Book Excerpt: 'Sex: A Natural History'

Aug. 27, 2002 -- Why do we find certain people and not others attractive? How do we select our mates, and why do we sometimes cheat on them? What about the battle of the sexes — are men really from Mars and women from Venus? In Sex: A Natural History, award winning science reporter Joann Rodgers explores the biology and psychology of what drives our sexual behavior. Read the introduction to Sex: A Natural History below.

'It is an old maxim of mine,' said Holmes, 'that when you have excluded the impossible, whatever remains, however improbable, must be the truth.' Answered Watson, 'Perhaps, you may have convinced me as to the motive, but you are yet to explain how it is done.'— Arthur Conan Doyle

People love sex. We have it every chance we get, in every position and season. We will take incredible risks, exhaust ourselves, even self-destruct to get it, do it, keep it. Once we experience its power and its pleasure — even when we only can imagine it — we seek it with the intensity of an addict after a fix. If nature were into efficient engineering of reproductive systems, interest in sex would end with a woman's last ovulation. Any self-respecting MIT graduate cum Harvard MBA would insist on a just-in-time inventory system: When you're out of eggs, you're out of sex. Instead, we, like every other living thing, are throbbing collections of protoplasm whose energies are ever in screaming search of sex. We want sex not just for reproducing, not just on purpose, but for pleasure and even just for the pleasure of its pursuit.

Consequently, if sex were a novel, it would be an Everyman story. The heroes and heroines would be every one of our genes, cells, tissues, hormones, organs, and most of all the chemistry of the brain and mind. The plot would track the quest to bring them together in harmony against all odds to merge a heavily guarded set of gametes while simultaneously bringing pleasure to participants. At the end, we would respect the characters if not altogether like them. A friend and colleague, two-time Pulitzer Prize-winning science journalist Jon Franklin, once said that truly heroic stories are full of "aha!" moments that show how ordinary characters resolve to imagine, then successfully execute creative, resourceful, lasting, and most of all practical solutions to fundamental, serious, life and death complications. He could have been summarizing the story of sex. There is no magical deus ex machina, no metaphysical or divine act that makes sex work. Just diligent, tenacious, randomly acquired, hard-earned tangles of internal constructions and functions, hardwired yet flexible enough in each one of us to meet unique circumstances.

Paradoxically, for most of us, how these elemental facts of life happen is as poorly understood as they are compelling. This is partly, at least, because Rube Goldberg couldn't reengineer a more clumsy, less "efficient" process. Sex in humans and other animals is an almost ludicrously complicated, infinitely resourceful, and varied network of anatomical, chemical, social, biological, and emotional signals and schemes for regenerating and perpetuating chromosomes, genes, and DNA.

But our lack of understanding is also because we seem squeamishly reluctant to think too hard about sex. Sex, according to Johns Hopkins University psychologist and sexologist John Money, is a thoroughly unloved human behavior. "Isn't it supremely ironic," he once told me, "that the very thing responsible for every parent and every child is a philosophical orphan? People thirst for a sexonomy, a factual set of fundamental principles that lets us talk and think about sex as a reflexive, built-in thing we do, no different than coughing, thinking, sneezing, or singing. Sex is here because we are members of the human species, not because of any act of yours or mine. And most people can't bring themselves to talk about it."

We prefer "doin' what comes naturally" to considering what "naturally" means. While excavating the secrets of life is notoriously difficult, those whose livelihood it is to try have become adept at peeking under the planet's blankets to find out just what "naturally" does mean. Sometimes hilariously so. En route to finding ways to get rid of a citrus crop pest called Diaprepes abbreviatus, an inch-long black beetle, University of Florida zoologist H. Jane Brockmann and Ally Harari of Ben-Gurion University in Israel spent long hours sorting out the mating rites of this bug and concluded simply that "everybody mounted everybody." Males mount females, males mount males and even mating couples. Females mount females and lure alpha males in the process. It turns out that the beetles have a hard time sorting out who is who. But nature doesn't play dice with anything so important as survival of a species, leading Brockmann, with classic understatement, to note that "when you see something like that, it demands an explanation."

Well, yes, but to those on the lookout for the natural signals of sex, these beetles' mating habits may also explain something about why Hugh Hefner made a splendid living exploiting the flirtatious dance of animal attraction between wealthy men and big-breasted young blondes. Or between men and women in general. The signals are different only in kind, not in category. As the beetle experts learned, the females who tried to mate with other females tended to wind up with the largest males, the insect equivalent of the men with wealth, which helped the girls in terms of their survival, because bigger, in the insect world, is indeed often better. The larger the male, the more likely he is to bring nuptial gifts, such as more food and a set of genes that helped him get them. It also turns out that while it's hard even for the beetles to sort it out, females are a bit bigger than males. So the males first follow their noses to the females, tracking a particular scent the girls give off; but because the boys still can't always tell who wears pink from who wears blue, they frequently mount another guy or, mistaking size for sex, head for a beetle couple already mating. Without any better navigational cue the males depend on sight, moving toward "big" beetles, which are more likely female but may well be a mating pair. And, if such a male is big enough, he can always push a smaller male off the back of the female of the pair he mistook for a big girl and have his way. In a male sexist world, that would be the end of it. But to the keen observers of these beetles, such visual signals suggest a far more important theme in the nature of sex: that it takes two to tango, and to choose to dance in the first place. It seems the females are also doing the choosing. How? By mounting each other. The bigger female, who mounts another female, attracts the biggest guys who, myopic as they are, think the female-female pair is one giant lady bug. Result: The biggest male gets the shrewdest female. To test all this further, the scientists spent more hours and days observing the reaction of male beetles to sets of large and small female-mounted pairs. The larger beetles went straight for the larger females. By attracting the biggest males, the most "fit" females got good genes (i.e., those from bigger, presumably more potent and wealthy males) and nuptial gifts such as food that male bugs tend to pass on during copulation. If all this doesn't remind us of the behavior of some two-legged animals, we haven't been paying attention. And if this isn't the nature of sex, and natural, nothing is.

Just why we so willingly endorse the mystique of sex but avoid confronting its mystery may rest in part in the difficulty of doing what the beetle watchers do-that is, spend a lot of time as objective voyeurs — but also in our universal vulnerability to its force. We adore telling stories about sex, but we don't want sex to tell us too much about ourselves.

Sex scares us. And why shouldn't it? "Making love," says psychologist David Buss, author of The Evolution of Desire, "is one of the most important, complex and perilous cooperative exchanges that any of us engage in during our lives. Loaded with promise and fraught with dangerous pitfalls, love affairs tax our ability at deal-making to the fullest, requiring the complete repertoire of psychological specializations that evolved for cooperation." University of Texas zoologist David Crews, who has studied sex behavior for decades, is more blunt. "Sexuality," he declares, "is all in your head and that's the most dangerous place it can be. It has so many components, but we only have one thought at a time. So usually we act on it and set down a course. We get confused, forgetting all the other aspects. We get into a lot of predicaments."

Why bother to parse any of this? Beyond idle curiosity, or concern about sexually transmitted disease, why probe? After all, no one needs an instruction manual; the mechanics aren't that hard to figure out, or even master. If the goal is to get, beget, or be gotten, even the sexually challenged get help from a vast inventory of how-to and advice books, medical texts, illustrated guides, devices, and therapists.

One reason to look further is that information at the clinical level is to a genuine knowledge of sex what a list of grapes is to a master wine maker — as remote from creation of a great or even a good vintage as a 1945 Chateau Mouton Rothschild is from fermented grape juice.

Another is that because sex is essential to collective survival, societies are obsessed with it, creating rules that irrationally inhibit and too often cruelly torment people. Debates over political "solutions" to sexual "problems" are well-served by information about the biology and psychology of sex. As archaeologist Timothy Taylor noted in his book The Prehistory of Sex: Four Million Years of Human Sexual Culture, sixteenth-century Spaniards were so outraged at the homosexuality and transvestism they encountered among the indigenous people they conquered that they destroyed almost all of the sculpture, pottery, monuments, and jewelry that depicted such practices, distorting greatly our view of sexual behavior and morality. "We cannot assume," Taylor adds, "that our modern way of thinking about sex — either biologically or socioculturally — is necessarily any more objective than any other way of thinking about sex. Even within the rather narrow Western tradition . . . from Plato to Shere Hite, it is clear that no one has ever had a monopoly on the truth about our bodies."

The most powerful and useful reason, however, is that whereas the evolutionary "goal" of sex is straightforward — a union between two suitable and compatible sets of chromosomes that assures survival of the species — its nature may be the clearest, brightest window through which to look, and mirror in which to reflect, the meandering and sometimes misleading connections between bedrock biology and behavior. Sex is, on the whole, the organizing theme, the ulterior motive of our inner and outer lives. So much is involved in the way we attach to others and reproduce ourselves that no other human process is likely to teach us more about the links between what we are and what to do.

Taken from the Latin secare, meaning to divide or cut (dividing in two implies reproduction), the word "sex" evokes not only copulation but the two sexes and their genitals. Yet sex is also about eros (from the Greek word for love), and thus about desire, motives, and intimacy. All of the kissing, touching, tickling, biting, teasing, positioning, penetrating, prolonging, and climaxing is the wrapper; the contents are at least as compelling.

Sex as we know and practice it today, in all of its expressions, from prostitution to bestiality, is part and parcel of our long, long, prehistoric legacy. In his Prehistory of Sex, Taylor exposes the hidden sexual treasures of museums and archaeological collections worldwide to support his contention that modern humans, from their earliest versions two hundred thousand to three hundred thousand years ago, had substantially invented everything there was to invent about sex, much about which we're still afraid to ask. Early man and woman, for instance, not only understood where babies come from but easily separated sex from reproduction, using contraception and abortion to control fertility. They developed animal husbandry, "oppressed" women to maximize population (harems are productive, after all), and probably practiced artificial insemination. They cross-dressed, recognized drag queens, established brothels, and created elaborately pornographic statues, fetishes, and paintings. "Starting around 5000 years ago, it is possible to document great variation in human sexuality in Eurasia: bestiality, homosexuality, prostitution . . . transvestism (male and female), transsexuality, hormone treatments, sadomasochism, autoerotic asphyxia . . . sex as an acrobatic and competitive pastime and sex as a transcendental spiritual discipline," he says. The artifacts that survived to fuel his research, all just a few thousands years old, "represent just a tiny part of the four-million-year saga of its prehistory," but include such compelling items as golden penis sheaths, the full erection of a Dordogne cave painter's depiction of a shaman in a trance, and other paintings of body piercing, anal sex, sadomasochism, and masturbation.

Our ancestors' sexual culture, then, is part of our sexual repertoire, too. But if at times surreal, sex is never unreal. Penises, personalities, vaginas, nipples, skin, neurons, phobias, temperament, facts, fallacies, and families all get in the act, and sometimes in the way. Our ovaries, testes, and gametes play out their complicated roles as long as we do our part to get them coordinated.

Predictably, it's the coordination of things right now that consumes our interest, our energy, and our time. Each thought of love and longing, each act of courtship and copulation, each flush of desire and arousal represents the sum and synergy of biological ingredients, always, always, in tune with particular circumstances and with those paramount drivers of sex, the mind and brain. Each sexual engagement is a moment in time that reveals what it is to be quintessentially and quite literally alive.

"Ideas," wrote novelist Henry Miller, whose Tropic of Cancer and other stories were once literally banned in Boston, "have to be wedded to action; if there is no sex, no vitality in them, there is no action." And it is in the process of sex that all the action takes place — the process, millions of years in the making, that snakes and lurches its way through an intricate dance beginning with the elegant, unheard intercourse of DNA molecules and concluding with what even poets and artists depict as a relatively graceless and disheveled display. "The act of procreation," wrote Leonardo Da Vinci in Dell'Anatomia, "and the members employed therein are so repulsive, that if it were not for the beauty of the faces and the adornments of the actors, and the pent-up impulse, nature would lose the human species." (Not likely, Leonardo.)

As choreographed by humans over the past 4 million years, sex vigorously endures. What's new is what's been learned about the nature of sex, the inner sex lives of our cells and ourselves. Understanding sex's nature lends meaning to the knowledge that both men and women have orgasms while dreaming, and that men universally are far more occupied with a woman's physical attractiveness than women are with a man's. To know sex at such levels is to better know ourselves.

Happily, the growing number of serious scientists engaged in the study of sex have a lot to tell us. For the best of them the story of sex isn't about mechanics featured in "sex education" film strips or some arcane account of genes and molecules, but indeed a way of organizing and thinking about all of the things we know or think we know about what is most central to human nature and behavior. If there is an overarching principle informing the story of sex, it's some version of what Martin Daly and Margo Wilson write in their book Sex,Evolution, and Behavior: "All social organization is in principle interpretable as the outcome of the sexual strategies by which animals try to reproduce themselves."

Over the past decade, those seeking sex's nature have overcome conceptual, social, political, and scientific obstacles and moved the field to a golden age of collaboration, sharing imaginative and even revolutionary ways of looking at the information they gather. To an astonishing degree, they have learned how to "ask" minds, brains, bodies, cells, and molecules — in creatures great and small — how sex works. To an equally astonishing degree, they are getting answers by learning how events that happen at one level of biology — in brain cells that make and react to chemicals, for example — influence, control, start, stop, and account for action at very distant levels. They are teasing out long hidden facts that tie sexuality to its biology and its circumstances.

Building on dramatic discoveries of the twentieth century, scientists are linking genes and the chemical blueprints they carry to both secare and eros. Their turf now ranges widely, from neuroscience and psychology to genetics, cell biology, endocrinology, and evolution — because nothing, no part of our conscious or unconscious lives, stays untouched by sex. When we're sexually attracted to someone, the allure plays itself out in our hormones, genes, and behavior, in our "hearts" and in our minds. When we encounter sex, it is simultaneously present in our bodies and brains, our memories, even in our culture. No wonder then that "Sex research covers everything from A to Z, anthropology to zoology," says Howard Ruppell, executive director of the Society for the Scientific Study of Sex. It embraces the enormous variation in human and animal sex behavior, not only between species but also between individuals in any one species. It brings psychiatrists and evolutionary biologists to the same meetings as zoologists and archaeologists to track each others' research and build on what each has learned. More than a half century ago, embryologist Frank Rattray Lillie of the University of Chicago first suggested that male hormones secreted in the womb were responsible for the fact that when cows have twins of opposite sexes, the female is sterile and behaves more like her brother the bull than other cows. Since then, scientists have systematically looked beneath the blankets and the skin to where the biochemical action of sex takes place. There is biological music that accompanies sex and the goal of science is to offer at least eight bars everybody can hum.

What universally characterizes the best of those contributing to our understanding of sex is their amazing ability to develop ways to even ask such questions as whether there might be molecules of monogamy or hormones for love. I am humbled by how hard it is to scientifically demonstrate what we lust after without a second — or even a first — thought. Most of us glibly throw out such lines as "men and women are different below the belt and in the brain" or "the brain is responsible for behavior," but scientific support for these sweeping generalizations required intellectual tours de force. Teasing out the secrets of the body-mind-brain-emotion-hormone-gonad circuits is a task that brings weak scholars to their knees. Just exactly how does a biologist ask a male bird or bird-watcher to objectively reveal what propels him to love or make love to a particular female?

They do it by exploiting the theoretical knowledge and technological means available now for the first time in history to map the brain, measure and regulate hormones, manipulate behavior, track DNA, and link specific complex traits to individual genes. With a discriminating eye on the work of Alfred Kinsey, William Masters, Virginia Johnson, and even Dr. Ruth, they have brought to sex research the tools of molecular biology, anthropology, social biology, brain imaging, and genetic engineering. Now they are moving quickly to apply these tools to mating in all its forms and functions, integrating discoveries from songbird brains and peacocks, one-celled creatures and fruit flies, worms, parasites, mice, and men. Riding the wave of technological and conceptual advances in their investigations of "inner sex," they've devised a way to collect and track individual daily urine samples to test for sex hormones and sexual cycles among gorillas in the wild. ("They won't pee in the bottle," one quipped.) They've knocked out individual genes to identify biological causes of sexual aggression and parenting, designed glass bottles so that everyone can watch fruit flies with a certain mutated gene seek same-sex liaisons, constructed cages that let virgin female mice sniff but not mate with males to see if the mere presence of the male within smell range triggers brain changes in the female (it does). They've fashioned toy dummy wasps to fool males into mating, strapped artificial weights to beetles to test the notion that males give females a substantial amount of body weight during sperm transfer and that females prefer heavier males. They've placed tiny microphones near insects to measure their "bedroom" sounds and made digital recordings of novel copulatory screams by macaques to analyze how the brain organizes to perceive and react to sex signals.

By tracing connections between brain activities, hormones, emotions, and anatomical structure, investigators are sorting out with some certainty how oxytocin, the natural hormone that triggers the uterine contractions of labor, keeps female prairie voles faithful to one mate; how it is that a woman in love can have orgasms simply by imagining a tongue on her clitoris or breasts; how Harlequin paperback romances and fantasies are so powerfully arousing; and why, for a nursing mother, the oxytocin, secreted by cells in a primitive part of her brain, makes her nipples tingle with pleasure as her milk "lets down" or at the thought of a lover's touch.

At the benches and in the field, sex scientists have learned that if you stare intensely into the eyes of an attractive girl you've just met in a bar, she'll likely avert her gaze because it's physically uncomfortable to do otherwise; and that when love is lost, it does feel like a punch in the gut. They also have learned that if you become her lover, she'll stare back; that what makes our hearts pound as we flee a mugger or build to a sexual climax is the same as what gives a long distance runner her "high"; and that the same muscles that constrict the anus of a novice bungee jumper are called into play to intensify the pleasure of orgasm. (Indeed, it may be the "thrill" of that constriction that impels the jumper's two hundredth plunge.) They've even amassed evidence that brain chemistry governs pair-bonding in neurochemical cycles during reproductive years, generating emotional attachment peaks and valleys that roughly correspond to prehistoric time requirements for child rearing as well as Americans' patterns of divorce.

Even if some of these findings fail to hold up to further scrutiny, they already have forged a perspective that has enriched sex research. Scientists are thinking "out of the box," searching for what David Crews calls "complementarities" instead of singularities. If it's true that like other animals, men and women have subliminal odors that attract the opposite sex, the pheromones we've been hearing so much about these days, that's interesting. But if it's also true that for any particular man, only some women attract him, then clearly something more enterprising is going on between the two sexes.

The story of sex is crowded with such accounts, some as irresistible as the sex drive: of what makes semen smell musty and taste sweet, and the sensual behavior these things create as well as reflect; of the erotic lure of the deep, thumping bass rhythms of a rock 'n' roll band; of the literally wilting effects of depression on the messages between brain and penis.

In telling the story of sex, others have done so from a particular point of view (feminism, for instance) or singled out a particular phenomenon (e.g., sperm competition) or expert theory as the dominant theme or basis of sex and sex behavior. There are also those that explore solely the evolutionary origins of sex. In this category is Matt Ridley's The Red Queen. Sex exists, Ridley writes, because it is the only reproductive design that can constantly mix genes and provide a necessary supply treadmill of potentially advantageous, disease-resistance genes that keep us from succumbing to the endless onslaught of bugs determined to kill us off. The process, like the subjects of the Red Queen in Lewis Carroll's Through the Looking Glass, "takes all the running you can do to keep in the same place." But at the end of the day, scientists don't really know precisely why sex exists.

Still others focus single-mindedly on hormonal chemistry or brain anatomy to fully explain some aspect of human behavior. The present story of sex, however, attempts a broader sweep at the rich stash of scientific discoveries about how sex gets done each step along the way to genetic survival. Astute scientific readers will note, and may even be disturbed by, the book's deliberate shifts between the evolutionary biologist's ultimate time tests of human behavior, those measured by persistence over eons; and the physiologist's moment-to-moment reconstruction of the biochemistry of now, the ebb and flow of action and reaction, brain receptors emptied and filled in response to some proximate event. ("Physiology" is derived from the Greek words for the science of natural events and causes). Certainly it would be a mistake for the reader to conclude from these shifts that, with only a few exceptions, physiological research in today's Homo sapiens has been, or ever can be, done to show that the latter absolutely supports the former, or vice versa. Nevertheless, I've moved without a stamped and valid passport between the two sciences, a liberty taken in the interest of keeping the story going. Trying to describe sex from so many different perspectives opens the text to attack from all sides. Indeed, thoughtful scientific readers, including Sue Carter Porges and James Weinrich, cautioned me properly against it. I fully recognize the danger of such speculative time travel, but risked it to offer fluid possibilities, a view of both camps and how they might be integrated.

Journalists, including myself, like to construct books at least partly out of a particular point of pique, and mine was founded on damage done to human sexual inquiry by the Freudian century. This book about sex contains only a single mention of Sigmund Freud, this one, a mention made only to say that it is possible, at last, to write a whole book about sex and never mention him at all. It's not so much a whine about his implausible impact as a testimonial to what rigorous scientists know about sex. A reporter's romp through the scientific literature of sex ought to tell a knowledgeable story about monogamy and its molecules; on courtship and its hormones; on gender and its genes; on sex behavior and its brain connections; on scholarly sex and good old fashioned "dirty" sex.

This account of sex also gives importance to the observation that the legendary battled of the sexes both begs the question of how we get to resolution of sexual conflict and denies the cooperative aspects of sex. And it appreciates that perhaps the largest differences in sexuality may not be between species so much as between individual males and females of all species. Especially the human species. There is, says Crews, who has exhaustively studied the role of hormones in the sexual behavior of animals, "enormous plasticity" in how each of us plays our sex scenes, based on novel circumstances and our physiological ability to adapt to such endless variety. "The variation among individuals within a sex is usually greater than the difference between the averages for each of the sexes," Crews says, and "[a]lthough the sexes differ in many traits, this often is a statistical phenomenon. Even as adults, each individual is capable of displaying the behaviors of the opposite sex and it is rare to find a truly sex-specific or sex-exclusive behavior at all." Indeed, it is altogether possible, even likely, that the complex string of traits we call sexual behavior came before the evolution of egg and sperm, penises and vaginas, the division into male and female. And more, that males emerged only after so-called hermaphrodite organisms — females that could reproduce themselves — first materialized.

Researchers may never fully pin down specific molecular tracks left by sexual exclusivity, sexual accessibility, sexual attractiveness, and emotional devotion, the strung-together quartet of human sexual strategies painstakingly identified by evolutionary psychologist David Schmitt. But, there is logic in the concept that the brain houses the mind, and the mind functions as interpreter of the slim space between our inner selves and the outside world we deal with in every moment of our lives. For biology's parishioners, the scientists in pursuit of those molecules, the mantra is "I wouldn't have seen it if I hadn't believed it!" rather than "I won't believe it until I see it." What makes the science of sex so alluring and so challenging is what makes biology, not physics, the twenty-first century's hottest and most complex science. Because unlike in physics, where a knowledge of mechanical and particle characteristics and behaviors leads to working models and provable causes (e.g., we stay rooted to the Earth because of gravitational force), in biology causes are never the sum of the parts, but rather the sum of parts interacting in particular organisms at a particular time in a particular environment. No one has seriously challenged Copernicus for centuries; evolutionary biology is still a "theory" to a majority of American citizens. Indeed, scientific information can arguably never tell us what is always or absolutely true; it can only, as biologist Ernst Mayr concluded after eight decades of inquiry, approximate what is possibly or probably believable.

That scientists get, for now, believable, even cogent answers that occasionally stand the test of challenge and analysis, time and reason is amazing, and I have tried to faithfully tell their stories. They in turn have relied on the mostly involuntary subjects of the story of sex: people and animals on whom experiments are made or who themselves are experiments of nature — the diseased and disordered and "different" across a zoo of species. Besides the humans, homage must be paid to prairie voles, whiptail lizards, red-sided garter snakes, Japanese quail, songbirds, possums, chimps, lobsters, blue-headed wrasse, one-celled algae, wild and inbred mice, rats, roaches, and dozens of other species who have "donated" their brains and bodies to science. After all, evolution has no direction toward perfection or superiority, so there's no way to say what is the best or more advanced model from which to draw lessons about the biology of human behavior or human nature. But I am persuaded by those whose work dominates sexual biology that there has perhaps been too little appreciation of how parallel are animal strategies — biological and emotional — for working out the "problem" of sex and too much emphasis on our self-consciousness to explain our sexual selves. Just as we did not evolve opiate receptors in the brain to enable addiction to heroin, we have not evolved as sexual beings to enjoy sex. Nature's driving force does not particularly "care" if we have love with our sex, or multiple orgasms, only that we successfully replicate. "If Arnold Schwarzenegger never has a child, in fact if he does not have a lot of children, then in terms of evolutionary fitness, he is a 90-pound weakling," notes molecular biologist Shirley Tilghman of the Howard Hughes Medical Institute at Princeton University.

A further statement about sex and sensibilities seems in order, especially if, like me, you grew up on Woody Allen's neurotic reflections on sex.

Almost universally, sex, even in fish, lizards, parasites, and peacocks, is a research orphan, or worse. Conducting human sex research requires a fighter pilot's skill and courage, and there's no parachute to safety. A potent piece of evidence for that statement comes from Diane deMauro, in her 1995 report "Sexuality Research in the United States: An Assessment of the Social and Behavioral Sciences." She notes that the work of the late Alfred Kinsey is nearly a half century out of date. Very few besides William Masters and Virginia Johnson, whose groundbreaking work is decades old as well, are lining up to make sexuality a "primary focus" of their research. (I found it remarkable that a bibliography of thirty-six key publications listed on the Web site of the Kinsey Institute was dominated by vintage books and articles. Only eight were published in the 1990s and six in the 1980s.)

Scholars know, often from bitter experience, that without the instincts of a cold warrior, studying sex can be dangerous and getting published punitive. Nearly every scientist I interviewed (more than fifty) volunteered that she or he had felt the threat of career-killing disapproval by public and private funding agencies, politicians, institutions, and the public. They prefaced every conversation with a plea that I take special care not to tie their observations too closely to human sex, even though they believe that, generally speaking, what happens in a mouse is relevant to what happens in a man.

Sometimes, the information I sought in published materials was deliberately buried by the authors in the arcane code language of their specialties, curbing visibility by all but their savvy peers. Without their help with the code, I might never have looked for, much less found, important information on female orgasm in articles about pupillary dilation as a measure of vagus nerve response to electrical stimulation.

However we get them, the facts summon us to become explorers of sex, to look "under the hood" for those events at which the brain meets behavior, the mind's capacity for love meets the body's compulsion to act, chemistry meets charisma and romance reproduction. When such information is melded with observations among our near and distant ancestors, and within the strict terms of evolution — if things work they tend to be copied — scientists can simultaneously train the microscope and the telescope on our most primal activity and create a picture that is both detailed and in perspective. They're giving us the forest and the trees.

No part of the human experience falls outside the purview of sex. Aggression and violence, culture and commerce all have an impact on, or are influenced by, sex. There's a sexual role for the senses and symmetry, dance and disease, fantasy, feelings, feminism and film, folklore and gender, genetics and humor, IQ and inhibition, language and love, medicine and memory, morality, pain, painting and perversion, chemistry and politics, racism and rape, religion and science. We evolved to think Renoir's paintings beautiful. The romanticism and eroticism they evoke for people everywhere are no random consequence of the processes that help to assure sex and reproduction.

Arguably, we invented cities, clothes, art, history, the work ethic, the family, and every human investment and venture because of sex, as a result of sex, to recruit sex, to be good at it, to master it, capture it, and ultimately to try and contain and control it. An economist looking at a societal organization chart might well conclude that while sex is freely distributed, it is never without cost, and that it may well be the stable currency required for every meaningful transaction. For sex is forever and completely tied to our identity, to the best and worst of who and what we are and of our stake in the future.

At a different level of discourse, we might consider sex research the way we think about the study of cave paintings at Lascaux, Chauvet, and Cosquer, which altered forever people's perspective on the development of modern human beings. Like the French cave art, vivid experiments of nature and modern science are extending our vision of sex and its role in all of life. To apply wishful or wistful political, social, or religious agendas to sex, to put facts into a frame that limits how we think people ought to behave, or once behaved, is to commit the same mistakes as those who argued that cave art was a prank, that early man was too "primitive" to have made art so compelling, creative, symbolic, and abstract. Believing — or not — that twenty-first-century human beings have or should have evolved beyond what "prehistoric savages" did adds nothing to our knowledge or understanding of what sex is, or how it works, any more than judging a drawing tells us what art is or how it came to exist. "There is great risk," evolutionary biologist David Haig warned me, "intellectually, scientifically, and socially in trying to draw moral lessons from sexual phenomena. All attempts to do so are a form of power grab or manipulation."

What else we can say with certainty is that sexual reproduction is so successful, so much the winning strategy in the survival sweepstakes that everything we see and understand about animal and human sex reflects solutions to quite literally life-threatening problems. If human sex were a Michelin guide for survival, every route and signpost; every superhighway and footpath; every mountain, valley, overlook, stream, and city; every detour, fork in the road, and attraction would represent field-tested, three-starred journeys chosen and endured by generations of tourists who never planned anything, including a trip home. There are no package tours, no guarantees, only origins and altered destinies. Getting there is not half the adventure, it's all of it. When we know how it all works, we increase our appreciation of the journey.

In any case, after nearly five years of sifting the work of scholars interested in the matter of sex, I am mostly still full of the wonder of it all. (Also, I am numb to double entendres gleefully exposed by friends and loved ones — and even my ceaselessly patient editor — after every innocent utterance, including "I'm coming!")

Meanwhile, the only certainty in our sexual futures is change wrought by failure and success, death, and adaptation of cosmic dimensions. Like every biological event, sex and the mechanisms that support it are subject to ongoing experiments of nature, some surely destined to be "dead ends" and some adaptive. Perhaps the "two-sex" system as the dominant animal mating strategy, and all "acceptable" sexual behaviors, are merely way stations en route to extinction, making way for different strategies, perhaps some that can better reconcile or embrace our spiritual, thoughtful, intelligent, emotional traits. Conceivably, heterosexual sex, and two genders, are a transitional set, much like the dinosaur archaeopteryx Melvin Konner wrote of in The Tangled Wing, "a piss-poor reptile and . . . not very much of a bird." Are we a "piss-poor mammal" and not very much of a "spiritual creature" working our way to something else?

Contrary to our egocentric wishes and vague memories of grade school Darwin, we cannot, with any ease, consciously or willfully change how our bodies and minds do sex to meet the whims of Western civilization, urban culture, religious beliefs, and environmental destruction. Our uniquely human influence on the process can be profound, of course. We perform in vitro fertilization and genetic engineering of single traits; we contracept and prohibit, regulate and rule-make. Nevertheless, secare and eros are our once and (foreseeable) future destinies. Nature's game rules still trump our will, and our personal version of sex cannot override biological or psychological legacy. Through sex and love comes our only immortality, through our children. (Or, more precisely, through our grandchildren, since from nature's perspective, parents' only real assurance of a genetic future is the fertility of our offspring, not ourselves.) The only sensible challenge — and real reward — is to catch nature's own act.

Excerpted with permission from Sex: A Natural History by Joann Rodgers © (Times Books / an imprint of Henry Holt & Co.)