Busting Myths About Human Nature

Race, monogamy, and other lies they told you.

Size Matters, or Does It?

A focus on genitals does not tell us much about being human

We know sex is complicated in primates and that human sexuality is the most complex sexuality of any animal on the planet. But for some reason we keep asking very simple evolutionary questions about sex and thinking we get good answers…turns out, they are usually wrong.

A recent flurry of news activity focused on an article published in the very prestigious journal, Proceedings of the National Academy of Sciences, entitled “Penis size interacts with body shape and height to influence male attractiveness.”  Their conclusions were that “flaccid penis size had a significant influence on male attractiveness.” And that “Height and shoulder-to-hip ratio also influenced a male’s relative attractiveness with taller men and those with a greater shoulder to-hip ratio being rated as more attractive by women.” The bottom line was that computer generated males with larger genitals and wider shoulders were slightly more attractive to the 105 self-reported heterosexual Australian women who they showed the computer generated images of naked men to (no, I am not making this up, read it for yourself).   

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The media ate it up and social media sites spread it across the globe because it is simple and fits nicely with popular cultural expectations about human nature (and penis size). This study probably tells us more about our cultural realities than our evolutionary histories. The researchers did not control for cultural contexts nor even mention (or consider?) the possibility that these women’s responses are affected by their experiences growing up in the world and not gut-level, “innate” responses revealing their evolved preferences.  Consider throwing in effects of enculturation, sexual histories, life experience, etc…, or even real bodies, into the mix and you can see that the “peer” review on this paper could have been better.      

Does this mean we can’t ask evolutionary questions about our sexuality?  Of course not.  We can and they can produce fascinating results.  But as I, and so many others, have posted, discussed and published we need to be careful, comprehensive and methodical about what we ask and how we ask it.

For example, should we really expect to find something out about human sexuality by looking at body dimensions, including genitals, outside of social context, histories, hormonal states, age groups, cultural views on sex and actual sexual activity?  No.  There is not going to be a single trait or a simple explanation for why human penises look the way they do or why any given women is going to find any given male attractive.  We are not even sure that the two are evolutionarily related. Yes, genitals are core to reproduction, but there are few mammalian species where female preference has influenced male penis morphology. In fact, there is very little known about how to measure relevant features of attraction, especially in humans. By “attraction” do we mean “wants to have sex with,” “wants to reproduce with,” “wants to be friends with,” “wants to be seen with” etc… In humans these are not necessarily tied to one another and thus we need to develop good and detailed questions before we jump in to testing our “evolved” traits.

So let’s re-ask the questions central to “does size matter?”  Does it matter in an evolutionary sense…that is, does size affect the ability to reproduce?  No, as long as you are physiologically functional (sperm count and such) and within the functional size range (which is very broad!) the actual size does not matter for reproduction. Could male genital form (and thus size variation) have been influenced by female choice?  Yes, but it is far from clear that this is the case, or why such variation would be relevant be given the full functionality across a broad size range in modern humans.  Before asking about sexual selection, we need to first figure out how the genitals fit in the overall development of the body. That is, whether or not male genital form is tied to our distinctive human bipedal morphology, fat deposition/tissue allocation, aspects of human vascular patterns, or some structural aspect of sperm delivery related to the female reproductive tract.  We might even be wrong in asking about the penis in the context only of males. Remember, both male and female genitals are from the same tissue masses and do not really begin to differentiate until 6 weeks into fetal development: the penis and clitoris are homologous structures.  There might be elements of developmental patterns and tissue allocations, in utero, that go a long way in explaining why our genitals look the way they do. These questions are not going to be answered by showing fake images of naked men to women and asking if it turns them on.

The better question regarding male genitals is not “does size matter,” but rather “how does this system work and how do our bodies and behaviors interface with one another to affect it?”

I admit this question is not as exciting as the penis size and attraction one, but it will get us much better, and more evolutionary relevant, answers about male genitals, and maybe even a little insight into sex.

Agustín Fuentes, Ph.D, is a professor in the Department of Anthropology at the University of Notre Dame.

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