Sex & Relationships

Why Do Women Orgasm?

Scientists are still mystified about the evolutionary advantages of many aspects of women's sexuality.

It is sometimes claimed that we have reached the "end of science," that we pretty much know all there is to be known, and as a result, from now on it is simply a question of mopping up. Not so. Some of the greatest, most challenging scientific conundrums are very close to home, among human beings: our species, our mysteries, ourselves. As it happens these enigmas are especially real when it comes to women’s sexuality.

Here are some examples:

The first notable mystery begins when a girl becomes a woman: menstruation. Although a few other species bleed slightly at midcycle, no other organism does so as prominently as Homo sapiens. Breast development is another perplexity. Although we take it for granted that women have conspicuous breast tissue even when not nursing, no other mammal is comparably bosomed. Only human beings are blessed (or, in the opinion of some, cursed) with prominent nonlactating breasts.

The mysteries continue. Go to a zoo and take a look at the chimpanzees, gorillas, or baboons (or indeed, nearly any nonhuman primate) -- there is no question when an adult female is ovulating. It is as clear as the bright pink cauliflower on her behind.  Not so for our own species. Given the great importance -- socially, biologically, evolutionarily -- of reproduction, and thus of ovulation, it is extraordinary and, as yet, unexplained why even now, in our medically sophisticated twenty-first century, it is exceedingly difficult to tell something so basic as when a woman is fertile. For reasons unknown human beings conceal their ovulation and are unique among mammals in doing so. Not only that, but in the great majority of cases, the exact time of a woman’s ovulation is even hidden from herself. Why the deep, dark secret? As with menstruation and nonlactating breasts hypotheses abound, but no one knows the answer.

Ditto for female orgasm. Its male counterpart is a no-brainer (almost literally!), since without ejaculation there would be no fatherhood and thus no evolutionary success.  But the data are quite clear. There is no correlation between female orgasm and female fitness in the evolutionary sense; in other words, orgasmic women are no more successful, reproductively, than their less fortunate, nonorgasmic "sisters." So why does female orgasm occur at all?

Proceeding along the trajectory of a woman’s life, we come to yet another mystery: menopause. By around age fifty a woman can anticipate that she will cease ovulating. Why does reproduction inevitably end, even for the healthiest women, at a time in middle age when many can anticipate several decades of continued life? This is especially perplexing since reproductive success is the name of the Darwinian game, and simple calculations show that producing just one additional child, compounded over time, would convey a huge evolutionary advantage. Perhaps it is not so surprising, then, that menopause -- a cross-cultural human universal -- is not shared by any other living thing except possibly for the short-finned pilot whale . . . but of course, you already knew that!

These and other traits are fundamental to being human, yet their basis is neither understood by scientists nor, for the most part, even acknowledged by the public as the puzzles that they are. In our book How Women Got Their Curves and Other Just-So Stories, we explored these and other sexual unknowns, describing some of the scientific hypotheses that have been proposed, while also suggesting some new ideas of our own. In short, there are more things in human sexual biology than are dreamt of in our philosophy or -- more to the point -- known by our science.

But don’t get the wrong idea, Horatio. Mystery is not the same as mysticism, and we are not referring to some sort of ineffable, spiritualistic claptrap beyond the reach of natural law or human understanding. Just as a "weed" is a plant that hasn’t yet been assigned a value, the various womanly mysteries are simply scientific questions waiting for answers. Our perspective, however, is one that many readers will find unusual.

Thus, when we ask "why" women conceal their ovulation, for example, we aren’t looking into the physiological mechanisms involved but, rather, we are asking how, if at all, individuals who keep this particular part of their biology under wraps have come to leave more descendants than others who were more public about their potential reproductive status. This approach is familiar to evolutionary biologists, less so to the thinking of most anatomists, physiologists, and so forth. Not that these scientists aren’t likely to be "evolutionary" in their thinking; rather, they are more prone to ask questions -- and to answer them -- in terms of immediate causal mechanisms. Instead, we raise questions -- and answer them -- in evolutionary terms. The underlying conceptual theme is therefore: "In what way has _______ (menstruation, nonlactating breasts, female orgasm, menopause, etc.) contributed to the fitness of human beings, especially among our ancestors, thus explaining why these traits evolved?"

Explanations for Menstruation and the Evolution of Breasts

Let’s look briefly at some possible explanations. It has been suggested, for instance, that menstruation serves a cleansing function, whereby periodic blood flow sluices away potentially dangerous pathogens, including, but not limited to, those introduced during copulation. This "explanation," like that of nearly all the perplexities we consider in How Women Got Their Curves and Other Just-So Stories, has generated fruitful debate but no definite answers. (We think that the absence of "closure" on these matters simply adds to their fascination, even as we remain hopeful that eventually such questions will be resolved, perhaps by someone reading this article!)

A big problem with the "cleansing hypothesis" is that menstrual blood actually contains nutrients that might encourage pathogen growth, and there is no evidence that intensity of flow correlates with number of male sexual partners or with pathogen load. Another possible explanation for menstruation is based on energy efficiency. Some calculations suggest that it is metabolically cheaper to slough off the uterine lining (which is energetically expensive to maintain) then regrow it, rather than to maintain a high level of vascularization. But on the other hand, why isn’t the uterus kept in a more efficient, low energy, less vascularized state until needed for nourishing an embryo?

An alternative explanation comes from the fact that human beings are unique among mammals in how much they invest in each offspring. This makes it especially important that an embryo, if it is to be brought to term, be a particularly capable one. For every successful pregnancy there are many "spontaneous abortions" caused by the failure of an early embryo to implant successfully. Moreover, a key aspect of early pregnancy takes place when such an embryo begins burrowing into the uterine lining and starts secreting a hormone that (ta daa!) inhibits menstruation. So perhaps -- our own suggestion -- menstruation is essentially a regularly repeating competence test, whereby evolution selects against embryos whose burrowing and secretory abilities are inadequate. Period.

As for the evolution of breasts, the obvious explanation is that they signal capacity to nourish offspring, so that bustier women would have been preferentially chosen by would-be fathers. But why hasn’t a similar process operated in other mammals? Moreover, there is no correlation between the size of breasts, while not lactating, and the eventual ability to produce milk. What appears as breast tissue is actually made up of fat with glandular structures only developing during pregnancy. Human breasts could therefore be a kind of biological deception.

On the other hand, they might provide accurate information, indicating a woman’s ability to accumulate and store calories, especially in a Paleolithic world before the advent of refrigerators. Or, they might be a means whereby potential mates are enabled to assess age and thus reproductive prospects: a kind of Goldilocks Principle in which relatively well developed breasts are found attractive if they are not too small (indicating someone too young to be reproductive) and not too droopy (indicating age and thus someone who might be menopausal) but "just right."

There are other possibilities, including an intriguing new idea in evolutionary theory known as the "handicap principle." According to this notion -- which up to now has been suggested as an explanation for elaborate and seemingly over-the-top male traits -- females prefer males whose excessive ornamentation demonstrates their superior genetic quality. You’ve got to be a pretty impressive peacock, for example, to be able to thrive despite growing and carrying around such a bizarrely over-built tail. Many women will attest that aside from their male-attracting function (and sometimes even then!) developed breasts are something of a liability. Just so. Perhaps they evolved as "handicaps," attractive to men for the same reason a colorful peacock’s tail appeals to peahens: an honest, albeit paradoxical, indicator of underlying genetic quality.

As with the other sexual mysteries of women, there are numerous additional biological hypotheses for the existence of prominent nonlactating breasts, including the bizarre suggestion by ethologist Desmond Morris that they serve as "buttocks substitutes," thereby facilitating face-to-face copulation and thus intense interpersonal bonding.

Ovulation, an Evolutionary Shell Game?

When it comes to concealed ovulation, one potential explanation is that it is essentially an evolutionary shell game. Women who hid their time of maximum fertility kept "their" men in a kind of sexual thrall. Among species in which ovulation is clearly signaled, males are free to copulate at this time, then essentially abandon the female, often in search of other short-term partners. But in Homo sapiens, in which ovulation is concealed, men wanting confidence of paternity are obliged to remain in attendance throughout the female’s cycle, engaging in regular sexual relations throughout the month; notably, human beings are also unusual among living things in the extent to which they copulate without much regard to ovulation or the details of a woman’s hormonal condition.

Paradoxically, another possible explanation for concealed ovulation goes precisely against the grain of the "shell game" or "keep him guessing" explanation. Thus, rather than promoting social and sexual bonding (cynics might say "bondage"), it has also been suggested that ovulation is hidden in our species because this facilitates women having sex with men other than their designated partner. After all, even the most dedicated man would likely have a hard time guarding "his" woman so closely as to be able to monopolize her sex life -- which would presumably be more possible if her ovulation were clearly signaled as in so many other primates. By obscuring their exact ovulation, ancestral women might therefore have actually given themselves more leeway to mate with other, more attractive males when and if they chose.

In addition to supporting greater potential choice of mates, concealed ovulation may have provided a counterintuitive evolutionary payoff. Thus, it is now well established that among many social species -- including, presumably, our own ancestors -- strange adult males comprise a major threat to the survival of infants. (After taking over a social unit, male langur monkeys, chimpanzees, gorillas, lions, and so forth often kill lactating infants -- which had been sired by their predecessor -- thereby inducing nursing mothers to begin cycling, and making these females potential recipients of the murderous male’s sexual attention.)

But, typically, newly ascendant males who had previously copulated with a female refrain from killing their offspring; it is as though they say to themselves, "Isn’t that my old flame from several months ago? And just look at that baby, he’s got my chin!" It has been suggested, in short, that female choice of multiple male sexual partners -- itself facilitated by concealed ovulation -- may be a means whereby our great-great-great … grandmothers fooled the men in their lives, inducing several to think that they might be our fathers, thereby taking out a kind of "infanticide insurance."

Other conceivable solutions to the mystery of concealed ovulation focus on its possible role as a social signal -- or, rather, an inhibitor of what would otherwise be such a signal. Thus, it could be argued that by concealing ovulation our early hominid ancestors obscured their reproductive status, thereby limiting possible aggressive competition from other, more dominant women. Consistent with this idea, there is growing evidence that -- contrary to earlier generalizations about the exclusive maleness of same-sex competition -- females generally and women in particular do in fact compete, albeit more subtly than the chest-beating, fangs-bared style more characteristic of males. Hence, it might well have contributed to a woman’s ultimate evolutionary success if she kept her reproductive status (almost literally) under wraps. A plausible hypothesis, except that it would seem stronger if younger, less dominant women concealed their ovulation, while older, socially and physically secure women flaunted theirs. But they don’t.

The above suggestions do not exhaust the many possible explanations for concealed ovulation. An especially intriguing one relates to another unique human trait, namely, consciousness. The idea was first suggested by Nancy Burley, an evolutionary biologist currently at the University of California, Irvine. It begins with the acknowledgment that among many people today, women want fewer children than men do, mostly due to the downsides of pregnancy and childbirth, especially in a pretechnological environment. Hence, ancestral women may well have been disinclined to have sex when they knew they were fertile: Call it the "headache hypothesis." Who, then, would have been more likely to reproduce? Those who did not know when they were ovulating -- a curious, counterintuitive situation in which natural selection would have favored individuals who were unaware of their reproductive status. This hypothesis has the added benefit of explaining one of the second-order mysteries of concealed ovulation: It is one thing to hide one’s time of maximum fertility from others -- notably men and possibly other women -- but why in Darwin’s name has selection worked to hide such important information from themselves?

The Significance of Orgasm and the Mystery of Menopause

Then, happily, there is orgasm. But it isn’t enough to say that female orgasm exists because it is pleasant or even wonderful. Most biologists agree that there must be a positive "adaptive significance" for it, or any other trait, to have evolved. There are no free lunches in biology. Here again possibilities abound, each surrounded by a rich trove of natural history and theory. For one, maybe female orgasm isn’t really adaptive at all but is a byproduct, a tag-along trait -- like nipples in men, which exist not because they are biologically significant in themselves but simply because they are selected for in women. Turning the logic around, perhaps orgasm is the female equivalent of male nipples: adaptive in men and available to women but of no special significance.

Or, maybe female orgasm serves as a carrot, inducing women to engage in sex. There is a problem, however. (This is a good place to note that nearly every hypothesis we discuss in How Women Got Their Curves and Other Just-So Stories has problems, each of which is worth explaining in turn.) Female orgasm seems to occur in only a very small number of animals. If orgasm is needed to induce copulation, why is it that the great majority of species don’t need it? Here is one possible answer: Human beings are probably unique in thinking ahead and, in the process, evaluating the consequences of their actions. Although some nontechnological people didn’t connect sexual intercourse with pregnancy and childbirth, most do, and presumably did. So perhaps orgasm was a biological inducement, whereby some women who would otherwise seek to avoid the pain and danger of pregnancy and childbirth -- recall Nancy Burley’s hypothesis for concealed ovulation -- were encouraged to get over some of their reluctance. But in that case why isn’t female orgasm more reliable, like its male counterpart?

Enter another candidate explanation: Could it be that female orgasm provides a kind of evaluation by which women assess the suitability of a prospective mate? Thus, when conducting research on grizzly bears, I noted decades ago that subordinate males constantly swivel their heads around while copulating, watching for the approach of dominant boars, who will chase away and replace their social inferiors. These dominant males, by contrast, do not appear anxious during sexual intercourse and, not surprisingly, they take considerably longer to ejaculate, unlike subordinates, who undergo the equivalent of premature ejaculation. If sow grizzlies experience orgasm -- and at present no one knows whether they do -- with what kind of partner would you expect it to occur?

Could it be, accordingly, that female orgasm has evolved as a means whereby a woman’s body tells her brain that she is interacting with a potentially desirable mate? Moreover, male grizzlies do not contribute to the rearing of their offspring; men do -- or at least they can. And this leads to a variant on the "orgasm as evaluation" hypothesis: If a sexual partner shows the attentiveness necessary to help a woman achieve orgasm, perhaps this indicates not only the social confidence associated with less hurried love-making but also the willingness to give of one’s self that could correlate with increased likelihood of being a caring coparent. In addition, both aspects of the "evaluation hypothesis" help make sense of the otherwise perplexing fact that female orgasm is notoriously inconsistent: It wouldn’t be much good as a means of partner evaluation if it occurred every time and with every partner.

It would be intellectually satisfying if female orgasm was a mechanism by which women increased the chances of actually being inseminated by an especially desirable partner, but this does not seem to be the case. There exists the inelegantly named "uterine upsuck hypothesis," which posits that orgasm generates uterine contractions, which in turn create a partial vacuum that draws semen toward the oviducts. This idea has been supported by some heroic laboratory research … but criticized by others, and it is based on an unacceptably small sample size: one woman! It has also been suggested -- although again the data are inconclusive -- that orgasm reduces the amount of "semen flowback" (the leakage of semen out of a woman’s reproductive tract) thereby increasing the likelihood that fertilization will be achieved by a partner helping the induce that orgasm. At present, however, no one knows.

Finally, as all women do if they live long enough, we come to menopause. We’ve already noted that reproduction is the sine qua non of evolutionary success, which makes it especially perplexing that women’s reproductive spigot is turned off at what seems an inappropriately early age. Nearly every other living thing keeps breeding until obviously debilitated by extreme old age. Men keep producing sperm into their eighth and even ninth decades. This in turn suggests one possible explanation: Maybe the rigors of pregnancy (necessarily more severe than the mere making of sperm) make it an evolutionary bad deal for women to keep breeding once having reached a certain age. But if so, why don’t other mammals experience menopause? Not only that, but natural selection should favor women who attempted to bear just one more child, no matter how old they were and even if they died trying because any who succeeded would be a step ahead of the competition.

Another potential explanation is the "prudent mother hypothesis," which proposes that women who keep making babies into late middle age might be more likely to succumb in the process and therefore be unavailable to care for their youngest, still-dependant child. Another idea -- known as the "grandmother hypothesis" -- is based on a different fact: By the time a woman reaches age fifty or so, her earlier offspring are themselves beginning to reproduce, and perhaps (given the age-related increase in "breeding costs" plus the prospect of helping out with the grandchildren) a woman’s ultimate fitness is better served by contributing to her children’s children rather than trying to make more of her own. Anthropological data show that in many traditional societies grandmothers in particular (much more than grandfathers) do a lot of biologically useful work.

Finally, there is this idea that relates to the most important new concept in evolutionary science, namely, the "inclusive fitness theory." The idea, summarized a bit misleadingly by the term "selfish gene," is that natural selection acts at the level of genes rather than individuals, and that accordingly genes joust with each other to get ahead. The paradox is that "selfish" success, on the part of genes, typically takes place via "altruistic" behavior directed toward others, as these others are likely to possess copies of the genes in question. Applied to menopause, the notion is that the elderly -- especially women -- contribute to the success of numerous others (including but not limited to their grandchildren) with whom they share genes, even if distantly. As a result their continued postreproductive survival represents "selfish" success of their genes mediated by "altruistic" contribution to others, e.g., the payoff of having someone who knows where water supplies persist even during a drought, or who is willing and able to teach others how to make valued items, avoid enemies, cultivate friends, and so forth.

We note that there are also "male mysteries." But for some reason they aren’t as sexy or as prominent as their womanly counterparts. Some of these perplexities include: Why are men so much hairier than women? And, paradoxically, why are they also more prone to go bald? If penile size doesn’t matter, why is it so widely considered -- at least by men -- to be important? Why do men have shorter lifespans than women? And why are they so notoriously reluctant to ask directions? In view of the fact that in most species males are the fancy, colorful sex, why are male Homo sapiens so drably ornamented compared to women? And -- most profound of all -- why do men’s underpants have that little trapdoor when no one uses it?