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Penis Size Interacts with Body Shape and Height to Influence Male Penis size interacts with body shape and height to influence male attractiveness Brian S. Mautza,1,2, Bob B. M. Wongb, Richard A. Petersc, and Michael D. Jennionsa aResearch School of Biology, Australian National University, Canberra, ACT 0200, Australia; bSchool of Biological Sciences, Monash University, Melbourne, VIC 3800, Australia; and cDepartment of Zoology, La Trobe University, Bundoora, VIC 3086, Australia Edited by Wyatt W. Anderson, University of Georgia, Athens, GA, and approved February 28, 2013 (received for review November 6, 2012) Compelling evidence from many animal taxa indicates that male The upright body posture and protruding, nonretractable gen- genitalia are often under postcopulatory sexual selection for char- italia of male humans make the penis particularly conspicuous, acteristics that increase a male’s relative fertilization success. There even when flaccid. This observation has generated suggestions by could, however, also be direct precopulatory female mate choice evolutionary biologists that the comparatively large human penis based on male genital traits. Before clothing, the nonretractable evolved under premating sexual selection (19, 20). Furthermore, human penis would have been conspicuous to potential mates. This novels, magazines, and popular articles often allude to the exis- observation has generated suggestions that human penis size tence of a relationship between penis size and sexual attractive- partly evolved because of female choice. Here we show, based upon ness or masculinity (21, 22). Many cultures have fashion items, female assessment of digitally projected life-size, computer-generated like penis sheaths and codpieces, that draw attention toward male images, that penis size interacts with body shape and height to genitalia (20), highlighting the potential for female choice to determine male sexual attractiveness. Positive linear selection was influence the evolution of male genitalia. There are numerous detected for penis size, but the marginal increase in attractiveness psychological studies directly asking females for their preference eventually declined with greater penis size (i.e., quadratic selection). regarding male penis size. The results are mixed, with studies Penis size had a stronger effect on attractiveness in taller men than finding that females prefer longer penises (23), wider penises in shorter men. There was a similar increase in the positive effect of (24, 25), or that penis size is unimportant (26). These studies, penis size on attractiveness with a more masculine body shape (i.e., however, all use self-reported, direct questioning and are greater shoulder-to-hip ratio). Surprisingly, larger penis size and therefore susceptible to biases of self-censorship and pressure to greater height had almost equivalent positive effects on male at- conform to socially desirable responses to sensitive issues (e.g., tractiveness. Our results support the hypothesis that female mate refs. 27–29). choice could have driven the evolution of larger penises in humans. The only scientific studies to attempt to test experimentally More broadly, our results show that precopulatory sexual selection whether flaccid penis length affects male attractiveness asked can play a role in the evolution of genital traits. women to rate five images created by modifying a single drawing of a male figure so that the test figures differed only in penis genital evolution | multivariate attractiveness | multiple cues length (30–32). These important studies were not designed to quantify directly the relative effect of penis length on attractiveness ale genitalia show great variation among closely related compared with other sexually selected male traits, such as height Mspecies (1). This variation is typically attributed to copulatory and body shape (30–32). Therefore, it is still unknown whether and postcopulatory sexual selection to increase male fertilization penis size affects attractiveness when there is substantive varia- success under sperm competition (2) or cryptic female choice (3). tion in other, arguably more important, body traits, or whether There might, however, also be premating sexual selection on male interactions between these traits and penis size determine net genitalia. Precopulatory processes can influence genital morphol- attractiveness. For example, does a given increase in penis length ogy (4, 5), but it is unknown whether these results are due to have an equivalent effect on the attractiveness of a short and tall direct female choice or sexual conflict. In species where genitalia man? In addition, the use of small photographs to quantify size- are externally visible, sexual selection might also act if females based preferences might lead to different estimates than those prefer males with specific genital morphology. Despite this po- obtained when viewing fully life-sized male bodies. tential effect, relatively little research has tested whether primary To address these issues, we presented a sample of heterosexual fl – Australian women with projected life-size, computer-generated sexual characters in uence male attractiveness (6 8). fi fi How female choice acts on any given male trait, and hence the male gures (Fig. 1). Each gure was an animated 4-s video in fl which the figure rotated 30° to each side to allow participants to strength and direction of selection, can be in uenced by several, fi fl EVOLUTION nonmutually exclusive factors. First, females use multiple cues more easily evaluate the gure.Wetestedfortheeffectsof accid penis size, body shape (shoulder-to-hip ratio), and height on male during the mate choice process (9). Overall male attractiveness is sexual attractiveness. The latter two traits have regularly been unlikely to be determined by individual traits (e.g., refs. 10 and 11), investigatedandareknowntoinfluence male attractiveness or so manipulating traits in isolation can lead to faulty conclusions reproductive success [height (15, 33–35), shape (18, 36, 37)]. Each about net male attractiveness (but see also ref. 12). Second, traits trait had seven possible values that were within the natural range within individuals are phenotypically and genetically correlated. These relationships can influence evolution via correlational se- lection (13). Third, there might be a size contrast effect such that Author contributions: B.S.M., B.B.M.W., R.A.P., and M.D.J. designed research; B.S.M., female assessment of attractiveness varies if the trait of interest B.B.M.W., R.A.P., and M.D.J. performed research; B.S.M. and M.D.J. analyzed data; and is viewed differently in relation to other traits, analogous to the B.S.M., B.B.M.W., R.A.P., and M.D.J. wrote the paper. Ebbinghaus–Titchener effect (14). For example, the same sized The authors declare no conflict of interest. penis might be perceived differently on short and tall men. Finally, This article is a PNAS Direct Submission. ’ fl afemales own phenotype might in uence her mate choice deci- 1Present address: Department of Biology, University of Ottawa, Ottawa, ON, Canada sions. Humans mate assortatively based on numerous traits, in- K1N 6N5. cluding height (15), facial symmetry (16), and body shape (17, 18). 2To whom correspondence should be addressed. E-mail: [email protected]. ’ Hence, it is likely that how a female rates a male s attractiveness This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. will partly depend upon her own phenotype. 1073/pnas.1219361110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1219361110 PNAS | April 23, 2013 | vol. 110 | no. 17 | 6925–6930 Downloaded by guest on September 30, 2021 size (Table 1). There were diminishing returns to increased height, penis size, and shoulder-to-hip ratio (quadratic selection: P = 0.010, 0.006 and < 0.0001) [“B” in Table 1] and, given the good fit of the linear and quadratic models, the optimum values appear to lie outside the tested range (i.e., maxima are >2SD from the population mean for each trait) (Fig. 2). A model using only linear and quadratic selection on the shoulder-to-hip ratio accounted for 79.6% of variation in relative attractiveness scores (centered to remove differences among women in their average attractiveness scores). The explanatory power of height and penis size when added separately to this model was almost identical. Both traits significantly improved the fit of the model (log-likelihood ratio tests: height: χ2 = 106.5, df = 3, P < 0.0001; penis: χ2 = 83.7, df = 3, P < 0.0001). Each trait, respectively, explained an extra 6.1% and 5.1% of the total variation in relative attractiveness. The effects of the three traits on relative attractiveness were not independent because of correlational selection (all P < 0.013) [“B” in Table 1]. Controlling for height, there was a small but significant difference in the rate of increase in relative attractiveness with penis size for a given shoulder-to-hip ratio (Fig. 3A). More com- pellingly, after controlling for shoulder-to-hip ratio, greater penis size elevated relative attractiveness far more strongly for taller men (Fig. 3B). Fig. 1. Figures representing the most extreme height, shoulder-to-hip ratio, and penis size (±2SD)(Right and Left) in comparison with the average Participant and Response Time Analysis. The average age of female (Center figure) trait values. participants was 26.2 ± 6.8 SD y old. The participants were 71.8% European, 20.9% Asian, and 7.3% from elsewhere with respect to ethnic origins. Female height was positively correlated with the (±2 SD) based
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