Sex: Not an Exact Science

In their drive to strip away the secrets of sex, scientists have eavesdropped on the conversations of college students, waved T-shirts soaked with men’s sweat under the noses of women, measured chimpanzees’ genitals and manipulated the genes of rodents and flies.

Sex and choosing a mate are so important that our entire cultural repertoire--from art and poetry to David Letterman jokes--may have evolved primarily to help us attract better mates, according to one recently published book.

In other words, we not only have sex on the brain; we have large brains because of sex.

“I believe most motivation, at some basic level, comes down to improving our ability to reproduce,” said Rachel S. Herz, assistant professor of experimental psychology at Brown University, who has studied the role of scent in sexual choice.

However, there is more than a whiff of dissension in the air when scientists from various academic backgrounds try to answer the nitty-gritty questions about the roots of human sexual behavior.

Academics in the growing field of evolutionary psychology say the experiences of our hominid ancestors--and the imperative to make sure one’s offspring thrive--can account for many quirks in human behavior, such as why so many wealthy men take young trophy wives and why some young women like older men with money.

Yet scientists who work with the molecular nuts and bolts that underlie human behavior say such theories are intriguing but built on scientific sand. Science is a long way from finding biochemical mechanisms that govern lust in animals, much less accounting for more complex emotions such as human love.

“Evolution shaped our behavior; there is no question on that,” said Benjamin Sachs, professor emeritus of psychology at the University of Connecticut. “But the particular ways in which evolution shaped our behavior are still subject to hypothesis and testing.”

To biologists, sex is a transaction that most of nature’s organisms use to shuffle their genes. A fresh deck of DNA helps each generation maintain genetic diversity, which in turn helps its progeny adapt to changing environments.

The genes that help the offspring survive are preserved through natural selection, a term coined by Charles Darwin to describe what has long been held as the key driver in the evolution of life.

Sexual selection, Darwin’s lesser-known mechanism of evolution, is the way parents ensure their offspring get dealt a good genetic hand. The classic example of sexual selection is the peacock’s ornate tail. Lugging around such glorious plumage obviously doesn’t increase the bird’s odds of survival if he is trying to outrun a hungry predator.

The humongous tail is there for one reason. The peahen digs it. The peacock’s multicolored tail is nothing more than a billboard advertising his genetic fitness.

Applying these principles to human behavior, evolutionary psychologists say people may think they are making rational and objective decisions about bedding or marrying Mr. or Ms. Right. But they are actually conditioned by evolutionary pressures to plot ways to get the best genetic partner to accompany their DNA into the next generation.

Linking animal and human sexuality and the selection of a mate, however, is always a dicey scientific proposition. Peacocks, after all, don’t have complex brains or cultures that produce love poems and TV series such as “Baywatch” that can influence perceptions of a mate’s appeal.

Researchers offer some seductive but still speculative evidence that the seat of love and lust may not be in the organs most cited by poets or sex-education teachers, but may literally be right under our noses.

“When it comes to the leave-the-bar-together level of attraction, women ranked smell more important than men did,” Herz said. “It’s not so much a case that she likes his smell, but it’s, ‘He doesn’t smell bad, so it’s OK.’ ”

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Why the meat-market sniff test? Studies suggest women may be able to sense genetic differences between themselves and a man they are thinking of sleeping with.

But smell is just one of the tests sexes give each other when they are looking for good genetic bargains, evolutionary psychologists say. Visually, women tend to prefer tall men, who tend to father more children. And men in almost all cultures prefer a woman whose waist is about 70% the circumference of her hips, a sign of fertility.

But those sensory tests may just be warmups for the big enchilada of sexual preference, the ultimate evolutionary turn-on: the human brain.

Geoffrey Miller, English author of “The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature,” suggests our unique ability to write and talk about sex as well as do it may have evolved from our ancestors’ success in fine-tuning pickup lines.

Survival value alone cannot be reason enough for the development of our huge brains, Miller argues. During the past 2.5 million years, the size of hominid brains tripled. Yet it wasn’t until about 10,000 years ago that the human population and our technological skills began to increase dramatically. If genes for large brains were selected by evolution because they helped us make tools, why is it that the best we did, until the rise of human civilization, was to make small improvements on stone axes? Evolution, after all, demands an instant payoff if genes for such things as big brains are to get passed on.

What happened is our brains became a sort of Stone Age entertainment system, Miller suggests. Just as peahens prefer males with showy fantails, human women wanted men who had brains enough to fill up the long Pleistocene nights with charm, wit and poetry. That preference spurred the growth of men’s brains, and women’s brains grew with them to better judge males’ intellectual firepower.

So is our romantic destiny really encoded in our genes, inherited from ancient hominids who used scent, looks and IQ tests to pick their soul mates?

Scientists have had some luck pinpointing genetic roots of behavior in animals. Last year, scientists managed to create a more caring and nurturing rodent from a notoriously promiscuous species by inserting a single gene from a monogamous species of vole, or field mouse.

But even those scientists caution against trying to draw a link between vole and human vows. The human mind is much more complex and the data does not exist to make the case that attributes such as intelligence evolved to attract the opposite sex.

Also, there is little evidence to support theories that humans possess a sort of functioning sixth sense that can detect pheromones, a sort of biochemical “come hither” signal used by other species, said Heather Eisthen, assistant professor of zoology at Michigan State University.

So-called vomernasal organs (VNOs) are found in some species. However, if that organ is tucked under the noses of humans, as some have suggested, it isn’t hooked up, said Vince Dionne, a professor of biology in Boston University’s marine-biology program.

“There are no neurons around what people think is the human VNO,” said Dionne, who studies scent in animals. “If there are no neurons, there are no sensory neurons that can tell the brain anything is happening.”

The sheer complexity of the human mind means there may never be a Nobel Prize for a scientist who can describe with certainty the molecular chain reaction that leads people to say, “I do.”

But Tom Wilkie, a molecular biologist at the University of Texas Southwestern Medical Center in Dallas, who studies the signaling between hormones and areas of the brain, says despite the difficulty of the task, the topic of sex is just too interesting to ignore.

But the human brain’s very creativity could end up stumping scientists’ effort to link genetics and complicated behavior such as courtship.

“It’s always going to be a murky area,” Wilkie concluded. “That’s the nature of the beast.”