There may be few questions of human sexuality more rancorous than those about the female orgasm. Scientists agree that women probably started having orgasms as a by-product of men having them, similar to how men have nipples because women have them. As Elisabeth Lloyd, a philosopher of science and theoretical biologist at Indiana University put it in her 2005 book The Case of the Female Orgasm: Bias in the Science of Evolution: "Females get the erectile and nervous tissue necessary for orgasm in virtue of the strong, ongoing selective pressure on males for the sperm delivery system of male orgasm and ejaculation." But why we ladies still have orgasms is hotly debated.
Male orgasms exist, it's widely believed, to encourage men to spread their seed. On face value, it would be easy to say that women orgasm for the same reason: to encourage them to have sex and make babies. But in practice, compared to male orgasm, female orgasm is very difficult to achieve. There's a lot of variation even within individual women, and 10 percent of women never have them at all. And, unlike male orgasm, female orgasm isn't a prerequisite for pregnancy.
So why do women have orgasms at all? There are two firmly opposed camps on this question. The first group proposes that it has an adaptive function in one of three categories: pair bonding, mate selection and enhanced fertility. I'll break these down. The pair-bonding theory suggests that female orgasm bonds partners, ensuring two parents for the offspring, while mate selection offers that women use orgasm as a sort of litmus test for "quality" partners. The enhanced fertility theory, meanwhile, proposes that uterine contractions during female orgasm help to "suck up" sperm into the uterus.
The by-product camp, on the other hand, claims that female orgasms are to this day an incidental by-product of male orgasm, not an evolutionary adaption. "There's no documented connection between women who have orgasm at all, or faster, having more or better offspring," Lloyd says.
The schism between the two camps deepened this month with the publication of a new study of twins and siblings in Animal Behavior that seems to rule out the by-product theory of female orgasm. Researchers Brendan Zietsch at the University of Queensland in Australia and Pekka Santtila at Abo Akedemi University in Finland asked 10,000 Finnish female and male twins and siblings to report on their "orgasmability" (their word, not mine). They looked for similarities in orgasm function between female and male twins. If the by-product theory of female orgasm is true, they say, this similarity should exist. Due to the inherent differences in orgasm between women and men, females were asked to report how often they had orgasms during sex and how difficult they were to achieve, while males were asked how long it took them to reach orgasm during the act and how often they felt they ejaculated too quickly or too slowly.
Zietsch and Santtila found strong orgasmability correlations among same-sex identical twins, and weaker yet still significant similarities between same-sex non-identical twins and siblings. However, they found zero correlation in orgasm function between opposite-sex twins. "We show that while male and female orgasmic function are influenced by genes, there is no cross-sex correlation in orgasmic function -- women's orgasmability doesn't correlate with their brother's orgasmability," explains Zietsch. "As such, there is no path by which selection on male orgasm can be transferred to female orgasm, in which case the by-product theory cannot work."
Zietsch says he doesn't have a favorite theory on the evolutionary function of female orgasm, but if forced to guess he'd say that it provides women extra reward for engaging in sex, thus increasing frequency of intercourse and, in turn, fertility. (There's no proof of this yet, though, as Lloyd points out.) Zietsch continues: "I've shown in another paper, though, that there is only a very weak association between women's orgasm rate and their libido, so the selection pressure on female orgasm is probably weak -- this might explain why many women rarely or never have orgasms during sex."
Lloyd and other proponents of the by-product theory agree that weak selection pressure could be acting on female orgasm, but not enough to maintain it over the eons of human evolution. Rather, if female orgasm bestows any reproductive benefits onto the human race, it would be by happy accident. Unsurprisingly, Lloyd has a lot of bones to pick with the recent study. Comparing different orgasm traits in women and men is a textbook case of apples and oranges, she says.
Kim Wallen, a behavioral neuroendocrinologist at Emory University and frequent collaborator with Lloyd, explains it thus: "Imagine that I wanted to compare height in men and women. In women I used a measurement from the top of the head to the bottom of the foot. In men I used how rapidly they could stand up. Would I be surprised that each measure was correlated in identical twins within sexes, but uncorrelated in mixed-sex twins? Such a result would be what was predicted and completely unsurprising. Zietsch and Santtila have done the equivalent of this experiment using orgasm instead of height."
Wallen also points out that previous research has shown that traits under strong selective pressure show little variability, while those under weak pressure tend to show more variability. With human orgasm this bears out in that men report almost always achieving orgasm during sex, while the ability to orgasm during intercourse varies widely among women. (Penis and vagina size – both necessary for reproduction -- show little variability, suggesting they are under strong selective pressure, Lloyd says, while clitoral length is highly variable.) Wallen asserts that Zietsch and Santtila, "chose to compare apples to oranges because the evidence is so strong that men's and women's orgasms are under different degrees of selective pressure, the very point they were trying to disprove." Yikes.
To their credit, Zietsch and Santilla acknowledged the limitations of their study, both in the paper and in Zietsch's email to me. More work obviously needs to be done. "Figuring out the function of female orgasm, if any, will probably require very large genetically informative samples, fertility data, and detailed information on sexual behaviour, orgasm rate, and the conditions and partners involved," Zietsch says. "I do have plans, but the debate probably won't be settled quite some time to come."
If, at this point, you're as frustrated as me, you might be wondering what we do know about female orgasm. Well, we're closer to knowing why they're so few and far between during sex. In a paper published online this January in Hormones and Behavior, Lloyd and Wallen found that the farther away the clitoris is from the urinary opening, the less likely it is that the woman will regularly achieve orgasm with intercourse. If this holds up in future experiments, Lloyd says, it would establish that a woman's ability to have an orgasm during sex rests on an anatomical trait that likely varies with exposure to male sex hormones in the womb. "Such a trait could possibly be under selection," she says, "but this would have to be investigated. So far, no selective force seems to appear."
Jennifer Abbasi is a science and health writer and editor living in Brooklyn. She has seen every episode of The X-Files. Have a question about the science of sex? Email Jen at firstname.lastname@example.org.