The first post on this topic can be found here.
In this second part I will look at the way a particular study is discussed in Jennifer Abbassi's good write-up:
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.Mmm. Did you get it? The answers had some similarities between same-sex siblings and same-sex identical twins and stronger similarities between identical twins. The answers of brothers and sisters or the answers of non-identical different-sex twins did not show such similarity, on average. All this is a roundabout (and common) way of trying to infer that something is an inherited trait, in the absence of direct evidence.
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.”
The critical response to the study is fairly obvious:
It's like comparing oranges and apples, right.
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.”
Except that ANY comparison of female and male orgasms might be exactly that, if we adopt a basic theory other than the by-product one. After all, researchers argue that the female orgasm serves functions such as strengthening pair-bonding or finding a high-quality mate, whereas the male orgasm is there just to spread seed. If the two types of orgasm have such different tasks, couldn't we argue that they are inherently different creatures and can never be compared in such simplistic ways?
Then there is this self-criticism by one of the study authors:
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.”My first post talks about the proof that would be required to show a correlation between orgasms and fertility or other outcomes and how difficult finding such a proof would be. But more generally, I'd like to know how studies measure libido.
It seems to me that the concept of libido, even if initially a biological one, has so many cultural over-pinnings that it's pretty much impossible to measure it in some simple quantitative way that lends itself to interpersonal comparisons. But I may just be ignorant.
Then we come to something quite fascinating:
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.Very interesting. Of course I immediately want to know if the female ability to orgasm is correlated with clitoral length or not. If it is not, the information about the clitoral lengths are of no consequence here. If it is, then I would expect to see lots of studies about that correlation.
The "selective pressure" concept is also interesting. Are we to interpret it here in purely a-cultural and a-historic terms? When, exactly was the selective pressure applied and by what phenomena?
My extreme lay-goddess interpretation of this is that men who couldn't orgasm failed to reproduce much but women who couldn't orgasm did reproduce. This assumes that the male orgasm is necessary for ejaculation. It also assumes that at some point in time there were men who could not orgasm at all.
But what are the reasons for the selective pressure differences? To what extent might they be cultural, caused by the ancient practice of women not having much say over getting married or having sex, say? Or are we expected to abstract away from culture altogether?
...the evidence is so strong that men’s and women’s orgasms are under different degrees of selective pressure...suggests that the selective pressure differences still operate. That, in turn, brings culture into the conversation.
Suppose*, just for the sake of a thought experiment, that female orgasms really are a happy accident, a by-product of male orgasms in humans. Could it not then be the case that those women who have a biologically lower orgasmic capability will now choose to have less sexual intercourse with men? If it is not much fun, why bother, unless you wish to have children? Many more women today are able to refuse intercourse if they wish than has been the case in the past.
You can see where I am going with this experiment. Any current differences in selective pressures by gender might diminish, if orgasmic** women have more intercourse, on average, than women who do not have orgasms, assuming that "more intercourse" leads to more children.
Yeah, it's a silly thought experiment. But somehow I get a similar flavor from this whole field. We know so very little and speculate so very much. And most everything that is speculated will then be debated in those public rancorous debates.
*I do not have a favorite among the theories discussed in these two posts.
**"Orgasmic" is here defined in the same way as it seems to be defined in the studies Abbassi describes. It is a narrow definition, having to do with a woman orgasming during penis-in-vagina sex. And no, I have no idea if the position must be a missionary one.