes is not a matter of choice. Straight men, it seems,
have neural circuits that prompt them to seek out women; gay men have those
prompting them to seek other men. Women¹s brains may be organized to select
men who seem likely to provide for them and their children. The deal is
sealed with other neural programs that induce a burst of romantic love,
followed by long-term attachment.

So much fuss, so intricate a dance, all to achieve success on the simple
scale that is all evolution cares about, that of raising the greatest number
of children to adulthood. Desire may seem the core of human sexual behavior,
but it is just the central act in a long drama whose script is written quite
substantially in the genes.

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 chromosome¹s proudest and almost only possession, sidetracks the
reproductive tissue from its ovarian fate and switches it into becoming
testes. Hormones from the testes, chiefly testosterone, mold the body into
male form.

In puberty, the reproductive systems are primed for action by the brain.
Amazing electrical machine that it may be, the brain can also behave like a
humble gland. In the hypothalamus, at the central base of the brain, lie a
cluster of about 2,000 neurons that ignite puberty when they start to
secrete pulses of gonadotropin-releasing hormone, which sets off a cascade
of other hormones.

The trigger that stirs these neurons is still unknown, but 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.

Several advances in the last decade have underlined the bizarre fact that
the brain is a full-fledged sexual organ, in that the two sexes have
profoundly different versions of it. This is the handiwork of testosterone,
which masculinizes the brain as thoroughly as it does the rest of the body.

It is a misconception that the differences between men¹s and women¹s 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 brain¹s 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.

It is no surprise that the male and female versions of the human brain
operate in distinct patterns, despite the heavy influence of culture. The
male brain is sexually oriented toward women as an object of desire. The
most direct evidence comes from a handful of cases, some of them
circumcision accidents, in which boy babies have lost their penises and been
reared as female. Despite every social inducement to the opposite, they grow
up desiring women as partners, not men.

³If you can¹t make a male attracted to other males by cutting off his penis,
how strong could any psychosocial effect be?² said J. Michael Bailey, an
expert on sexual orientation at Northwestern University.

Presumably the masculinization of the brain shapes some neural circuit that
makes women desirable. If so, this circuitry is wired differently in gay
men. In experiments in which subjects are shown photographs of desirable men
or women, straight men are aroused by women, gay men by men.

Such experiments do not show the same clear divide with women. Whether women
describe themselves as straight or lesbian, ³Their sexual arousal seems to
be relatively indiscriminate — they get aroused by both male and female
images,² Dr. Bailey said. ³I¹m 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.²

Dr. Bailey believes that the systems for sexual orientation and arousal make
men go out and find people to have sex with, whereas women are more focused
on accepting or rejecting those who seek sex with them.

Similar differences between the sexes are seen by Marc Breedlove, a
neuroscientist at Michigan State University. ³Most males are quite stubborn
in their ideas about which sex they want to pursue, while women seem more
flexible,² he said.

Sexual orientation, at least for men, seems to be settled before birth. ³I
think most of the scientists working on these questions are convinced that
the antecedents of sexual orientation in males are happening early in life,
probably before birth,² Dr. Breedlove said, ³whereas for females, some are
probably born to become gay, but clearly some get there quite late in life.²

Sexual behavior includes a lot more than sex. 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 ³can last 12-18 months,² 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.
Brain imaging studies show that a particular area of the brain, one
associated with the reward system, is activated when subjects contemplate a
photo of their lover.

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.

Researchers have devoted considerable effort to understanding homosexuality
in men and women, both for its intrinsic interest and for the light it could
shed on the more usual channels of desire. Studies of twins show that
homosexuality, especially among men, is quite heritable, meaning there is a
genetic component to it. But 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 mother¹s side.

But Dr. Bailey believes the effect, if real, would be more clear-cut. ³Male
homosexuality is evolutionarily maladaptive,² he said, noting that the
phrase means only that genes favoring homosexuality cannot be favored by
evolution if fewer such genes reach the next generation.

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 don¹t 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 ³a maternal immune response to succeeding
male pregnancies,² Dr. Bogaert wrote last year in the Proceedings of the
National Academy of Sciences. Antimale antibodies could perhaps interfere
with the usual masculinization of the brain that occurs before birth, though
no such antibodies have yet been detected.

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.

The effect supports the idea that the levels of circulating testosterone
before birth are critical in determining sexual orientation. But
testosterone in the fetus cannot be measured, and as adults, gay and
straight men have the same levels of the hormone, giving no clue to prenatal
exposure. So the hypothesis, though plausible, has not been proved.

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. Researchers had long assumed that
steroid hormones like testosterone and estrogen did all the heavy lifting of
shaping the male and female brains. But 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 women¹s
neurons presumably perform that role by other means.

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 X¹s
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.

³It¹s popular among male academics to say that females preferred smarter
guys,² Dr. Arnold said. ³Such genes will be quickly selected in males
because new beneficial mutations will be quickly apparent.²

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, ³will have more variable brain phenotypes,²
Dr. Arnold writes, because women¹s 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. Women¹s care
in selecting mates, combined with the fast selection made possible by men¹s
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.

Who can doubt it? It is indeed desire that makes the world go round.