A paper published by Zietsch and Santtila (2011) has recently caused a bit of a stir in the science-blogging community. Heavy-weights such as PZ Meyers, Scicurious and Greg Laden have posted regarding the paper with various levels of criticism. The paper, published in Animal Behaviour, and entitled 'Genetic analysis of orgasmic function in twins and siblings does not support the by-product theory of female orgasm' was published by a scientist I have come to know over the past year or so.

It's always exciting to see someone you know make a splash, and I've had some interesting conversations with Dr. Zietsch regarding the criticisms. I'm a small-fry in the blogging community, but I offered Dr. Zietsch this forum to respond (if he felt so inclined). 

The following are not my words, it is the work of Dr. Zietsch. I suspect he'll be paying attention to comments, but I would like to stress that the author of the post that follows is not the author of the majority of posts that belong to the this blog. 

[Ed: PZ's rebuttal, here]

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My recent paper testing the by-product theory of the female orgasm (Zietsch & Santtila, 2011) has attracted a lot of attention, including critical blog entries by Greg Laden and PZ Myers. Rather than describing the study here, you can read the abstract and full paper here. Greg Laden doesn’t appear to have read the article at all – for example, he claims that we show that “the similarity in orgasmic function between closely related individual is not predicted by the genetic relatedness. Ladies, it turns out that your genes did not make you come”, whereas actually we show the opposite, that genetic variation does underlie orgasmic function in both men and women. He goes on to say “This paper is so wrong that I'm not even really going to critique it”, so I’ll take the same approach to his blog and leave it at that.

On the other hand, PZ Myers clearly did read the paper, and has identified important limitations, i.e. the use of self-report data and the fact that we assessed orgasmic function (susceptibility to orgasm in response to sexual stimulation) with different questions in males and females. Well, I say he identified them, but they were explicitly acknowledged in the limitations section of the paper itself:

A limitation of this study is the use of self-report to measure orgasmic function, which may be subject to considerable measurement error. Furthermore, for reasons described earlier, orgasmic function was assessed differently in males and females, which could potentially lower the cross-sex correlation. As such, it is possible that a true cross-sex correlation exists that we did not detect despite our large sample; however, the cross-sex correlations could not be higher than the correlations between same-sex DZ twin pairs, which were low for both males and females. These low samesex DZ twin correlations suggest genetic nonadditivity, which lessens the likelihood of any substantive cross-sex selection pressure, since nonadditive genetic variance is only weakly subject to selection and can correlate only weakly between opposite-sex siblings. Nevertheless, it is not known what strength of selection pressure would be required to maintain female orgasm; perhaps even extremely weak cross-sex selection pressure could be sufficient. As such, some caution is warranted in interpreting these findings, and the by-product theory cannot be definitively ruled out.

Indeed, we measured susceptibility to orgasm in response to sexual stimulation (let’s call it ‘orgasmability’) by assessing the likelihood of orgasming during sexual activity in women and the time taken to orgasm during sexual activity in men. That’s because during sex, men tend to reach orgasm faster than women and generally cannot continue once it is reached. Even when women reach orgasm faster than their man, they can generally continue intercourse until he reaches orgasm, sometimes achieving more orgasms. As such, women’s orgasm during sex is time limited – if she doesn’t reach it in relatively quick time, she might not have it at all, whereas men can go until they finish. That’s why we measure likelihood of orgasm in women and time to orgasm in men, consistent with countless other studies and definitions of orgasmic ‘dysfunction’ in men and women in DSM-IV.  

I would argue that likelihood of orgasm in women and the time to orgasm in men both reflect the susceptibility to orgasm in response to sexual stimulation, and so with nearly 2000 pairs of opposite-sex twins and siblings, we would have been able to detect a cross-sex correlation if one existed. However, PZ says it’s apples and oranges, which presumably means he thinks that how quickly a woman tends to reach orgasm during sex is uncorrelated with her likelihood of orgasming. I think that’s implausible. However, those with a different opinion could prove their point and provide compelling evidence for the by-product theory if they demonstrate an opposite-sex correlation in orgasmic function, measuring it in whatever way they see fit.

Myers makes some other points that suggest that although he read the paper, he didn’t read it very carefully, since he misses its main point. The by-product theory, as described in detail by Lloyd (2005), says that female orgasm is currently (and always has been) maintained by ongoing selection on the male orgasm. Selection can only operate on additive genetic variation, so if the male orgasm has zero heritability (i.e. zero additive genetic variation) as Myers suggests, then there is no selection on it and therefore no indirect selection on female orgasm. (Hidden genetic variation with no phenotypic effects in males but expressed in females, which Myers alludes to, is irrelevant here because it’s invisible to selection, assuming no direct selection on female orgasm). He goes on to talk about males and females sharing orgasm-related circuitry and genetic apparatus (which nobody denies) – but, to be repetitive, selection only acts on genetic variation – for selection on the male orgasm to act on the female orgasm, additive genetic variation in male orgasm needs to correlate with additive genetic variation in female orgasm. If such a correlation exists there would be a correlation between opposite-sex siblings, and that’s what we tested – as described in this passage from the introduction of the paper:

There seems little doubt that the female orgasm was originally possible because of the developmental homology with structures and processes that generate the male orgasm, and this is consistent with all existing evolutionary theories of female orgasm. However, the by-product theory argues that female orgasmic function is currently (and always has been) maintained only by ongoing selection on male orgasmic function. As Lloyd (2005, page 110) put it: ‘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’. That is, heritable individual variation in male orgasmic function (susceptibility to orgasm in response to sexual stimulation) is subject to strong selection pressure and, to the extent that there is genetic correlation across sexes, heritable individual variation in female orgasmic function is also subject to that same selection pressure (see Amundson 2008, page 444). Given previous findings of substantial heritable variation in male and female orgasmic function (Dawood et al. 2005; Dunn et al. 2005; Jern et al. 2009; Witting et al. 2009), the by-product theory strongly predicts a correlation between the orgasmic function of opposite-sex siblings. A zero opposite-sex correlation would mean that selection pressure on the male orgasm could not be transmitted to females, and would indicate that entirely different genetic influences underlie variation in male and female orgasmic function. This situation would pose a serious problem for the by-product theory, that female orgasmic function is currently (and always has been) maintained only by ongoing selection on male orgasmic function.

I’m certainly not biased against the by-product theory – actually, I find it intuitively appealing, and I had expected to find a positive cross-sex correlation to support it. I also published another paper this year testing existing adaptive explanations of female orgasm, and found little support for (and substantial evidence against) most of those explanations. What I’ve done is use large, genetically informative samples to test adaptive and non-adaptive theories of female orgasm. As with most research, there are limitations to this approach, which I note in the papers – but science generally doesn’t progress via single, perfect, definitive studies, but rather accumulation of imperfect evidence from different sources until the picture becomes clearer and an answer becomes apparent. We are definitely nowhere near understanding the evolutionary basis of female orgasm, but that will only change through the scientific process of generating hypotheses, testing them, and reporting the results (and their limitations). 

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Dr. Brendan Zietsch 

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Zietsch, B., & Santtila, P. (2011). Genetic analysis of orgasmic function in twins and siblings does not support the by-product theory of female orgasm Animal Behaviour DOI: 10.1016/j.anbehav.2011.08.002