Penis Size: Does it matter and why?

A study just published in the Proceedings of the National Academy of Sciences explores the question of penis size and female preference in humans. The study involved making a set of 3D models of human males of various relative body sizes, and fitting them out with various size flaccid penises. These were shown to a sample of Australian women to get their reactions.

The assumption of this study is that at some time in the past humans did not wear clothing, so that information about penis size in men would be available to women who could observe flaccid penises and then choose sexual partners. That assumption is limited, perhaps flawed, in at least three ways:

1) We have no idea when men started to cover their penises on a regular basis. Ethnographically, there are very few cultures where men walk around with exposed penises, though there are several cultures in which men attempt to highlight and perhaps exaggerate the sizes of their mating equipment using various techniques. Since foraging people around the world, who stand in as models for the human "paleolithic," often cover both male and female groin areas, it stands to reason that the practicer of covering up is old, even if it has not always been practiced. Archaeological evidence of early Homo (African and Asian Homo erectus/ergaster) strongly suggest that our ancestors, well before they became Homo sapiens, lived in a fairly wide range of habitats suggesting but not proving that clothing was developed as far back as just under 2 million years ago. If there was a period of universal exposure of the entire body, it may well have been much earlier than the evolution of anything looking like modern human culture and mating systems.

2) It is highly unlikely that human or pre/proto-human females would determine mating preference on the sole or primary basis of the details of the experience of copulation, assuming that some degree of paternal investment in offspring or the female herself was important. A better model of human mating suggests that females would look for a wide range of features, mostly behavioral, in long term male partners, and these longer term relationships would have more of an effect on selection (for a particular size penis) than a single variable.

3) There is not strong reason to believe that if females were interested in penis size as a factor in copulation that they would use flaccid penises to make assessments. The correlation between erect penis size and flaccid penis size is poor. In addition to this, in a social group in which no one wears clothing, other sources of information about erect penises would certainly be available. Penises would be erect at random times now and then, and in a social system where females make short term decisions about copulation, there would certainly be long term availability of information via the usual linguistic channels, after the evolution of language or proto-language, which would presumably be early(ish) in human evolution.

However, given these caveats, it may be reasonable to carry out the experiment reported in this paper because, well, why not?

The researchers note that human flaccid (visible) penis size is notably larger than that of our relatives, the great apes. This suggests that visual evaluation of penises was a selective force in human evolution. From the abstract of the paper:

Compelling evidence from many animal taxa indicates that male genitalia are often under postcopulatory sexual selection for characteristics that increase a male’s relative fertilization success. There could, however, also be direct precopulatory female mate choice based on male genital traits. Before clothing, the nonretractable human penis would have been conspicuous to potential mates. This observation has generated suggestions that human penis size partly evolved because of female choice. Here we show, based upon female assessment of digitally projected life-size, computer-generated images, that penis size interacts with body shape and height to determine male sexual attractiveness. Positive linear selection was detected for penis size, but the marginal increase in attractiveness eventually declined with greater penis size (i.e., quadratic selection). Penis size had a stronger effect on attractiveness in taller men than in shorter men. There was a similar increase in the positive effect of penis size on attractiveness with a more masculine body shape (i.e., greater shoulder-to-hip ratio). Surprisingly, larger penis size and greater height had almost equivalent positive effects on male attractiveness. Our results support the hypothesis that female mate choice could have driven the evolution of larger penises in humans. More broadly, our results show that precopulatory sexual selection can play a role in the evolution of genital traits.

What have we learned from this study? Perhaps, mainly, something about the reaction a certain subset of Australian women have to male penis size. However, we can also guess that human sexuality, including details such as this, are a product of our rather complex and difficult to parse culture. I am uncomfortable linking these results to either the behavior of paleolithic humans or to a model of sexual selection, given that human sexuality today is so diverse and clearly constructed from exposure to enculturation and lived experience. Is this scientific evidence that when people say things like "size does not matter" or "it's how you use it that counts," they are kidding? Perhaps. In Australia. But probably not.

To me, a more interesting study would look at biological and cultural variations in the relationship between flaccid penis size and erect penis size, and how information about these things would be made available in different normative cultural settings. For instance, I would predict that if penis size matters in relation to either female mate choice or male-male competition, this relationship would be strong (and flaccid penises generally larger) in societies where men don't cover up, but uncorrelated (with little selection on flaccid penis size) in societies where men do cover up.

See also this at The Stochastic Scientist.

Mautz, B., Wong, B., Peters, R., & Jennions, M. (2013). Penis size interacts with body shape and height to influence male attractiveness Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1219361110