These studies, however, all use self-reported, direct questioning and are therefore susceptible to biases of self-censorship and pressure to conform to socially desirable responses to sensitive issues (e.g., refs. The results are mixed, with studies finding that females prefer longer penises ( 23), wider penises ( 24, 25), or that penis size is unimportant ( 26).
There are numerous psychological studies directly asking females for their preference regarding male penis size. Many cultures have fashion items, like penis sheaths and codpieces, that draw attention toward male genitalia ( 20), highlighting the potential for female choice to influence the evolution of male genitalia. Furthermore, novels, magazines, and popular articles often allude to the existence of a relationship between penis size and sexual attractiveness or masculinity ( 21, 22). This observation has generated suggestions by evolutionary biologists that the comparatively large human penis evolved under premating sexual selection ( 19, 20). The upright body posture and protruding, nonretractable genitalia of male humans make the penis particularly conspicuous, even when flaccid. Hence, it is likely that how a female rates a male’s attractiveness will partly depend upon her own phenotype. Humans mate assortatively based on numerous traits, including height ( 15), facial symmetry ( 16), and body shape ( 17, 18). Finally, a female’s own phenotype might influence her mate choice decisions. For example, the same sized penis might be perceived differently on short and tall men. Third, there might be a size contrast effect such that female assessment of attractiveness varies if the trait of interest is viewed differently in relation to other traits, analogous to the Ebbinghaus–Titchener effect ( 14). These relationships can influence evolution via correlational selection ( 13). Second, traits within individuals are phenotypically and genetically correlated. 10 and 11), so manipulating traits in isolation can lead to faulty conclusions about net male attractiveness (but see also ref. Overall male attractiveness is unlikely to be determined by individual traits (e.g., refs. First, females use multiple cues during the mate choice process ( 9). How female choice acts on any given male trait, and hence the strength and direction of selection, can be influenced by several, nonmutually exclusive factors. Despite this potential effect, relatively little research has tested whether primary sexual characters influence male attractiveness ( 6 – 8). In species where genitalia are externally visible, sexual selection might also act if females prefer males with specific genital morphology. Precopulatory processes can influence genital morphology ( 4, 5), but it is unknown whether these results are due to direct female choice or sexual conflict.
There might, however, also be premating sexual selection on male genitalia. This variation is typically attributed to copulatory and postcopulatory sexual selection to increase male fertilization success under sperm competition ( 2) or cryptic female choice ( 3). Male genitalia show great variation among closely related species ( 1). More broadly, our results show that precopulatory sexual selection can play a role in the evolution of genital traits. Our results support the hypothesis that female mate choice could have driven the evolution of larger penises in humans.
Surprisingly, larger penis size and greater height had almost equivalent positive effects on male attractiveness. 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). Penis size had a stronger effect on attractiveness in taller men than in shorter men. Positive linear selection was detected for penis size, but the marginal increase in attractiveness eventually declined with greater penis size (i.e., quadratic selection). 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. This observation has generated suggestions that human penis size partly evolved because of female choice. Before clothing, the nonretractable human penis would have been conspicuous to potential mates. There could, however, also be direct precopulatory female mate choice based on male genital traits. 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.
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