For years, scientists have been debating the function of female orgasm. Nowthey've finally figured it out. For women, the psychology of sexual satisfaction turns out to be much more sophisticated than most (male) scientists have been willing to concede. Of course.

Ever since Alfred Kinsey and Masters and Johnson made the subject of human sexual response safe for respectable scientists, laboratory studies of the physiologic "hows" of sexual arousal have flourished. Volunteers have been prodded, filmed, tape-recorded, interviewed, measured, wired, and monitored, quantifying for the annals of science the shortened breath, arched backs and feet, grimacing faces, marginally intentional vocalizations, and jumping blood pressure of human orgasm.

While physiological details abound, fewer scientists have attempted to answer the "why" questions about human orgasm. To those who view human behavior in an evolutionary framework, which we believe adds an invaluable perspective, male orgasm is no great mystery. It's little more than a physiologically simple ejaculation that is accompanied by a nearly addictive incentive to seek out further sexual encounters. The greater the number of inseminations a male achieves, the better his chances of being genetically represented in future generations.

Compared with the more frequent and easily achieved orgasm men experience, women's sexual climax has remained a mystery. After all, women do not need to experience orgasm in order to conceive. So what is the function of orgasm in females?

Darwinian theorists who made early attempts to address female orgasm proposed that orgasm keeps a woman lying down after sex, passively retaining sperm and increasing her probability of conception. Others suggested that it evolved to create a stronger pair bond between lovers, inspiring in women feelings of intimacy and trust toward mates. Some reasoned that orgasm communicates a woman's sexual satisfaction and devotion to a lover.

Most recently, evolutionary psychologists have been exploring the proposition that female orgasm is a sophisticated adaptation that allows women to manipulate—even without their own awareness—which of their lovers will be allowed to fertilize their eggs.

Male Nipples?

The diversity of evolutionary hypotheses reflects one general attitude: that the quickened breath, moaning, racing heart, muscular contraction and spasms, and nearly hallucinatory states of pleasure that orgasm inspires constitute a complex physiologic event with apparently functional design. But critics of adaptationist hypotheses have long argued that evolution is more slipshod than purposeful. A few, including Harvard evolutionist Stephen lay Gould, have insisted that female orgasm probably doesn't have a function.

Instead, Gould argues, female orgasm is incidental, caused by an anatomical peculiarity of embryonic development. In embryos, the undifferentiated organ that later becomes the penis in males becomes the clitoris in females. Antiadaptationists like Gould--whose thinking uncannily parallels Freud's belief that women spend their life in penis envy--hold that the clitoris is, biologically speaking, an underdeveloped penis; it can let women mimic male orgasm, but it has no functional relevance or evolutionary history of its own.

Well known for his emphasis on chance events and structural constraints as major players in the evolutionary process, Gould sees the supposed functionlessness of female orgasm as a classic illustration why scientists ought not automatically assume that a trait has adaptive significance. He criticizes other evolutionists for overemphasizing natural selection and functionality, and concludes that female orgasm is like the male nipple--nothing more than developmental baggage.

Many evolutionists have rejected Gould's notion that women's orgasms are developmentally contingent on men's. Unlike a male nipple, adaptationists have pointed out, the female orgasm does something. It inspires strong emotions that can affect bonding and sexual preferences, making women more likely to prefer the company of one mate over another.

Only during the past few years have studies begun to yield evidence that may resolve the baggage-versus-adaptation debate over women's orgasms.

Sperm Competition, with Women Judging

Clues for a reasonable adaptation hypothesis were readily available by the late 1960s, when The British Medical Journal published an exchange of letters about the muscular contractions and uterine suction associated with women's orgasm. In one letter, a doctor reported that a patient's uterine and vaginal contractions during sex with a sailor had pulled off his condom. Upon inspection, the condom was found in her cervical canal! The doctor concluded that female orgasms pull sperm closer to the egg as well.

Yet, it was only three years ago that two British biologists, Robin Baker and Mark Bellis, tested the so-called upsuck hypothesis. They were building upon ideas articulated by evolutionary biologist Robert Smith, who suggested that since women don't have orgasms every time out, female orgasm favors some sperm over others. Baker and Bellis sought to learn just how female orgasms might affect which of a lover's sperm is used to fertilize a woman's eggs.

They asked volunteers to keep track of the timing of their orgasms during sex, and, after copulation, to collect male ejaculates from vaginal flowback--a technical term denoting a distinct form of material that emerges from the vagina several hours after sex (scientists have devised a way to collect it). The team counted sperm from over 300 instances of human copulation.

They discovered that when a woman climaxes any time between a minute before to 45 minutes after her lover ejaculates, she retains significantly more sperm than she does after nonorgasmic sex. When her orgasm precedes her male's by more than a minute, or when she does not have an orgasm, little sperm is retained. Just as the doctors' letters suggested decades earlier, the team's results indicated that muscular contractions associated with orgasm pull sperm from the vagina to the cervix, where it's in better position to reach an egg.

Baker and Bellis proposed that by manipulating the occurrence and timing of orgasm--via subconscious processes--women influence the probability of conception. So while a man worries about a woman's satisfaction with him as a lover out of fear she will stray, orgasmic females may be up to something far more clever--deciding which partner will sire her children.

Good Men Are Hard To Find

Meanwhile, other researchers were making discoveries about the nature of male attractiveness. Behavioral ecologists had noted that female animals, from scorpion flies to barn swallows, prefer males with high degrees of bilateral body symmetry, called developmental stability in the parlance of science.

Development, or the translation of genes into parts of the body, can be perturbed by stresses such as disease, malnutrition, or genetic defects. One measure of developmental instability is deviation from bilateral symmetry in traits like hands, eyes, and even birds' tail feathers. Males whose immune systems are strong, and who forage well, develop with high symmetry, so females who choose symmetrical suitors are securing good genes for their offspring.

Evolutionary biologist Randy Thornhill and psychologist Steve Gangestad at the University of New Mexico in Albuquerque have tested whether humans also share this preference. And indeed they do. In their studies, women consistently identify as most attractive males whose faces (and other body parts) are most symmetrical.

But this, it turns out, is more than a matter of mere aesthetics. A large and growing body of medical literature documents that symmetrical people are physically and psychologically healthier than their less symmetrical counterparts.

Thornhill and Gangestad reasoned that if women's orgasms are an adaptation for securing good genes for their offspring, women should report more orgasms with relatively symmetrical mates. Collaborating for a second time, the two, along with graduate student Randall Comer, devised some very interesting studies to test this idea.

First they enrolled 86 sexually active heterosexual couples from among the undergraduates. The average age of the partners was 22 and the couples had been together an average of two years. Then the researchers had each person privately--and anonymously--answer questions about his or her sexual experiences.

The researchers took facial photographs of each person and analyzed the features by computer; they also had them graded for attractiveness by independent raters blind to the study. They measured various body parts to assess bilateral symmetry--the width of elbow, wrist, hand, ankle, and foot bones, and the length of the second and fifth fingers. Earlier studies had suggested all of these were associated with health.

Indeed, the hypothesized relationship between male symmetry and female orgasm proved to be true, the researchers recently reported in the journal Animal Behavior (Vol. 50, December). From data on sexual behavior provided by the women, those whose partners were most symmetrical enjoyed a significantly higher frequency of orgasms during sexual intercourse than did those with less symmetrical mates. Even the data on sexual experience provided by the men showed the women had more orgasms with the most symmetrical men.

Of course, symmetry is a relative thing, and a relative rarity at that. No one is perfectly symmetrical, and very high symmetry scores were few and far between in this sample, as in others. In consolation, Thornhill and Gangestad point out that the differences they are measuring are subtle, and most require the use of calipers to detect.

What's Love Got To Do With It?

It's important to note what did not correlate with female orgasm during sex. Degree of women s romantic attachment did not increase the frequency of orgasm! Nor did the sexual experience of either partner. Conventional wisdom holds that birth control and protection from disease up orgasm rates, since they allow women to feel more relaxed during intercourse. But no relationship emerged between female orgasm and the use of contraception.

Nor can the study results be explained by the possibility that the symmetrical males were dating especially uninhibited and orgasmic women. Their partners did not have more orgasms during foreplay or in other sexual activities. Male symmetry correlated with a high frequency of female orgasm only during copulation.

The findings support evolutionary psychologists' "good genes" hypothesis: Women have orgasm more often with their most symmetrical lovers, increasing the likelihood of conceiving these men's children. Well, that's how it would have worked for millennia, before condoms and the Pill.

And it is for the precontraceptive stone age that our brains seem to be built; the agricultural and industrial revolutions are flashes in the geological pan, far too recent in evolutionary terms to have fundamentally changed the way we experience emotions or sex. To argue, as may champions of chance like Gould, that sexual attraction has remained completely arbitrary throughout evolution seems increasingly unwarranted.

Cheating Hearts

Here's the cruelest part of Thornhill and Gangestad's findings: The males who most inspire high-sperm-retention orgasmic responses from their sexual partners don't invest more in their relationships than do other men. Studies show that symmetrical men have the shortest courtships before having sexual intercourse with the women they date. They invest the least money and time in them. And they cheat on their mates more often than guys with less well-balanced bodies. So much for the beleaguered bonding hypothesis, which wants us to believe that women with investing, caring mates will have the most orgasms.

The women who took part in the study were no saints, either. They sometimes faked orgasm. Their fakery was not related to male symmetry. Faking, however, was more common among women who reported flirting with other men. Clearly earlier theories were not too far off the mark when they proposed that a man looks for cues of sexual satisfaction from his mate for reassurance about her fidelity. Faking orgasms might be the easiest way for the woman with many lovers to avoid the suspicions of her main partner.

Baker and Bellis found that when women do engage in infidelity, they retain less sperm from their main partners (their husbands, in many cases), and more often experience copulatory orgasms during their trysts, retaining semen from their secret lovers. Taken together, these findings suggest that female orgasm is less about bonding with nice guys than about careful, subconscious evaluation of their lovers' genetic endowment.

Exhibit B

Patterns of female orgasm point to one important conclusion about our evolutionary past--that sexual restraint did not prevail among women. But that's only part of the evidence. Exhibit B is male ejaculation.

Baker and Bellis found that the number of sperm in men's ejaculate changes, and it varies according to the amount of time that romantic partners have spent apart. The longer a woman's absence, the more sperm in her husband's ejaculate upon the couple's reunion. Males increase ejaculate size, it seems, to match the increased risk that a mate was inseminated by a competitor.

In an ancestral environment of truly monogamous mating, there would have been no need for females to have orgasm or for men to adjust ejaculate size. Both are adaptations to a spicy sex life.

Male Bias

Darwin proposed that female animals' preferences have shaped male ornaments such as peacocks' tails. But his audience--largely male scientists--laughed off his theory of sexual selection on the grounds that females (human or otherwise) are too fickle to exert the necessary selection pressure.

Today, evolutionary biology is no longer so completely a male discipline. But many male evolutionists nevertheless carry old biases. The notion that female orgasm is anything other than a developmental legacy leaving females able to imitate "the real thing" will be difficult for some to accept. But as uncomfortable as it may make many of us men--including male scientists--a woman's orgasm appears to be a more complex and discriminating comment about her lovers' merits than are our own.

Explosive Findings!

If we use his study's findings to understand how we humans are designed to behave in the sexual domain, says Randy Thomhill, Ph.D., then we are better equipped to deal with problems that arise in relationships. He points to the following results as among those we should take to heart:

o A woman's capacity for orgasm depends not on her partner's sexual skill but on her subconscious evaluation of his genetic merits.

o Women's orgasm has little to do with love. Or experience.

o Good men are indeed hard to find.

o The men with the best genes make the worst mates.

o Women are no more built for monogamy than men are. They are designed to keep their options open.

o Women fake orgasm to divert a partner's attention from their infidelities.