There is perhaps no larger mystery in the science of human reproduction than the role of the female orgasm. Sensations of pleasure generally aren’t random and are usually the result of some evolutionary pressure driving us to engage in adaptive behaviours. When it comes to sex we know where babies come from, we know why mammals have breasts, and we know more than enough about erections and ejaculation but we can’t for life of us work out just what the role of the female orgasm is in all this.
Where the male orgasm is concerned, there is no great mystery in decoding an evolutionary function. Ejaculation and orgasm in males go hand in hand, and are inherently linked to reproductive success. Not so in the case of the female orgasm where researchers are yet to discover any conclusive links between its occurrence and a subsequent reproductive advantage. Female orgasm doesn’t increase fertility (Zietsch & Santtila, 2013), doesn’t increase the number of children born, nor does it confer any obvious survival advantage (Barash & Lipton, 2009). Add to all this the fact that orgasms are surprisingly rare during penetrative sex. In her book, The Case of the Female Orgasm, Elisabeth Lloyd (2005) reviewed orgasm rates reported by women across over 100 studies and found that only around 25% of women orgasm during all penetrative sexual encounters. Furthermore, around a third of women reported orgasming either infrequently or not at all, with the remaining falling somewhere between these two extremes. This leaves us asking, why does the female orgasm happen so rarely during sex? More to the point, why does the female orgasm happen at all?
Two main camps have emerged in trying to explain the evolution of the female orgasm: adaptationists and non-adaptationists. As one might imagine, adaptationists view the female orgasm as an evolved module that provides some advantage, while non-adaptationists espouse the position that the orgasm is essentially a by-product of other biological processes.
Both male and female orgasms rely on the development of similar erectile tissue (i.e. the glans penis and the glans clitoris) and as result they likely share an evolutionary origin (Puts, Dawood, & Welling, 2012). Proponents of the by-product hypothesis argue that this shared development is the result of strong selection pressure on male orgasms (Symons, 1979). In contrast, the female orgasm is believed to have no adaptive purpose and a frequently used analogy to explain this is the existence of the male nipple. Female mammals require nipples in order to breast-feed their young and as a result evolution has invested a lot of time in ensuring that these structures develop properly under enormous selective pressure. As a result, males develop nipples as well even though they aren’t required for nursing young.
The lack of an obvious link between female orgasms and any advantage (apart from more pleasurable sex) and the inconsistency of orgasms during sex seems like compelling evidence for the by-product hypothesis. However, there are a couple of reasons that the by-product doesn’t adequately account for the female orgasm. For something that evolution shouldn’t care about, the orgasm sure is intricate. Female orgasms are reported to be more psychologically elaborate and complex than the male counterpart, with increased intensity (Mah & Binik, 2001, 2002) rather than reduced, as might be expected in a by-product. The clitoris also responds to sex hormones across the lifespan (Huffman, 1969), unlike the penis which ceases reacting to after puberty (Shabsigh, 1997). The clitoris is also enormously innervated, accounting for its incredible sensitivity (Winkelmann, 1959), which is an incredibly complex process for an organ that is not under selective pressure.
Perhaps most damning of all are the results of a recent genetic analysis of male and female orgasm (Zietsch & Santtila, 2011). Firstly, the researchers compared measures of orgasm sensitivity across male and female twins and found no correlation. Secondly, the study found unique genetic components underlying orgasms in each sex. Both of these findings conflict with the by-product hypothesis.
So maybe the orgasm does serve some evolutionary function?
Adaptationists believe that the enormous variation in orgasm frequency might reflect some sort of selection process assessing the quality of mates. This is referred to the mate-choice hypothesis and argues that the female orgasm is discriminatively responding to some inherent genetic quality in certain mates (Alcock, 1980; Smith, 2006; Thornhill, Gangestad, & Comer, 1995). In contrast to penetrative sex, almost all women can reach orgasm through masturbation, which might suggest that orgasms occur only during sex with a genetically compatible partner.
Interestingly, one study has observed that women report greater sexual satisfaction with partners whom possess different immune system genes (Garver-Apgar, Gangestad, Thornhill, Miller, & Olp, 2006). A diverse immune system is linked to better resistance to disease and has been hypothesised to drive some aspects of human mating based on a genetic advantage that might be conferred to offspring. Physical attractiveness has similarly been suggested to indicate mate quality, and females report more orgasms when they find their partners more attractive (Shackelford et al., 2000). Of course, providing orgasms is a highly valued trait and this could just mean women find men more attractive when they’re better in bed. With the exception of several similar findings (for a review see Puts et al., 2012), there has been very little consistency in finding other male traits that are likely to induce orgasm (including penis size).
Between the two theories above we’re not much closer to a definitive evolutionary account, as neither seem to be able to accurately capture the remarkable complexity, puzzling inconsistency, or overall impact of the female orgasm. Compounding this confusion is the possible distinction between vaginal and clitoral orgasms, the failure to investigate manual stimulation of the clitoris during sexual intercourse, and difficulty determining how much an orgasm actually influences female satisfaction. Given that so many women do not experience orgasm during penetrative intercourse, how important is the orgasm for sexual satisfaction overall? Is it more likely to induce penetrative sex if it occurs during foreplay? When it comes to the female orgasm, there are probably more questions than answers and for now it might just be best to put down the textbooks and get on with it.
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Zietsch, B. P., & Santtila, P. (2011). Genetic analysis of orgasmic function in twins and siblings does not support the by-product theory of female orgasm. Animal Behaviour, 82(5), 1097-1101. doi: 10.1016/j.anbehav.2011.08.002
Zietsch, B. P., & Santtila, P. (2013). No direct relationship between human female orgasm rate and number of offspring. Animal Behaviour, 86(2), 253-255. doi: 10.1016/j.anbehav.2013.05.011