A genuine spandrel, unavoidable when an arch is inscribed in a right angle; hence, a by-product rather than a result of direct "selection." (Wikipedia)
In our search for the adaptive significance of female orgasm, we must acknowledge an important possibility, analogous to what scientists call the “null hypothesis,” which is to say, the possibility that nothing especially interesting is going on. Much of evolutionary biology involves searching for the adaptive significance of things, whether structure, physiology, or behavior. But we must always consider the chance that a trait in question has no adaptive significance at all, that it does not owe its existence to the direct action of natural selection. Maybe its a “spandrel,” something that exists for reasons other than its direct, adaptive significance.
This prospect has been raised with regard to female orgasm, first by anthropologist Donald Symons, then by paleontologist/author Stephen Jay Gould, and also by philosopher of science Lisa Lloyd. Their argument is that female orgasm does not owe its existence to any biological payoff associated with it, but is rather a non-adaptive tag-along trait, unavoidably linked to what really matters, namely, male orgasm. It is a claim that must be taken seriously, if only because to be sure, not all traits have been selected for in themselves; some occur simply because of an unavoidable connection with something else (in which case the “something else” typically is adaptive).
At first blush, it might seem absurd and also demeaning to claim that women’s orgasms are merely secondary byproducts to the Real Thing — namely, orgasm in men. Indeed, it is reminiscent of Simone de Beauvoir’s contention that women are the “second sex,” an eternal Other compared to men who as genuine Subjects, are where the Real Action resides.
The “by-product” hypothesis points, however, to a powerful metaphor: the case of male nipples. Men don’t lactate, so why do they have nipples? The classic and almost certainly correct answer is: because women have nipples, and they lactate, and the complex developmental pathway during human embryology that eventually gives rise to nipples in women necessarily engenders nipples in men, too, where they are unnecessary, unselected for, and even downright silly.
Although no one claims that orgasms are silly, those who deny adaptational status maintain that they are like male nipples, without biological significance in themselves, and that they exist only because they are tightly bound to something strongly selected for in the opposite sex.
The by-product hypothesis has superficial plausibility. Embryonic development is typically a package deal, with various traits dependent on preceding stages. Since male and female embryos undergo a common developmental pathway, it would be a major fitness burden to interfere and redesign an unnecessary independent pathway that led to nipple-less men. After all, if nipples don’t do any good, at least they don’t do any harm, so presumably they are simply along for the ride. If they caused trouble, perhaps if they frequently became cancerous or if they required a lot of metabolic energy to create and maintain, they doubtless would have been selected against, but they ain’t broke—merely irrelevant—so evolution hasn’t fixed them. The same applies, or so it is claimed, to female orgasm.
This idea isn’t quite as ridiculous as it seems. Just as male orgasm and ejaculation occurs when the penis is suitably stimulated, female orgasm is intimately linked to stimulation of the clitoris, and the penis and clitoris both derive from the same undifferentiated embryonic tissue, called the genital ridge. Both penis and clitoris are therefore richly endowed with nerves and with parallel brain mechanisms that respond—orgasmically—to enough of the right input from them. It’s just that in one case (men) that response is adaptive, whereas in the other (women) it is essentially an evolutionary hitchhiker.
Although the by-product hypothesis deserves respectful attention, it is almost certainly incorrect. For one thing, a potential parallel with male nipples proves nothing. Men’s nipples are small and inconspicuous, as befits a by-product upon which selection has been reduced. By contrast, female orgasm is complex, highly elaborated, and downright Technicolor compared to its relatively feeble male counterpart. Rather than existing as an insignificant, pale imitation of where the genuine action is supposed to be (in men), female orgasm shows every sign of having been structured and fine-tuned by evolution. Given its multidimensional intensity and repeatability, it seems downright absurd to relegate female orgasm to the role of mere tag-along hitchhiker while crediting male orgasm as the Real McCoy.
For another thing, women’s orgasms are unlike men’s nipples in that they aren’t always there. There are no nipple-less men, but lots of anorgasmic women. Nor are there any men who develop nipples some of the time, although there are lots of women—the great majority—who experience orgasm only sometimes. Male nipples, by all accounts an unavoidable consequence of human embryology, are persistent, albeit nonfunctional. Female orgasms are not consistent; paradoxically, as we shall see, this may be part of their ultimate functionality.
As to the clitoris, it has, by some estimates, more than 8,000 nerve endings, exceeding that found anywhere else in the human body and approximately double that of the penis. Its only known function is sexual sensation and orgasm. If the clitoris (and thus the orgasmic consequence of stimulating it) exists merely as an unavoidable by-product of selection to produce the penis, why is the former better endowed neuronally than the latter? In fact, a case could be made that male orgasm isn’t nearly as necessary as many people assume; there is no reason, for example, why ejaculation couldn’t be as unexciting as, say, urination—something biologically necessary and which is therefore prompted by feelings of urgency but without any orgasmic sturm und drang.
It has also been claimed that women’s orgasm couldn’t possibly be adaptive since it isn’t consistently evoked during heterosexual encounters. Indeed, according to a recent study, about one third of women report never being orgasmic during sexual intercourse, compared to only about 20 percent during masturbation; in other words, about 67 percent are orgasmic, at least on occasion, during intercourse, whereas 80 percent regularly experience orgasm as a result of masturbation.
This rather inelegantly named “intercourse-orgasm discrepancy” supposedly bespeaks the evolutionary irrelevance of orgasm. One of my biological colleagues has suggested that anyone who seriously thinks of female orgasm as merely an evolutionary shadow of male orgasm is either a woman who has never had one or a man who has never been with a woman who has had one. But as we shall see, the intercourse-orgasm discrepancy actually relates quite well to the “evaluation hypothesis,” which we shall meet in a future post.
For now, note that plenty of things that occur intermittently are nonetheless adaptive. Wolves are only intermittently successful when they initiate a hunt—sometimes the moose gets away—but going hunting is clearly adaptive for a hungry wolf! In fact, psychologists understand that a degree of unpredictability actually adds to the effectiveness of a stimulus: So-called “intermittent reinforcements” are often more potent than is a guaranteed payoff. The fact that some orgasms “get away” doesn’t make them any less biologically relevant.
For many women orgasms are more reliably evoked digitally or orally, rather than by an inserted penis, and primarily via the clitoris rather than the vagina. This, too, supposedly makes it less likely that orgasm evolved with heterosexual intercourse in mind. But in fact, it is entirely consistent with biological reality for a trait, having evolved in one context, to function also in another. The fact that orgasm is more readily induced by oral, digital, or mechanical stimulation of the clitoris than by a penis entering the vagina suggests, among other things, that evolution isn’t always maximally efficient, not that orgasm isn’t an adaptation.
If the clitoris were lodged inside the vagina instead of just outside, then women would doubtless find themselves more inclined to climax as a result of vaginal penetration. But because of embryologic constraints, the genital ridge—which eventually differentiates into either clitoris or penis—doesn’t end up there. So, resourceful human beings (as well as several species of nonhuman primates) have discovered other, more efficient ways to stimulate the clitoris, thus achieving an outcome also available via other means. In short, female orgasm is indeed not well designed to be reliably elicited by penile–vaginal intercourse. Under various scenarios, it might be more adaptive yet—and certainly, more convenient for those involved—if women could achieve orgasm by hearing music, wrinkling their noses, eating chocolate, and so forth, but insofar as orgasms are keyed to penises and clitorises, that’s what natural selection has had to work with. Penis and vagina are a nice lock-and-key pair, satisfying both evolutionarily and personally. The fact that for women, fingers or tongue upon clitoris are typically even more satisfying doesn’t undermine the adaptive significance of either heterosexual intercourse or of the female sexual response, however evoked.
Many women don’t achieve orgasm, but perhaps they could, in the right circumstances. Some, presumably, cannot do so at all. This, too, doesn’t mean that orgasm isn’t an adaptation. Most people can run and jump, whereas some can’t — and of these, some probably never will. But this doesn’t mean that the capacity to run and jump isn’t adaptive, and that the suite of anatomy and behavior enabling most healthy people to do so hasn’t been produced by natural selection. It turns out that women experiencing sexual dysfunction are liable to have substantially reduced clitoral innervation via the pudendal nerve.
Accordingly, just as there are natural athletes and the rest of us—even though a degree of athleticism has almost certainly been promoted by evolution—there may well be natural sexual athletes, too. And, unfortunately for them, there are those whose neuronal functioning is suboptimal, not necessarily because evolution doesn’t favor the higher performers, but rather because many biological traits are distributed across a continuum.
Not everyone is an Olympic champion; indeed, as we shall soon see, those who are—that is, women who achieve orgasm with every sexual encounter—are probably less likely to be the focus of natural selection than are those who fall in the “sometimes” category.
Coming soon: orgasm as an inducement for polyandry, or monogamy?
