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WHY IS SEX FUN? The Evolution of Human Sexuality.<i> By Jared Diamond</i> .<i> BasicBooks: 166 pp., $20</i> : SEX ON THE BRAIN: The Biological Differences Between Men and Women.<i> By Deborah Blum</i> . <i> Viking: 330 pp., $24.95</i>

<i> Robert Lee Hotz, author of "Designs On Life: Exploring the New Frontiers of Human Fertility," is also a science writer for The Times</i>

An evolutionary biologist might explain the suburban passion for sport utility vehicles as a mating ploy meant to signal the owner’s reproductive fitness. The oversized automobiles are commodious enough for any number of offspring and, like the peacock’s tail, show that the owner can divert any amount of energy to ornamental display.

So too, an anthropologist might trace the human tribe’s obsession with the sex life of celebrities to the primordial biology of the human species. Certainly, the reproductive behavior of the dominant animal in any mammalian hierarchy is under constant scrutiny by those on its lower rungs. What else could explain the persistent public fascination with rumors about Michael Jackson’s offspring, Demi Moore’s breast implants or Bill Clinton’s alleged extramarital affairs? Surely something atavistic is at work.

From our demand for waterproof cosmetics to our sense of a rightful place in the cosmos, we are shaped by the biology of sex far more than we would like to admit. Indeed, the debate over biologically based gender differences has caused no end of cultural and political conflicts as, spurred by changing economic conditions and the technological innovations of birth control, we seek the social equilibrium of a new sexual norm. Yet when we look at ourselves in the mirror of biology, we see that of all the works of nature, human sexuality is perhaps the strangest.

As a matter of fundamental biology, almost no other species, for instance, feels so strongly about having sex in private. Few are so enamored of recreational sex or so receptive to it. And while most animals widely advertise the moment of ovulation--when the female of the species is most primed for fertilization--the precise timing of ovulation among human beings is hidden biologically, even from the woman experiencing it. The sexual oddity of the species touches the human male: By the standard of other primates or, indeed, of any detectable reproductive need, the human penis is unnecessarily large: five times the size of the bull gorilla’s or the orangutan’s, even though those animals are twice the size of human beings. And the oddity encompasses the human female too: Why do women--apparently the only creatures to do so--experience menopause?

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Two new books attempt to offer scientific insights into human sexuality. The first, by distinguished UCLA physiologist Jared Diamond, examines sex through the lens of evolutionary theory; the second, and by far a more ambitious work by Pulitzer Prize-winning science writer Deborah Blum, draws on new biological and behavioral research to probe the fundamental riddle of gender: How can we be so different when we are so much alike?

In Diamond’s view, the relentless adaptive forces of evolution have shaped us in response to a war for reproductive advantage. He is convinced that the unusual nature of human sexuality was essential to the development of the intelligence and upright stature we consider the hallmarks of our species.

In his newest book, Diamond has crafted an engaging display of evolution’s hidden logic to explain how such puzzling sexual characteristics may have been the unlikely result of adaptive selection. Women undergo menopause not because they have outlived their reproductive usefulness, Diamond argues, but because they gain a greater reproductive advantage by caring for their grandchildren than if they continued to conceive from their own aging store of ova. And the human penis may have achieved its size to signal sexual prowess--not to women, Diamond suggests, but to other men, to discourage competition.

In human affairs, however, biological ends are accomplished through social means. The proper social roles of the two genders in courtship, reproduction and parenting--and the biology that may underpin them--are a source of enduring friction; only now has science come to have an inkling of the remarkable subtlety with which biology both knits together and separates the human genders.

Exploring the biological differences between men and women, Deborah Blum, who teaches journalism at the University of Wisconsin at Madison, has centered her inquiry squarely on the confluence of biology and culture to illuminate the natural forces at work within us. The result is superbly crafted science writing, graced by unusual compassion, wit and intelligence, that forms an important addition to the literature of gender studies. She offers considerable scientific expertise and, more important, a humane and unflinching discussion of the biological issues that have set men and women against each other since time out of mind.

Indeed, many researchers who attempt to explore gender differences, she notes, are accused of sexism simply for expressing an interest in the topic. Such suspicion is well-founded. The historical record of sexual science is a sorry one. It is not so long since the standard medical treatment for a woman’s moodiness was the removal of her reproductive organs. Certainly, more people in the 20th century have died as a result of misconceived notions of human biology, such as eugenics and erroneous racial theories, than from any thermonuclear device.

As a scientific discipline, Blum demonstrates, the study of the biology of gender is only now coming into its own, emerging from a thicket of prejudices and misconceptions to pose some of the most provocative questions today in neuroscience, biochemistry, genetics, psychology and behavioral studies.

Neuro-imaging studies, for instance, appear to show that men and women really do think differently. Medical PET scans show that they even appear to daydream differently, with different parts of the brain being activated during rest states. Other behavioral studies appear to confirm that men are more aggressive, better able to organize power networks and more prone to violence; women are more nurturing, more resilient and more prone to gossip. Other tests show persistent gender differences in mathematical and spatial abilities.

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Yet so malleable is the neural structure of the developing human brain--male or female--that it is impossible to know whether such differences arise from genes and the inborn biochemistry of gender, or are the effect of gender expectations and rigid childhood conditioning. By adulthood, such attitudes would be integrated into the physical structure of the brain. Moreover, without knowing it, we already may have derailed the subtle reproductive mechanisms designed through the trial and error of evolution.

As a case in point, researchers recently attempted to study how women react to the subtle chemical clues about a man’s immune system contained in his scent. They discovered that most women found the scent sexiest when the male immune system was most different from their own, a biochemical signal perhaps of some reproductive advantage as a potential mate. Women using birth control pills, however, consistently chose men whose immune systems most resembled their own. In other words, they were drawn to the “wrong” man.

It is understandable to view such findings through the prism of our vulnerabilities. But gender biology, Blum writes, has extraordinary promise if its insights can be given an objective hearing. “It opens possibilities. It doesn’t close them. And it doesn’t segregate men from women. If anything, it does the opposite, emphasizing how intricately woven together we are in the design of evolution.”

If there is a danger here, it lies in experimental half-truths and our greed for self-knowledge. We are too anxious to race ahead of scientific certainty to draw conclusions about the biological roots of human character. The impulse to act on half-knowledge is for many policy-makers almost irresistible. Here, Blum warns that researchers have posed fundamentally open-ended questions about gender and that they have taken only the first step toward answering any of them.

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The truth of even simple relationships between gender biology and behavior is elusive. Take the apparent links between violence and maleness. Is the aggression of boys, as some new research hints, merely a matter of a biochemical imbalance between trace levels of copper and zinc? Or is it associated with the extra copy of the male chromosome that sometime occurs as a reproductive error during conception? Or is excessive testosterone, the hormone found in such high levels among men, at fault?

Men’s testosterone levels, however, drop dramatically when they are in happy marriages, so much so that some researchers speculate that women instinctively use monogamy to control male behavior. Other new research shows that many women in high-pressure professions show a rise in the level of testosterone. Does this mean they also have become more aggressive? More male? Or just more stressed?

If the 20th century has demonstrated any single scientific principle conclusively, it is that biology is not destiny. Yet we would be wise, Blum argues, to be informed by the biology of our gender. It is integral to our nature, be it male or female; it is designed so by evolution. To deny it would be as mistaken as to continue being confined by it.


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