Hearts are clearly for pumping blood, lungs are for drawing air into the body. But sometimes scientists are unsure of the biological functions of anatomical structures, especially when those structures belong to species that are long extinct. Many species of hadrosaur, also known as duck-billed dinosaurs, had large hollow crests on the tops of their heads. What were these for? Suggestions have included a form of snorkel, an air-tank to enable underwater exploration, and a resonating chamber to amplify calls. We should not suppose, though, that every biological structure must have its own function, as though organisms were composed of neatly designed interlocking elements.
What are the nipples of male humans for? The most tempting answer is that they have no function at all. Male nipples play no role in the survival and reproduction of men. Female nipples, on the other hand, play an obvious biological role in lactation. Although some genes are specific to men, and others are specific to women, the great majority of the genes that are involved in development from egg to adult are common to both sexes. Males have nipples because males and females develop through broadly similar processes, and females need nipples to nourish their young. Male nipples are an evolutionary side effect of female lactation.
What about women’s orgasms? What are they for? In a wonderful case study, the philosopher of science Elisabeth Lloyd argues that various forms of bias have affected scientists’ work in this domain. Lloyd is happy to acknowledge that the pleasure women get from sex has the biological function of encouraging sexual activity, and thereby reproduction. Her target is instead the specific functionality claimed for orgasm, rather than that claimed for sexual pleasure in general. Lloyd argues that the most plausible hypothesis for female orgasms is that they, like male nipples, have no function with respect to survival and reproduction. Instead, they are best thought of as evolutionary side effects—this time, of the physiological structures underpinning male orgasms. Lloyd is open to the idea that data might eventually be produced demonstrating that women’s orgasms do have a biological function. Her claim is merely that as things stand (or rather, as things stood back in 2005 when her book was published), evidence favors the “side-effect hypothesis.”
In endorsing what I am here calling the side-effect hypothesis, Lloyd is not asserting that women’s orgasms are unimportant, or imaginary, or only mildly enjoyable. Some commentators have attacked Lloyd on the grounds that her skepticism about the biological function of female orgasms devalues them. These attacks are unfair. The abilities to play the piano, to solve complex equations, and to write prose are also unlikely to have functions with respect to survival and reproduction, but there is nothing unreal or frivolous about them. Someone who suggests that sprinting ability, but not footballing skills, assisted the survival and reproduction of our ancestors does not thereby imply that Usain Bolt is a more significant sportsman than Lionel Messi. In order to draw attention to the fact that she regards orgasms as real and valuable, Lloyd has largely dropped her original language of female orgasms as “by-products.” That evoked unfortunate images of industrial waste or jars of Marmite. Instead, she now tends to refer to the female orgasm as a “fantastic bonus.”
It is not possible to summarize all of Lloyd’s evidence in favor of the side-effect hypothesis here, but we can get a flavor of it. Her basic case draws on the facts that, for women, sexual intercourse is often not accompanied by orgasm (even though the women in question are entirely capable of having orgasms) and that orgasms are instead most readily produced by masturbation. This means that female orgasm has no obvious direct link with reproduction. She quotes with approval the American biologist and sexologist Alfred Kinsey’s remarks on how intercourse often fails to elicit orgasm: “It is true that the average female responds more slowly than the average male in coitus, but this seems due to the ineffectiveness of the usual coital techniques.”
Lloyd goes on to argue that there is little or no credible evidence supporting the various suggestions that have been put forward for biological functions for female orgasms. The zoologist Desmond Morris, for example, suggested back in 1967 that female orgasm helped to solve the potentially fatal problems posed to our bipedal species by gravity. As he put it: “There is . . . a great advantage in any reaction that tends to keep the female horizontal when the male ejaculates and stops copulation. The violent response of female orgasm, leaving the female sexually satiated and exhausted, has precisely this effect.” Orgasms tire women out, and cause them to stay lying down. Thanks to this, fertilization is not threatened. A similar hypothesis was put forward in the 1980s, when Gordon Gallup and Susan Suarez suggested that “the average individual requires about five minutes of repose before returning to a normal state after orgasm, and some people even lose consciousness at the point of orgasm.”
Lloyd responds by pointing out that the “average individual” Gallup and Suarez specify here turns out not to be a woman at all; instead it is the average man who needs five minutes of rest after orgasm, as determined by Kinsey and colleagues in 1948. She also provides evidence indicating that men and women do not respond to orgasms in the same ways: while men might typically need a lie-down, women often continue in a state of arousal after orgasm. Responding to Morris’s image of female orgasm keeping the woman prone, Lloyd points out that this presupposes that the orgasmic woman is lying down. She then draws our attention to further research (available when Morris wrote his own piece) indicating that the most effective position for clitoral stimulation, and hence orgasm, during intercourse is when the woman is on top of the man. Under those circumstances, orgasm would seem to encourage, rather than prevent, the draining effects of gravity.
The views of Morris, Gallup, and Suarez are fairly old, and one might think of them as easy targets. Lloyd considers many other theories of the female orgasm, including the far more recent “upsuck” theory, a hypothesis that remains influential today. The basic idea of the upsuck theory is that female orgasm increases the chances of fertilization, because orgasm results in ejaculated sperm being sucked by the uterus into the reproductive tract.
Lloyd recognizes that there is a study, done on just one woman, suggesting that pressure in the uterus drops after orgasm, which might indicate potential for a sort of vacuum suction effect. But she questions the idea that this results in any sperm being sucked into the cervix, or the body of the uterus. For example, she cites a study by Masters and Johnson— pioneers in the 1950s and ’60s of the laboratory-based study of intercourse—that reported “[no] evidence of the slightest sucking effect,” and she notes that the contractions of the uterus that accompany orgasm may push sperm out rather than sucking it in. She concludes her review with the comment that “three studies suggest no upsuck related to orgasm, and the one study that does consists of a total of two experiments done on the same woman, which document not upsuck itself but a change in uterine pressure.”
Although Lloyd claimed there was no good evidence back in 2005 in support of biological functions for female orgasms, she was not foolish enough to suggest that such evidence could never appear. Ten years have passed since her skeptical assessment. Even so, the very best verdict we can come to for proponents of biological functions for female orgasms is that the question remains unsettled. For example, a 2012 review goes against Lloyd’s skeptical view, informing readers that “a variety of evidence suggests that female orgasm increases the odds of conception.” The authors of that review lean quite heavily on a particular version of the upsuck theory: they claim that orgasm promotes the release of the hormone oxytocin. They also report that, in general, oxytocin promotes the “transport” of sperm through the cervix.
Back in 2005, Lloyd raised an important challenge for this idea: orgasm is not the only way to cause the release of oxytocin, and the amount of oxytocin that orgasm releases is small.
Oxytocin levels also increase through sexual stimulation alone, even when orgasm does not occur. The question, then, is whether the boost to oxytocin levels that seems to arise from orgasm is enough to make a significant difference to sperm transport, given that nonorgasmic sexual stimulation appears to raise oxytocin levels all by itself.
Recent work by the sexual physiologist Roy Levin has ended up reinforcing Lloyd’s critical treatment of the “upsuck” hypothesis in forceful terms. Levin calls the upsuck theory a “zombie hypothesis”—an idea that simply refuses to lie down even when (from the perspective of the evidence) it is well and truly dead. He notes that the experiments used to show a link between oxytocin release and sperm transport involved injecting women with around four hundred times as much oxytocin as would normally be released in orgasm. So Lloyd’s question of whether orgasm releases enough oxytocin to make a difference to sperm transport is a good one. Alongside many other criticisms, Levin also argues that sexual arousal results in the cervix moving into a position well away from the location of ejaculated semen, with the result that even if orgasm produced a suction effect, the cervix would not be close enough to the semen for any of it to be sucked up. His conclusion is blunt: “There is no uncontroversial empirical evidence for the human female’s orgasm having any significant role in facilitating sperm uptake by enhancing either its rate or the amount transported or both in natural coitus.”
Lloyd concludes, then, that there is no good evidence supporting any story of female orgasm’s functionality, and Levin concurs. Why, though, have researchers been so enthusiastic in embracing hypotheses of function, in spite of the poverty of evidence? Lloyd makes two suggestions. First, she suggests there is a bias in favor of adaptationism. Very roughly speaking, the adaptationist is one who assumes that the organism can be atomized into distinct traits, each with its own function with respect to survival and reproduction—rather in the manner that an exploded diagram of a washing machine reveals a variety of parts, each of which has a job to do. As we have seen, there is no guarantee that every trait must be explained in this way—it is certainly implausible to think male nipples have biological functions—but researchers on female orgasm seem to have shown a particular enthusiasm for hypotheses framed in terms of biological function, which has led them to overstate evidence in favor of their views, and to overlook evidence against them.
Second, and more interesting, Lloyd suggests that researchers have tended to assume that female sexuality must be like male sexuality: male orgasm has an obvious reproductive function, it is reliably elicited in sexual intercourse, it often results in a period of tiredness. These sorts of assumptions have been projected onto female orgasm in a way that obscures abundant evidence showing how female orgasm and intercourse are only loosely connected. For women, intercourse results in orgasm comparatively rarely, masturbation results in orgasm far more reliably. Indeed, some of Lloyd’s earlier work on sex research in primates demonstrates how the presumption that female sexuality must be linked closely to reproduction has closed off important areas of research.
Female bonobos (the species formerly known as “pygmy chimpanzees”) often engage in something called “genito-genital rubbing”: two females hold each other and “swing their hips laterally while keeping the front tips of their vulvae, where the clitorises protrude, in touch with each other.” The question of whether this is same-sex sexual behavior, or whether instead it is social behavior of a nonsexual kind, seems like a sensible one to ask. But Lloyd points out that this question was closed off from serious inquiry when some researchers stipulated that behavior in nonhuman primates is sexual only when it occurs in oestrus—that is, only when the animal is in a fertile phase of its menstrual cycle and certain hormone measures are high. Since genito-genital rubbing occurs during nonfertile periods, it follows that genito-genital rubbing cannot be sexual. Evidently this is not an important experimental result. It is a trivial consequence of stipulating that behavior can be sexual only if it occurs during a period of fertility.
Excerpted from "The Meaning of Science" by Tim Lewens. Published by Basic Books. Copyright 2016 by Timothy Lewens. Reprinted with permission of the publisher. All rights reserved.