Penis size figures prominently in discussions of the evolutionary biology of human reproduction and mate choice. Yet serious research lags well behind a pop culture driven by myths that few bother to question. A reappraisal yields some unexpected insights.

Mythologizing human penis size

Perhaps Desmond Morris did not start everything, but his 1967 book The Naked Ape definitely fanned the flames. Introducing the male of the species, he wrote: “He is proud that he has the biggest brain of all the primates, but attempts to conceal the fact that he also has the biggest penis, preferring to accord this honour falsely to the mighty gorilla.” Other biologists have endlessly repeated that claim over the past 50 years. Witness Steve Jones in The Language of Genes (1993): “There is also the question — as yet unanswered by science — as to why, in penis size, man stands alone.” The myth flourished with ever-growing hyperbole. The Amazon blurb for Jared Diamond’s 1997 book Why is Sex Fun? rhetorically asks: “Why is the human penis so unnecessarily large?” Unnecessarily? Is that why we see hordes of men seeking penis reduction?

For hard facts, let us turn to reproductive biologist Alan Dixson’s comprehensive studies of the primate penis. In 2009 and 2012, he published information for 48 primate genera, listing the following erect penis lengths in inches: 3.4 in orangutan, 2.6 in gorilla, 5.8 in chimpanzee, 6.8 in bonobo and 6.6 in men. A male bonobo accordingly has a longer penis than a man. Dixson cited human penis size from the impressive sample of 2310 summarized by Gebhard & Johnson (1979). But 12 smaller samples listed by Promodu and colleagues in 2007 indicate a cross-cultural average of only 5.7 inches. So broader comparison suggests that a male chimpanzee also has a longer penis than a man. Nevertheless, the human penis is the widest compared to other primates. Indeed, average girth, flaccid or erect, is close to average length.

Hard data also conflict with other myths regarding human penis size. Strikingly, no clear differences between human populations are evident in the table of Promodu and colleagues or in the extensive online compilation by EthnicMuse. Contrary to repeated claims, African men do not have a particularly large penis, although there is a trend for Asian men to have somewhat smaller-than-average penis lengths (possibly reflecting smaller average body size). Also contradicting popular belief, a 2002 study by Shah & Christopher found no significant correlation between penis size and shoe size.

The influence of body size

For comparative biologists, the negative finding for shoe size reported by Shah & Christopher is, in fact, unexpected. As a rule, all bodily structures of any species are adapted to fit overall body size, so in principle, the penis should tend to be longer in bigger men. In fact, an early study by Loeb (1899) did report a fairly strong correlation between penis length and body height. And some more recent studies similarly reported significant findings. For example, a 2001 paper by Ponchietti and colleagues examining penis size and body size in 3,300 young men found highly significant correlations between flaccid penis length and body height and weight, although the latter relationship was negative. In one intriguing 2002 study, Spyropoulos and colleagues reported a significant correlation between penis length and index finger length. Yet several studies found no significant relationship between penis size and body dimensions.

As always, some disparities between studies may reflect sample size effects. It is also generally more appropriate to examine penis size relative to a linear dimension, such as body height, as relationships with body weight within species are complicated. However, it seems that disparities also arise because the penis is measured in different ways. It may be either flaccid or erect. To sidestep practical difficulties of measuring the erect penis, many investigators instead measure the length of the flaccid penis when stretched. Stretched and erect penis lengths are quite similar, so the former is often used as a proxy. But there is a fundamental problem because flaccid penis length is weakly correlated at best with the length of the stretched or erect penis. Many investigators have preferred to use stretched penis length as their basic indicator, so this may account for the puzzling lack of correlation with body size reported by some investigators. It is notable, for example, that Ponchietti and colleagues found significant correlations when examining flaccid penis length, whereas Shah & Christopher measured stretched penis length in their study that found no correlation with shoe length. However, that cannot be the whole story as a smaller-scale study by Siminoski & Bain (1993) did report a significant correlation between stretched penis length and shoe size.

Penis size and female choice

Increase in male attractiveness with penis size for computer-generated models slows above the average (pink vertical).

Concern about penis size is widespread among men. Many believe themselves to be under-endowed. But evidence that women prefer a longer or wider penis is equivocal. In addition to problematic measurement of penis size, confounding variables pose difficulties. However, a very recent study by Mautz and colleagues used a sophisticated approach that avoided many previous pitfalls. They created computer-generated male figures with variable penis length, body height and shoulder-to-hip ratio. Then, each of 53 randomly selected figures was projected as an animated 4-second rotating video to 105 Australian women, who rated their attractiveness as potential sexual partners. Larger penis size and greater height accounted for 6.1 percent and 5.1 percent of total variation in male attractiveness, respectively. But shoulder-to-hip ratio accounted for a considerably greater proportion of variance in attractiveness (79.6 percent). Interactions between variables were also identified. For instance, the influence of penis length on attractiveness was greater for taller figures and for those with a higher shoulder-to-hip ratio. It is notable, however, that the relationship between penis length and attractiveness was not linear. The effect on attractiveness began to decrease above a penis length of only 3 inches.

Although the study by Mautz and colleagues is a welcome step forward, many questions remain unanswered. Is the flaccid penis or the erect version subject to female choice? Furthermore, Mautz and colleagues varied only three features in their computer-generated figures, yet potential confounding factors, notably proportionality, abound. Prominent among these are more subtle effects of overall body size. The findings actually suggest that a mismatch between penis size and body size may decrease attractiveness. Note also that that greatest effect of penis length on attractiveness was found with a small penis, not a large one. In fact, the most striking implication of the study is that the shoulder-to-hip ratio—giving a triangular upper body form at higher values—has a far greater effect than either penis length or body height. So the take-home message regarding female choice is that body-building might be a far better investment than penis enlargement.

References

Diamond, J.M. (1997) Why is Sex Fun? The Evolution of Human Sexuality. New York: Basic Books.

Dixson, A.F. (2009) Sexual Selection and the Origins of Human Mating Systems. Oxford: Oxford University Press.

Dixson, A.F. (2012) Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes and Human Beings (Second Edition). Oxford: Oxford University Press.

EthnicMuse Penile Anthropometry by Country:

https://ethnicmuse.files.wordpress.com/2013/07/penile-anthropometry.pdf

Gebhard, P.H. & Johnson, A. B. (1979) The Kinsey Data: Marginal Tabulation of the 1938-1963 Interviews Conducted by the Institute for Sex Research. Philadelphia: W.B. Saunders.

Jones, J.S. (1993) The Language of Genes. London: HarperCollins.

Loeb, H. (1899) Harnröhrencapacität und Tripperspritzen. München. medizin. Wochenschr. 46:1016-1019.

Mautz, B.S., Wong, B.B.M., Peters, R.A. & Jennions, M.D. (2013) Penis size interacts with body shape and height to influence male attractiveness. Proc. Natl. Acad. Sci. U.S.A. 110:6925-6930.

Morris, D. (1967) The Naked Ape: A Zoologist's Study of the Human Animal. London: Jonathan Cape.

Promodu, K., Shanmughadas, K.V., Bhat, S. & Nair, K.R. (2007) Penile length and circumference: an Indian study. Int. J. Impotence Res. 19:558-563.

Shah, J. & Christopher, N. (2002) Can shoe size predict penile length? BJU Int. 90:586-587.

Spyropoulos, E., Borousas, D., Mevrikos, S., Dellis, A., Bourounis, M. & Athanasiadis, S. (2002) Size of external genital organs and somatometric parameters among physically normal men younger than 40 years old. Urology 60:485-489.

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