Theories about the evolution of human sexuality have spawned some intriguing ideas. One of the more peculiar ones is that oral sex has an evolutionary function, namely to detect recent infidelity by one's partner. Cunnilingus for example, is supposed to allow a man to detect the presence of another man's semen in or around the woman's vagina. A recently published study aimed to test this theory and found that a man's interest in performing cunnilingus was correlated with his partner's attractiveness. The authors argued that more attractive women are more likely to be targeted by other men for mate poaching, and that partners of such women have more reason to be concerned about sperm competition, and therefore use oral sex to detect possible infidelity, albeit unconsciously. They concluded that their results confirm their hypothesis that oral sex functions to detect infidelity. Although interesting, their results are inconclusive because they did not appear to consider a more obvious explanation for their findings.
Is cunnilingus just about pleasure or does it have a darker purpose?
According to many evolutionary psychologists, men have been concerned with detecting and counteracting infidelity by their partners through human history. Sperm competition occurs when the sperm from two or more males occupy a woman’s reproductive tract at the same time, and hence compete to fertilise her ovum. Scholars have claimed that men have evolved a variety of mechanisms to deal with the threat of sperm competition (Pham & Shackelford, 2013). For example, some research has found that men ejaculate a greater volume of sperm when they have been separated from their partner for a good period of time compared to when they have been in each other’s company for the same amount of time. This is presumed to occur because there is a greater risk that the woman may have been unfaithful in her partner’s absence.
Pham and Schackelford (2013) argued that men with more attractive partners are at a greater recurrent risk of sperm competition because other men are more likely to woo them into having affairs. Therefore, men with more attractive partners have more reason to be concerned about and more likely to engage in behaviour aimed to detect infidelity. The idea that cunnilingus, oral sex performed on a woman, could function to detect infidelity was proposed in a 2006 book, but this study is the first to test this empirically. The idea is that oral sex may allow a man to detect the presence of another man’s semen through smell or taste. Pham and Schackelford’s study did not test whether men can actually detect semen in this manner (admittedly a difficult thing for a research study to test). What they did test were the hypotheses that men with more attractive partners (presumed to present a greater “recurrent risk of sperm competition”) would be more interested in performing oral sex, and that they would perform it for a longer duration “to better detect rival semen.” Contrary to what has been claimed elsewhere, the authors did not claim that men consciously perform oral sex because they think their partner has been unfaithful. It is possible for a behaviour to serve an evolutionary function without a person knowing what that function is. They simply need to want to do it, even if they do not know why.
As side-note I’d like to point out that there is a common misconception often advanced by people who really ought to know better that evolutionary psychology assumes that everything that people do is somehow an evolutionary adaptation and that evolutionary psychologists cannot or will not acknowledge that some behaviours are simply by-products of other adaptations with no special function of their own. This is a gross misrepresentation of what evolutionary psychology is about[i] and in fairness to the authors of the study they were attempting to actually test whether or not their hypothesis about the adaptive function of oral sex is valid, rather than just assuming it is. It is quite possible that oral sex has no evolutionary function in itself. Humans are a highly sexed species compared to most mammals (Diamond, 1998) and engage in many non-procreative sexual acts, perhaps for pleasure alone. Oral sex might simply be a by-product of this interest in sex that humans have. However, if it can be shown that this particular behaviour appears to serve a definite purpose that has an evolutionary history, a reasonable case can be made that it has an adaptive function.
To test their hypotheses, the authors recruited heterosexual males in committed relationships that had lasted at least one year. These were asked a series of questions about how attractive they thought their partners were (to themselves and to other men); about their relationship satisfaction; and their most recent sexual experience. Participants were asked to rate their interest in and duration of oral sex compared to what is “typical” for them. I thought the wording of these questions was somewhat peculiar. One man’s “typical” level of interest in oral sex might be quite different from another man’s, so asking the questions in this way would seem to make individual responses difficult to compare. Their reasons for asking about the participants’ most recent experience in particular was also not made clear.
The results were much as the authors’ expected. “Recurrent risk of sperm competition” (attractiveness) predicted interest in performing oral sex independently of relationship length, relationship satisfaction, and duration of intercourse. The latter three were not significant when recurrent risk/attractiveness was taken into account. Recurrent risk and duration of intercourse each predicted duration of oral sex independently of relationship length and relationship satisfaction. This indicates that the more attractive a man found his partner, the more interested he was in performing oral sex, and the longer he performed it for. The authors took this as evidence in support of their hypothesis that oral sex functions to detect infidelity when recurrent risk of sperm competition is high. However, there is an alternative explanation seems more obvious that the authors seem to have overlooked. This is that the more attractive a man finds his partner, the more interested he would be in performing sex acts in general with her. That is, greater attraction would produce greater sexual excitement generally, and hence greater willingness to engage in a variety of sexual acts.
I think it is also worth noting that recurrent risk/attractiveness had rather modest sized correlations with interest in oral sex and duration of performance (.26 and .24 respectively). These are not trivial sized correlations compared to most findings in psychology, but they do suggest that other factors besides partner attractiveness are related to a man’s willingness to perform oral sex. For example, it has been argued that heterosexual men demonstrate their masculinity through their ability to “master” women’s bodies, and that this may be manifested by skill in bringing a woman to orgasm through oral sex (Backstrom, Armstrong, & Puentes, 2011). Additionally, reciprocity may play a role. That is, men perform oral sex with the expectation of receiving it in return.
To their credit, the authors do consider some alternative explanations of their findings that could be considered in future studies. One of these is that men perform oral sex to increase the woman’s sexual satisfaction. Research has found that women are more sexually satisfied the more frequently they receive oral sex. Other research has found that the more sexually satisfied a woman is, the less likely she is to be unfaithful. Female sexual satisfaction was not assessed in this study, and the authors acknowledge that future research should consider whether the relationship between attractiveness and male interest in oral sex remains after taking into account desire to satisfy the partner. This seems to me like a very reasonable alternative explanation.
The other alternative they considered, which I consider to be much more speculative, is based on the idea that a woman retains more sperm in her uterus when she has an orgasm. Hence men might perform oral sex to increase the chance the woman will have an orgasm, and therefore retain more of the man’s sperm. This idea is based on a study by Baker and Bellis (1993) which actually claimed to have found that female orgasm increased sperm retention, but only when it occurred between one minute before and 45 minutes after ejaculation. Orgasm occurring more than one minute before ejaculation had no effect on sperm retention according to this study. If Baker and Bellis are correct, performing oral sex would not be effective in increasing sperm retention unless the timing was very specific.[ii] In any case, Pham and Shackelford did not assess whether female orgasm occurred.
I would argue that although the Pham and Shackelford study is an interesting one, the results are an inconclusive test of their hypotheses because there are alternative explanations for their findings. Some of these alternative explanations, such as those involving female satisfaction, and my own hypothesis that female attractiveness generally increases male interest in sexual activity seem like more obvious explanations. This does not necessarily mean that the authors are incorrect, only that more research is needed to test these different explanations. For example, studies might assess whether men with more attractive partners are also more interested in other activities associated with sexual foreplay, such as kissing and so on. It would then be possible to test whether interest in oral sex is independent of interest in these other activities. It is also possible that oral sex might serve a combination of functions and that all of these hypotheses have a grain of truth.
Finally, it might be a good idea to consider the woman’s perspective. Pham and Shackelford seem to portray women as passive recipients of male interest and do not appear to consider female agency. For example, they talk about attractive women as targets for mate poaching and consider “recurrent risk of sperm competition” purely in terms of the woman’s attractiveness to other men. While it may be true that men are more likely to target attractive women for affairs, it is also the case that the woman actually has a say in the matter. Some women are more likely to be unfaithful than others and this may be related to her character and choices as much as her looks. Furthermore, if the function of cunnilingus was to detect whether a woman had been sexually active with another male, it would seem reasonable that if she had in fact been unfaithful she might try to avoid receiving cunnilingus to avoid detection. Pham and Shackelford’s study does not consider the woman’s desires and her actual willingness to be unfaithful. Future studies might consider whether men are more likely to perform oral sex on a woman who may present a “recurrent risk of sperm competition” due to her own desires and her actual willingness to be unfaithful.
[i] Stephen Jay Gould seems to have originated this particular canard about evolutionary psychologists being “pan-adaptationists” who are too blind to see that many features of the human psyche have no evolutionary function in themselves. See this article by Tooby and Cosmides, leading figures in the field, which shows how Gould completely misrepresented their work, in which they explicitly stated that most human behaviours are probably by-products without an evolutionary function.
[ii] It is also worth noting that the Baker and Bellis study has been strongly disputed by Elisabeth Lloyd on the basis that the sample size was too small to draw any reliable conclusions. The findings by Baker and Bellis do not appear to have been replicated so their claims might be taken with a grain of salt.
Pham and colleagues performed a companion study considering women's interest in fellatio, with rather different results. I have written a critique of this study here.
© Scott McGreal. Please do not reproduce without permission. Brief excerpts may be quoted as long as a link to the original article is provided.
This article also appears on Psychology Today on my blog Unique - Like Everybody Else.
Backstrom, L., Armstrong, E. A., & Puentes, J. (2011). Women's Negotiation of Cunnilingus in College Hookups and Relationships. Journal of Sex Research, 49(1), 1-12. doi: 10.1080/00224499.2011.585523
Baker, R. R., & Bellis, M. A. (1993). Human sperm competition: ejaculate manipulation by females and a function for the female orgasm. Animal Behaviour, 46(5), 887-909. doi: http://dx.doi.org/10.1006/anbe.1993.1272
Diamond, J. (1998). Why Is Sex Fun? : Basic Books.
Pham, M., & Shackelford, T. (2013). Oral sex as infidelity-detection Personality and Individual Differences, 54 (6), 792-795 DOI: 10.1016/j.paid.2012.11.034