The Case of the Female Orgasm: Bias in the Science of Evolution

  • Elisabeth A. Lloyd
Harvard University Press: 2005. 320 pp. $27.95, £18.95 0674017064 | ISBN: 0-674-01706-4

For men, orgasm is an intimate part of reproduction: ejaculation doesn't usually happen without it. Presumably, male orgasms evolved because, in the past, males who experienced sexual pleasure were more likely to have sex, and so were more likely to sire children. But what about orgasm in women? Women can become pregnant without orgasm; indeed, some women bear lots of children without ever experiencing one. So how has the female orgasm evolved?

There are two basic possibilities. The female orgasm may have evolved under natural selection on females, which is to say that females who have the capacity to reach orgasm have historically had more surviving children than females who do not. Alternatively, it may have evolved as a by-product of natural selection on something else. A number of evolutionary biologists have hypothesized about the former, imagining various ways that orgasm might have enhanced female reproductive success. Elisabeth Lloyd, a philosopher of science at Indiana University, prefers the second.

In The Case of the Female Orgasm, Lloyd champions the notion — first advanced by Donald Symons in 1979 — that orgasm in women is an accidental consequence of the fact that the clitoris develops from tissue that in a male embryo will become the penis. This would mean that women have orgasms just because men do, not because it enhances their reproductive success. Lloyd also mounts a scathing attack on those who have speculated about how orgasm might have been subject to natural selection on females. She accuses them of failures of logic, shoddy data analysis, and a tendency to ignore data they don't like. She says they commit these sins because they are hostage to a variety of unexamined assumptions, the most egregious being ‘adaptationism’ — an (in her view) absurd and unjustified commitment to natural selection as an explanatory force in evolution.

Sex talk: it raises a smile, but scientists still cannot explain why most women have orgasms. Credit: DARREN STAR PRODUCTIONS/KOBAL COLLECTION

Lloyd cites several facts to support her contention that female orgasm is a by-product. First, there are no data showing that women who reach orgasm during sex have greater reproductive success than women who do not; moreover, orgasm is unnecessary for conception. Second, during the past 50 years, surveys of Western women have found that although a minority always reach orgasm during copulation, some never do, and everyone else does only sometimes. Third, most women find it easier to reach orgasm through manual stimulation than through stimulation from the penis. Finally, some other female primates, such as the stump-tailed macaque, the bonobo and the chimpanzee, can reach orgasm.

But none of these comes close to dealing a hammer blow to natural selection. Consider the fact that the clitoris develops from the same tissue as the penis. This tells us something about the origin of the clitoris, but little about why it is still here. Once something has arisen, it can still be subject to natural selection. It may be that the clitoris has been modified to help women achieve orgasm. Then again, it may not: we don't know.

Or consider the matter of orgasm and reproductive success. There are no data showing that orgasm enhances reproductive success; but nor are there data showing that it doesn't. What conclusion can we draw? None: absence of evidence is not evidence of absence.

Or consider the fact that not all women experience orgasm during sex. Lloyd equates variation in phenotype with proof that natural selection has not acted. But this need not be so: we all have eyes, yet we cannot all see equally well. No one would argue that eyes have not evolved under natural selection on vision.

The sad fact is that, for now, all statements about the evolution of the female orgasm are conjectures in an empirical vacuum. To advance the debate, we need data.

The most obvious approach would be to ascertain whether there is (or was) a link between orgasm and reproductive success. Measuring the relationship between a given trait and reproductive success is difficult in any organism. It is obviously impossible to know whether orgasmic women have tended to have more children than anorgasmic women. The best we can do is try to infer.

The fact that orgasm is not necessary for conception rules out the obvious way that orgasm could enhance reproductive success — but it could have more subtle effects. For example, could orgasm during sex induce ovulation? In mammals such as ferrets and cats, ovulation is induced by stimulation from the male; might it be facultatively induced in humans? As far as I know, such an effect has not been reported for any primate, but then, as far as I know, no one has looked for it.

We also need to know far more about the nature of orgasm. Orgasm is the result of two phenomena: contractions in the pelvic region, and the perception of pleasurable sensations by the brain. Yet we have little understanding of how the two components relate to each other. Moreover, a distinction is often made between clitoral and vaginal orgasm. Whether these are physiologically different — let alone whether they evolved under different selection pressures — is unknown. Indeed, the neuroanatomy of the genital region is poorly understood, and we have scant data on how much it varies among women. Brain scans suggest that different parts of the brain may be involved in orgasm for males and females — which would be consistent with natural selection acting on females directly — but the sample sizes are as yet too small to draw confident conclusions.

To understand the significance of the variation in women's experience of orgasm, we need to know what causes this variation. Is it due to genetic differences in genital anatomy? To differences in the way brains perceive pleasure? To psychological or cultural factors? Or to a physical incompatibility between a woman and her partner(s)? In other words, do some women lack a capacity for orgasm, or is the capacity there but never realized? Again, data are lacking. A recent twin study (K. M. Dunn et al. Biol. Lett. doi:10.1098/rsbl.2005.0308; 2005) suggests there may be a genetic component — if your identical twin has orgasms it's likely that you do too — but whether the effect is due to physiology or psychology is unclear.

And we need to know more about when other primates experience orgasm. Do females in other species have orgasms with some males but not with others? No one knows. Here, too, we need to know how pelvic contractions translate into brain waves. And we need to investigate males as well as females: it is often simply assumed that males in other species have orgasms. Data on other primates will help us to understand the relationship between male and female orgasm, and whether the female orgasm evolved before the split between humans, chimpanzees and bonobos.

In short, it's time to collect data. Without it, the debate will remain like sex sometimes is: furious, empty and anticlimactic.