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.
Bias in the Science of Evolution: Male orgasms exist, it's widely believed, to encourage men to spread their seed. On face value, Cant female orgasm would be easy to say that women orgasm for the same reason: 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 Cant female orgasm camps on this question. The first group proposes that it has an adaptive function in one of three categories: 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 Cant female orgasm 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. 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, 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 Cant female orgasm to achieve, while males were asked how long it took them to reach orgasm during the act and how often they felt they ejaculated Cant female orgasm 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.
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. 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: In women I used a measurement from the top of the head Cant female orgasm 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.