Sex: Peak Experience

For all the toe-curling, heart-racing, oxytocin-pumping ecstasy that it delivers, the function of female orgasm remains a big question mark. Considered by some the greatest mystery in the evolution of human sexuality, scientists have spent decades trying to explain its origin. Male orgasm, after all, serves an obvious purpose: the perpetuation of the species; none of us would be here without it. But women can and do conceive without having orgasms. So why are they endowed with the physiological capability?

The most forceful explanation of recent years comes from philosopher Elisabeth Lloyd, who, in her 2005 book, The Case of the Female Orgasm, argues that there is a scientific bias toward seeing an adaptive purpose for it, and that every prominent evolutionary theory of female orgasm is in fact flawed. The most plausible explanation, she posits, is what's known as the "byproduct theory"—that because male and female embryos develop similarly in the early months of gestation, the same developmental processes that result in the male orgasm incidentally produce its female counterpart as well. Rather than serving any grand evolutionary purpose, this theory goes, female orgasm is analogous to male nipples.

Since Lloyd's book appeared, however, scientists have continued searching for an adaptive reason for female orgasm, and new evidence has slowly accrued to suggest that it might be more than a blissful byproduct after all. The most prominent explanation is that orgasm enables women to covertly evaluate and select high-quality males in order to ensure the fitness of resulting offspring. "There is still no definitive evidence either way," says David Puts, an evolutionary anthropologist at Pennsylvania State University and a leading researcher on the origins of sexuality, "but as new evidence accumulates, it generally seems to support the mate-choice hypothesis."

Puts offered some of that evidence in a study published in Evolution and Human Behavior in 2012. It reported on an experiment in which the female in 110 heterosexual couples on a college campus rated her partner's masculinity and level of dominance, while computer software objectively measured the male partner's facial masculinity and symmetry, markers of attractiveness and genetic quality. The results showed that women with more attractive partners had more frequent orgasms during or after their partner's ejaculation, a time window believed to be optimal for sperm retention. Women partnered with particularly masculine and dominant partners, meanwhile, also reported more frequent orgasms. Because of the correlation between orgasm and men of high genetic quality, researchers cautiously concluded that the results supported the mate-choice theory.

For female orgasm to serve a function in mate selection, it should also increase the odds of conception, an assumption that is hotly debated yet has also gained some scientific support. In 2007, a team of researchers led by obstetrician Georg Kunz at St. Johannes Hospital in Dortmund, Germany, examined the effects of oxytocin—the hormone that causes uterine contractions and is released into the female bloodstream after orgasm—on sperm transport. Study participants received a dose of oxytocin followed by a vaginal injection of a marked, sperm-like substance. Supporting the theory that female orgasm evolved to enhance fertility, oxytocin increased the flow of the substance to the dominant (egg-producing) follicle's oviduct during the fertile phase of the menstrual cycle, a process believed to increase the odds of conception.

There's evidence that orgasm abets fertility another way—by reversing uterine pressure from outward to inward. The inelegantly termed process, "sperm insuck," is thought to push sperm into the uterus. Still others have suggested that the purpose of women's orgasm is to reverse the "vaginal tenting" that occurs during sexual arousal—a physiological change in which the uterus is pulled upward—by bringing the cervix back in contact with the sperm pool.

Yet scientists have been unable to reach any consensus, as other evidence emerges to cast doubt on theories like the enhanced-fertility function of female orgasm. In one paper, published last year in Animal Behaviour, researchers Brendan P. Zietsch and Pekka Santtila reported a study of 8,000 female twins and siblings. After controlling for a variety of factors, they found no correlation between their subjects' orgasm rates during intercourse and number of offspring. The pair, who also published a 2011 paper that challenged the byproduct theory, is quick to add that their more recent paper didn't eliminate the possibility of any adaptive explanation, but did contradict the enhanced-fertility theory.

"The correct evolutionary explanation of female orgasm might be one that has yet to be thought of," Zietsch says. "It remains a mystery." If researchers can agree on anything, it might be precisely that.

David Barash, an evolutionary psychologist and the author of Homo Mysterious: Evolutionary Puzzles of Human Nature, has been a vocal critic of the byproduct theory and personally hews toward a version of mate-choice theory that suggests that the effort and selflessness it typically takes a man to bring a woman to orgasm is what the female orgasm evolved to evaluate. It may signal his inclination to be involved in "something beyond 'bim, bam, thank you, ma'am,' which may say something about his willingness to invest subsequently in the outcome of children," he says.

Still another adaptive possibility Barash would like to see further considered is whether female orgasm offered prehistoric women, who faced a high mortality risk during childbirth, a subjective motivation to copulate. "Orgasm may be that extra carrot dangled in front of them while they're thinking, I really don't want to have sex, because I could ultimately die."

Puts, for his part, continues to focus on ascertaining which characteristics of a mate are related to women's orgasm frequency. He also hopes to see more work done to clarify whether female orgasm promotes conception. For instance, tracking radio-labeled particles through women's reproductive tracts to see how orgasm affects their movement "could provide strong evidence that orgasm helps transport sperm to the ovum," he says. The long story of understanding female orgasm has yet to reach its climax.

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