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.
Footnotes
[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.
Further reading
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.
Other posts
about sex and psychology
References
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
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