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How Women Got Their Curves and Other Just-So Stories: Evolutionary Enigmas

David P. Barash, Ph.D. and Judith Eve Lipton, M.D.

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Paper, 224 pages,
ISBN: 978-0-231-14665-4
$25.00 / £17.50

April, 2009
Cloth, 224 pages,
ISBN: 978-0-231-14664-7
$29.95 / £19.95

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Excerpt from Chapter 5: The Enigmatic Orgasm

It used to be conventional wisdom among biologists that human beings are unique in experiencing female orgasm, but no longer. Nonetheless, female orgasm remains both a marvelous phenomenon and a contentious, unsolved mystery among evolutionary biologists. Given the longstanding and widespread sexual repression of women in both Western and Eastern societies, it is not surprising that only recently has anorgasmia (failure to experience orgasm) been identified and treated. Nonetheless, the real biological mystery isn’t why some women don’t climax, but why some do....

Anthropologist-primatologist Sarah Blaffer Hrdy suggests that female orgasm evolved as a spur to having sex with many different males. "Based on both clinical observations and interviews with women," writes Hrdy, "there is a disconcerting mismatch between a female capable of multiple sequential orgasms and a male partner typically capable of one climax per copulatory bout." A potential consequence of this "mismatch" is that females would be inclined to seek multiple partners in order to achieve their orgasmic potential. As for why this potential exists at all, Hrdy suggests that it is ultimately driven by the fitness benefit of taking out an anti-infanticide insurance policy, as proposed earlier for the evolution of concealed ovulation. Thus, female orgasm and its requirement of sustained stimulation may have provided the proximate mechanism under­pinning the ultimate payoff deriving from having sex with multiple partners. Here are Hrdy’s own words: "It is possible that as in baboons and chimps the pleasurable sensations of sexual climax once functioned to condition females to seek sustained clitoral stimulation by mating with successive partners, one right after the other, and that orgasms have since become secondarily enlisted by humans to serve other ends (such as enhancing pair-bonds)."

Picture humanity’s mother studiously going from one sexual partner to the next, maintaining and motivated by unsatisfied sexual tension while transitioning among males, egged on in her search for "sustained clitoral stimulation" by the hope that the next guy will finish what the previous one hadn’t quite managed. Or maybe if she had already climaxed, she might nonetheless be inspired to encounter multiple males by the simple fact that female orgasm is rekindlable (whether this is the right word for the experience is not certain, but it clearly is a bona fide phenomenon). If so, then the ultimate motivation for such behavior would have been the fitness bonus of taking out an anti-infanticide insurance policy, proximately motivated by the prospect of an orgasm. Or several.

Nonhuman female primates—notably macaques, baboons, and chimpanzees—do in fact mate sequentially with a number of different partners, and if stimulation from these encounters is cumulative, orgasm might be a proximal payoff. It might accordingly be part of a complex reward system among many animals, not just human beings, that induces females to mate with many different males. In addition, it might have evolved in nonhuman primates for one reason (e.g., infanticide insurance), then been maintained among modern human beings for another.

For a distinctly nonsexual example of a trait’s adaptive significance changing over time, consider feathers. It is clear from the structure of fossil dinosaurs and birds that the earliest feathers did not evolve in the service of flight because the earliest feathered reptiles lacked hollow bones and were for the most part earthbound. Far more likely, feathers helped primitive "birds" to keep warm and only subsequently became elaborated as aeronautic devices. Among modern birds—especially males—feathers are further adapted as sexually selected traits (remember the peacock). A trait’s current adaptive significance can therefore be quite different from its original function.

Even if female orgasm evolved as a mechanism that induced women to mate with multiple men—a questionable if intriguing hypothesis—it doesn’t mean that human cultural traditions would necessarily welcome it. Thus, the hideous practice of "female circumcision," still widespread in much of northern and eastern Africa, may owe its existence to a recognition that female sexual desire can lead to multiple partners: in order for a woman to be considered marriageable, it is necessary to guarantee her fidelity by curtailing her orgasmic potential, if not eliminating it altogether. Hrdy has made the interesting proposal that female orgasm may thus be a relic, adaptive among our primate ancestors but potentially disadvantageous—even dangerous—to some women today. Thus, insofar as orgasm might even occasionally induce women to seek out additional sex partners beyond their designated husband, this consequence in itself might have serious (and certainly fitness-reducing) results. In much of the world, the penalty for a woman’s having sex with more than one man (especially if she is married) is quite severe, sometimes including death.

As with menstruation, concealed ovulation, the existence of nonlactating breasts, and the other evolutionary enigmas yet to be explored, the conceptual waters surrounding female orgasm are muddy indeed. Thus, another potential evolutionary payoff of orgasm would seem to push in precisely the other direction from Hrdy’s hypothesis, toward monogamy rather than multiple partners. Let’s return to her suggestion, with the intervening phrases deleted: "It is possible that . . . orgasms have since become secondarily enlisted by humans to serve other ends (such as enhancing pair-bonds)."

There is some evidence that women are more likely to climax with familiar partners because they are more likely to feel (and to be) safe and thus comfortable and relaxed, to be able to make their needs and preferences clear, and more likely to have them met. Put this all together, and a case might be made that rather than being an inducement for polyandry, as Hrdy proposes, female orgasm is an evolutionary sweetener for its opposite, monogamy, as Hrdy also proposes! In this regard, it is altogether consistent to have it both ways because, as already noted, a trait can evolve for one reason, then be employed for another. (In any event, if there were evidence that chimpanzee females are more likely to achieve orgasm by mating sequentially with multiple males, this would strengthen Hrdy’s infanticide insurance hypothesis for female orgasm, but diminish confidence in the monogamy hypothesis; at present, there are no data either way.)

A variant on the monogamy hypothesis is also worth considering. Aside from its possible role in promoting pair bonding via the female’s inclination, it is not unreasonable to suppose that female orgasm might promote monogamy by acting on the male as follows: if female orgasm serves as a sexual reward, inducing ancestral women to engage in a higher frequency of copulations than would otherwise occur, and if their mates respond positively to an enhanced sex life vis-à-vis that experienced by partners whose females are nonorgasmic, and if part of this response involves a greater commitment to a monogamous pair bond with the women in question, and if this commitment results in greater evolutionary success for these women, then natural selection may have favored female orgasm. That’s a lot of "ifs," but—pardon the expression—if they all fall into place, so would a case for the validity of this particular hypothesis.

The Monogamy Hypothesis also leads to the prediction that monogamous species should be heavily represented among animals that experience female orgasm, but this is not the case. The best-documented nonhuman examples of female orgasm—bonobos, chimpanzees, certain species of macaque monkeys—are notoriously nonmonogamous.

Perhaps the most persistent hypothesis regarding female orgasm speaks to its most straightforward potential adaptive value: fertilization. Maybe there is some truth after all in the quaint, quasi-Victorian notion, often hinted at but rarely stated explicitly in romance novels, that when a woman truly "gives herself" to her lover, she is more likely to conceive. It would be lovely, if true, not only for its poetic appeal, but because if female orgasm led to greater likelihood of fertilization, it would constitute a satisfying book-end match for its male counterpart: he ejaculates when sexually aroused, thereby introducing sperm into the female, and she, if similarly aroused, engages in a kind of internal process that underscores the cooperative or complementary nature of the whole enterprise. Of course, if this correlation were indeed true, then orgasm wouldn’t belong in a book about evolutionary enigmas because its adaptive significance would be clear.

But it isn’t (clear, that is).

Not that there aren’t possibilities by which female orgasm might make fertilization more likely—aside from Desmond Morris’s horizontal hypothesis. One theory long favored by many biologists is that the contractions associated with orgasm produce a negative intrauterine pressure, which in turn helps draw sperm up and toward the fallopian tubes. The resulting uterine upsuck hypothesis is linguistically ugly, but logically appealing. Such appeal was enhanced nearly four decades ago by research using a radio telemetry device inserted into the uterus, which in fact showed the production of a vacuum cleaner–like negative pressure following orgasm. Biologists eager (maybe overeager) to embrace an adaptive significance for female climax have cited this study many times—despite the fact that it involved a grand total of one woman!

By-product hypothesis advocate Elisabeth Lloyd points out this egregiously small sample size in her book The Case of the Female Orgasm. It is an interesting example of how certain research findings can become reified and embedded in the scientific literature as received wisdom despite the fact that their actual empirical backing may be exceedingly weak. In this regard, sex researchers and evolutionary biologists alike owe a debt of gratitude to Dr. Lloyd, who—a philosopher of science herself—did what is all too rare among practicing empiricists: she actually read the original literature that is so often cited without being critically examined. The article on intrauterine pressure was admittedly in its own way heroic, on the part of the researchers as well as the subject, but it simply cannot (yet?) be accepted as valid. It isn’t necessarily invalid, but with an "n of one," as scientists put it, the case is far from closed. Even worse, follow-on studies attempting to show direct uptake of tiny tracking particles following orgasm have been unsuccessful.

Another possibility presents itself. What if a woman’s orgasm increases the likelihood of fertilization by reducing the amount of seminal "flowback"—that awkward but undeniable phenomenon whereby after a man ejaculates, variable amounts of semen leak out of the woman’s reproductive tract? Again, at least one study supports this conjecture. A small number of couples surprisingly agreed to collect seminal flowback after sexual intercourse and to report whether female orgasm had occurred and, if so, at what time relative to the man’s ejaculation. The key finding was that female orgasm occurring within one minute before and forty-five minutes after ejaculation was associated with less flowback than when there was no orgasm or when orgasm occurred at other times. In other words, it appears that by climaxing, women retain more of their partner’s semen.

As with the uterine upsuck hypothesis, the semen retention hypothesis appears plausible, although—once again—its empirical foundation is shaky at best. As Lloyd points out, the data aren’t terribly reliable, the sample is small, and the statistical analysis is seriously flawed. And so both hypotheses remain much like female orgasm itself: exciting but elusive.

The (re)search goes on. Female orgasm might facilitate fertilization in other ways. In addition to possibly giving sperm a physical boost along their way via negative pressure and keeping them from dribbling out (via reduced "flow­back"), it may provide a variety of biochemical benefits, even contribute to the sperm’s anatomical development (which is known as capacitation). According to researchers David Puts and Khytam Dawood, orgasm may also promote fertilization by

"facilitating interaction between sperm and oviductal epithelium, which may prolong sperm longevity, increase the number of capacitated sperm [sperm capable of fertilizing an ovum], or lengthen the interval over which at least some sperm in an ejaculate are capacitated. . . . Female orgasm may allow the earlier entry of sperm into the cervix by resolving the ‘vaginal tenting’ of sexual arousal, which elevates the cervix from the posterior vaginal wall, removing it from the semen pool. . . . Female orgasm also causes patterns of brain activation and hormone release associated with increased uterine contractions, lower uterine pressure, and movement of semen into the uterus. Peristaltic uterine contractions transport sperm in rats, dogs, cows . . . and probably humans . . . and appear to be caused both by a hormone released during orgasm and by stimulation of brain areas activated during orgasm. In women, orgasm activates the cingulate cortex and medial amygdala . . . , and electrical stimulation of these areas in experimental animals induces uterine contractions. . . . Orgasm also activates the paraventricular nucleus (PVN) . . . , and both PVN stimulation . . . and orgasm have been found to cause oxytocin release into the bloodstream. . . . Oxytocin, in turn, induces uterine contractions . . . , changes uterine pressure from outward to inward, and increases the transport of a semen-like fluid into the uterus and oviducts."

The situation is complex and—at least at first encounter—more than a bit paradoxical. For instance, sperm arrive more quickly at the fallopian tubes in a woman who is not sexually aroused! The "vaginal tenting" associated with orgasm raises the cervix away from the posterior wall of the vagina and makes it more difficult for sperm to proceed. As it turns out, however, this delay actually promotes fertilization because freshly deposited sperm cannot penetrate the human egg; they must first undergo capacitation, an anatomical and chemical change that takes one to four hours. Hence, an orgasmically imposed speed bump in the sperm’s migration actually makes fertilization more likely rather than less.

A useful summary statement comes from a recent scholarly book describing the physiology and anatomy of orgasm (and which, incidentally, was cleverly published with a cover that looks like brown-paper wrapper): "While it is wide­ly recognized that a woman’s orgasm is not essential to pregnancy . . . [it] may bring into play a combination of physical processes that promote pregnancy."

The hormone oxytocin is especially interesting in this regard. Oxytocin is a fascinating chemical involved in inducing uterine contractions associated with giving birth and then helping to induce the postpartum mother to accept and nourish her offspring. It has also been implicated in heterosexual pair bonding in some rodents and might be involved in human pair bonding as well.

For those who agree with the idea that evolution does not play randomly (nonadaptively) with its creations, a modification of Einstein’s famous dictum that God does not play dice with the universe, there is fortunately yet another hypothesis regarding the adaptive significance of female orgasm. Better yet, it is not only especially promising, but also encompasses several of the preceding points.

Recall that one of the arguments in support of the by-product hypothesis is that female orgasm is so unreliable that it can’t be adaptive. After all, eating is adaptive, and so people get reliably hungry if they are deprived of food. Breathing is adaptive, so their lungs expand and contract reliably. If female orgasm were adaptive, goes the argument, then it too would be predictable. Because it happens only sometimes, it can’t be a product of evolution. But wait. This is like saying that because lions do not succeed every time they hunt zebras, it must be nonadaptive for them to hunt! Lots of things are highly adaptive yet don’t always work when called on to do so. It is adaptive to find a mate, but not every romantic effort is crowned with success; it is adaptive to reproduce, but some couples—despite their best efforts—are childless. Moreover, one of the most compelling explanations for female orgasm as an adaptation not only is compatible with its elusiveness, but in a sense relies on it—more specifically, on the occasional disconnect between body and mind, and how people often find themselves using the former to inform the latter.

The Evaluation Hypothesis

The poet John Donne, writing of one Elizabeth Drury, waxed enthusiastic about how her body not only spoke, but seemed to think:

Her pure, and eloquent blood

Spoke in her cheeks, and so distinctly wrought

That one might almost say, her body thought.

It seems unlikely that any body literally thinks, even the anatomically eloquent Ms. Drury’s; that’s what brains are for. But if the body does think, then as befits good thought, it does so in silence. And we can’t be sure that John Donne, a now-dead white male who wrote several centuries ago, was gesturing toward female orgasm in any case. By contrast, the twentieth-century Portuguese writer and feminist icon Anaïs Nin was less inhibited, writing of "Electric flesh-arrows . . . traversing the body" and noting how "a rainbow of color strikes the eyelids. A foam of music falls over the ears. It is," she announced, "the gong of the orgasm."

With or without an accompanying gong, orgasms sometimes may appear to speak, at least to the person who resides inside that body’s brain and who might well profit from the information thereby provided.

First suggested by David P. Barash nearly three decades ago, the idea is that orgasm might be a way a woman’s body speaks to her brain, "telling herself" that she has been having sex with a suitable partner—that is, one who is not worried about being displaced by a competitor, who is self-confident and unhurried enough to be satisfying to her. When Barash was a graduate student more than ten years earlier, he observed that when subordinate male grizzly bears copulate, their heads are constantly swiveling about on the lookout for a dominant male, who, should he encounter a couple in flagrante, will likely dislodge his lesser rival and take its place. Not surprisingly, subordinate males ejaculate very quickly, whereas dominants take their time. If female grizzly bears were to experience orgasm, with which partner would you expect it to be more likely? And is it surprising that premature ejaculation is a common problem of young, inexperienced men lacking in status and self-confidence? Moreover, is it surprising that women paired with such men are unlikely to be orgasmic?

Research on a large captive group of Japanese macaque monkeys is also suggestive. The technical article’s title neatly summarizes its finding: "Female Orgasm Rate Increases with Male Dominance in Japanese Macaques." During 238 hours of observations in which 240 copulations were observed, female orgasmic responses occurred in 80 (33 percent). Of these orgasms, the highest frequency took place when high-ranking males were copulating with low-ranking females, and the lowest between low-ranking males and high-ranking females.

So maybe a woman’s orgasm isn’t elusive because it is a vestigial by-product, fickle and flaky, sometimes on and sometimes off like a light bulb that isn’t firmly screwed into its evolutionary socket. Maybe, instead, it is designed to be more than a little hard to get, adaptive precisely because it can’t be too readily summoned, so that when it arrives, it means something. The evaluation hypothesis would therefore predict precisely the variability that devotees of the by-product hypothesis consider incompatible with adaptation. It is consistent with several of the hypotheses already described, notably the copulatory reward hypothesis, because it specifies that individuals would be especially rewarded for copulating with partners who are particularly likely to evoke orgasmic responses.

The evaluation hypothesis is even compatible with the fact that orgasm is more reliably evoked by masturbation than by sexual intercourse; potential partners warrant evaluation, whereas there is no comparable pressure to assess one’s own masturbatory technique. Moreover, any information made available in the former case can certainly be used to fine-tune the latter. Almost certainly, masturbation is not an adaptation for reproduction in either males or females; rather, it occurs just because the wiring exists—in both sexes—for orgasm based on stimulation, even in the absence of a sexual partner. "If it ain’t broke, don’t fix it" becomes "if it’s available and feels good, use it."

The evaluation hypothesis yields some testable predictions. One, which seems so obvious as to be unworthy of testing, is that women should find orgasms not only pleasurable but important in the context of a sexual relationship. Don’t scoff: if a woman’s climax is merely an irrelevant, tag-along by-product, then it needn’t be accorded any more attention than men give to their nipples. In a survey of 202 Western women of reproductive age, 76 percent reported that experiencing orgasm with a partner was between somewhat important and very important; only 6 percent said it was somewhat unimportant to very unimportant.

If orgasm helps women evaluate their partners, then it would seem to be compatible with an attitude of control and independence. In much of the world, women tend to associate sex with submission, and, interestingly, the more they do so, the more they experience impaired arousability and orgasm frequency, suggesting that orgasms have something to do with autonomy and selfhood, but in an erotic context.

Two more predictions. Compared to their less impressive fellows, socially dominant men should be better lovers—that is, more likely to evoke orgasms in their partners. And for women, experiencing orgasm with a particular partner should lead to preference for that partner. In short, after having had an orgasm, a woman should likely want more and thus have additional sex with the partner in question. The evolutionary outcome is that in the absence of reliable birth control, a woman would more likely be impregnated by this person. Preference for sex with a sexually satisfying lover seems so obvious that it, too, might appear a foregone conclusion, but just because it is obvious doesn’t make it any less true or significant. In addition, it is at least possible that causation actually runs the other way: once a woman has a preference for a particular partner (for whatever reason), she might be more likely to be orgasmic with him or her. It might be possible to disentangle these factors, but not easily.

And what about faking orgasm? Many women do it, although the subject has received virtually no research attention beyond the commonsense observation that most men find it gratifying to think that their partner has been "satisfied." Why do men feel this way? The most romantic answer is that if you love someone, you care about his or her happiness, and it isn’t genomics science to realize that orgasms make people happy. A more biocynical view—and, not coincidentally, more bioaccurate—would look for an evolutionary payoff for the faker.

Several such payoffs can be imagined. One is that because a woman’s orgasm often stimulates a man’s, she may be inclined to fake hers just to hurry up his and "get it over with." Or by pretending to be sexually satisfied, a faker might keep her partner more content, self-confident, and thus more likely to maintain the relationship—albeit at the cost of foregoing the kind of intimate feedback that might otherwise make such fakery unnecessary. Alternatively, or­gasmic pretense might increase the man’s confidence regarding paternity of any offspring, building on his likely assumption that a sexually satisfied woman wouldn’t have sought to mate with someone else.

As we have seen, one consequence of being a mammal, with fertilization occurring within the female’s body, is that although women can be entirely confident of their genetic relationship to their children, men have to take their wives’ word for it. If by being orgasmic—or just seeming to be—women help men move from "maybe" to "definitely," then one might predict that husbands of seemingly orgasmic women would be more likely to invest in their children. Yet another possible payoff, the most cynical of all, is that faked orgasm is a way of generating a false sense of security, which would diminish the likelihood that the man will engage in "mate guarding," thereby facilitating a woman’s ability to engage in extrapair copulations (or EPCs, in the evolutionist lexicon), during which, it is fair to note, her orgasms might even be genuine.

Relatively little is known about actual rates of extrapair copulations, which isn’t surprising given the frequently dire consequences of men’s discovering that "their" children aren’t genetically theirs. Nonetheless, the availability of DNA fingerprinting is rapidly changing this situation, and current estimates are that rates of extrapair paternity are about 2 percent in many human populations and about 10 percent in traditional societies, in which birth control is relatively unavailable and which probably most closely approximate the condi­tions in which Homo sapiens evolved.

Fake orgasm, or something remarkably like it, seems to occur in at least one group of nonhuman animals—certain freshwater salmonid fish that engage in external fertilization. In the best-studied case, that of brown trout, females excavate a depression in a stream bed, whereupon a dominant male typically chases other males away and begins to quiver by vibrating his trunk muscles; subordinate males array themselves nearby. When the female is ready to spawn, she quivers violently, which induces comparable quivering by the male, culminating in the two individuals releasing eggs and sperm, pretty much simultaneously. Sometimes, however, the female quivers but keeps her eggs to herself. Among the various explanations tested to explain this finding, the researchers concluded it was most plausible that female brown trout use "fake orgasm" to trick additional males into joining the spawning pair (her conspicuous quivering attracts additional suitors, some of whom eventually ejaculate). This, in turn, might benefit the female either by increasing the genetic diversity of her offspring or by literally causing a fight to break out among those suitors, after which she gets to spawn—for real, this time—with the winner.

Human orgasms are private affairs (pornographic movies excepted); salmo­nid fish, by contrast, are altogether shameless. It seems most likely that people use fake orgasms either professionally to titillate viewers of X-rated movies or, on a personal level, to enhance the pair bond or to facilitate a woman’s going outside that bond. Among brown trout (which neither produce nor patronize "adult" movies), there is also no pair bond to enhance, so it seems that fake orgasm is strictly a tactic whereby the female gives herself a wider reproductive field from which to choose.

As befits a phenomenon as subtle yet powerful as female orgasm, there is a subtle yet powerful variation on the evaluation hypothesis. What, exactly, is being evaluated? One relevant dimension involves social dominance and self-confidence, à la grizzly bears, but it isn’t the only one. Another would include sufficient access to resources to orchestrate interactions that are private, safe, and gratifying—in a word, romantic—and thus appealing to women’s evolved evaluation mechanisms. In addition, if orgasms are somehow evaluative, one thing they might evaluate is the partner’s genetic quality. A prediction, accordingly, is that female orgasms should be more readily elicited by genetically desirable men.

This correlation does seem to hold. One study has found that women are significantly more orgasmic when paired with men who are more symmetric. Maybe this outcome occurred because of something about the women rather than about the men: Might it be that women who are more orgasmic are differentially drawn to men who are more symmetric? Not so. The same study found that when masturbating, women are equally likely to experience orgasm regardless of how symmetrical their (temporarily absent) partner. Another intriguing result from the same research is that women are more likely to experience ostensibly "high sperm retention orgasms"—that is, climaxes that occurred in close temporal proximity to the man’s—when the man is more symmetrical. All these discoveries are consistent with the notion that female orgasm might be a mechanism for "cryptic female sperm choice" by selectively retaining sperm from men who are more genetically desirable or by enhancing the likelihood that a woman will copulate repeatedly with such men, or both. This two-pronged interpretation was first proposed more than twenty years ago and, if anything, now appears more validated than ever.

Here, then, is a prediction: insofar as female orgasm is involved in mate selection by the female—whether by selective retention of sperm or of the guy who produces them—its existence should correlate with species in which females copulate with more than one male. With strict monogamy (as reported for certain South American primates such as the pygmy marmoset) or rigid harem structure (as among gorillas, in which females mate exclusively with the dominant silverback), orgasm should be relatively unimportant. This seems to be the case.

There doesn’t appear to have been any comparable research examining the effect of partner symmetry on climax during female-female encounters. It seems reasonable, however, to predict a similar finding: once natural selection has equipped women with a tendency to respond more orgasmically to genetically superior partners—assuming, of course, that it has done so—and once certain individuals find themselves with a same-sex preference (for whatever reason), the likelihood is that they will apply their already-evolved orgasmic inclinations to their preferred partners regardless of gender. It would be helpful if a statistically adequate sample of lesbian lovers would be willing to record their orgasmic histories and if they would team up with researchers equipped with measuring calipers to determine symmetry.

Is it "merely" a Just-So Story to suggest that sexual satisfaction is more frequent and more readily obtained with partners that are more appealing? In a sense, yes. But once a Just-So Story generates predictions and the gathering of evidence—whether the outcome supports or refutes the "story"—it has become a hypothesis being tested scientifically. And the correlation between "sex appeal" and evolutionary payoff is precisely the kind of "obvious" notion that cries out to be evaluated and that sometimes provides gratifying surprises as well as directions for future research.

At the same time, there is more than one dimension to what is "appealing," and of them, body symmetry actually seems rather unimpressive. Admiring a beautiful man or woman, how many people are likely to exclaim, "How wonderfully symmetric"? However, it is entirely possible that an assessment of symmetry figures prominently—albeit unconsciously—in such perceptions. Moreover, symmetry is easy to measure and, so far at least, surprisingly important. Also worth evaluating, however—and perhaps even more important—is the partner’s inclination toward the woman.

Male mammals are, in a sense, roving inseminators. Sperm are abundant and cheap, and males, as a result, are primed by evolution to be quick on the draw and not terribly selective as to targets. Their modus operandi is shoot first and ask questions later, if at all. But in certain species, human beings most especially, males have more to contribute: they can be providers, protectors, helpmates, and partners, not just lovers. In addition, a man’s behavior as a lover may yield some clues as to his inclination in these other crucial dimensions. According to a simple game theory model, males can be caricatured as either Cads or Dads. Cads are superficially attractive, but lack parental follow-through; they’re inclined to love ’em and leave ’em. Dads are, as their name implies, more likely to stay the course and to take the kids to soccer games, but less flashy and perhaps with less instantaneous sex appeal. Might it be that the elusive orgasm has been tuned to help transcend first appearances and encourage women to respond to men who aren’t simply out for a quick sexual encounter—that is, to respond in favor of those who are likely to be Dads? If so, how might this work?

If female orgasm were unlocked quickly and easily, then any Cad could do the trick, then be on his way. But, of course, it isn’t. Women are somewhat slower to rouse, often requiring extensive foreplay and direct, focused attention to the clitoris, which, after all, isn’t within the vagina and thus isn’t likely to be stimulated by a hurried and selfish sexual "technique." This requirement, in turn, may have set the stage for a woman to assess whether her partner demonstrates an inclination to be lovingly generous, predisposed to meet her needs rather than selfishly focus only on his own pleasure. If so, then maybe he’ll also be inclined to pay for the kids’ orthodonture.

"Cryptic choice" of sperm would be especially important in environments in which men tend to provide relatively little investment, in which case orgasm may serve to promote preferential fertilization by "good-quality" sperm or to promote bonding with high-investing men. The Evaluation Hypothesis is thus multipronged and consistent with the fact that nonorgasmic women or women who experience orgasm only rarely are as likely to reproduce as those who are fully and reliably orgasmic. The hypothesis deals with mate selection and ultimately with quality of offspring, not quantity.

Another prediction: if female orgasm is a kind of bioassay of a partner’s dadlike tendencies, then there should be a correlation between skill as a lover and inclination or ability to invest as a parent. Someone should test this. Even if such an association exists, however, it is nonetheless possible that women are more sexually responsive to Dads out of gratitude to their daddy-prone behavior. A clearer test, therefore, would be possible, albeit difficult: see whether men whose sexual style involves a notably focused attention to pleasuring their partner turn out, perhaps years later, to be just as focused on taking good care of their children.

Such a finding would actually go counter to one of the widespread assumptions in evolutionary thinking: a woman who perceives her mate to be of low genetic quality may employ a strategy of garnering resources from her primary mate, but at the same time having extrapair sex with a male who is of higher genetic quality. These circumstances could have selected female design favoring retention of sperm from men who possess phenotypic markers of good genes. In other words, women are said to be primed by their biology to settle down with Dads, who offer the long-term, fitness-enhancing prospect of resources and commitment, but to have sex on the sly with Cads, who offer the short-term prospect of sexual excitement, resulting from an equally fitness-enhancing promise of alluring genes to be inherited by their children. Just so. Or maybe not. Whatever the reality, it seems clear that without postulating "stories" of one sort of another, we’ll never know the truth. As it is, someday we will.

***

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About the Author

David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington. An early pioneer of sociobiology, he is the author of twenty-four books, including, with Judith Eve Lipton, The Myth of Monogamy: Fidelity and Infidelity in Animals and People.Judith Eve Lipton is a clinical psychiatrist specializing in women's health and, with David P. Barash, the author of Gender Gap: The Biology of Male-Female Differences and Making Sense of Sex: How Genes and Gender Influence Our Relationships.

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