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			<h1 class="headline"><!-- ISI_LISTEN_START -->Pas de deux of sexuality is written in the genes</h1>
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					<div id="author" style="float: left;"><strong><a href="http://www.iht.com/cgi-bin/search.cgi?query=By Nicholas Wade&amp;sort=publicationdate&amp;submit=Search">By Nicholas Wade</a></strong> </div>
					<div id="pubDate" style="float: right;">Published: April 10, 2007</div>
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	                <p>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.</p> 
<p>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. Women&#039;s 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.</p> 
<p>So much fuss, so intricate a dance, all to achieve success on the simple scale that is all evolution cares about, that of raisingthe 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.</p> 
<p>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&#039;s 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.</p> 
<p>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.</p> 


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<p>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.</p> 
<p>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.</p> 
<p>It is a misconception that the differences between men&#039;s and women&#039;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&#039;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.</p> 
<p>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.</p> 
<p>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.</p> 
<p>"If you can&#039;t make a male attracted to other males by cutting off his penis, how strong could any psychosocial effect be?" said J. Michael Bailey, an expert on sexual orientation at Northwestern University.</p> 
<p>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.</p> 
<p>Such experiments do not show the same clear divide with women. Whether women describe themselves as straight or lesbian, "Their sexual arousal seems to be relatively indiscriminate &#8212; they get aroused by both male and female images," Dr. Bailey said. "I&#039;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."</p> 
<p>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.</p> 
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