How and Why Patterns of Sexual Dimorphism in Human Faces Vary Across the World 2 3 4 Karel Kleisner1*, Petr Tureček1, S
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1 How and why patterns of sexual dimorphism in human faces vary across the world 2 3 4 Karel Kleisner1*, Petr Tureček1, S. Craig Roberts2, Jan Havlíček3, Jaroslava Varella Valentova4, Robert 5 Mbe Akoko5, Juan David Leongómez6, Silviu Apostol7, Marco A. C. Varella4, S. Adil Saribay8 6 7 8 1 Department of Philosophy and History of Science, Faculty of Science, Charles University, Prague, 9 Czech Republic 10 2 Division of Psychology, University of Stirling, Stirling FK9 4LA, UK 11 3 Department of Zoology, Faculty of Science, Charles University, Prague, Czech Republic 12 4 Department of Experimental Psychology, Institute of Psychology, University of Sao Paulo, Sao Paulo, 13 Brazil 14 5 Department of Sociology and Anthropology, Faculty of Social and Management Science, University 15 of Buea, Buea, Cameroon 16 6 Human Behaviour Lab, Faculty of Psychology, El Bosque University, Bogotá, Colombia 17 7 University of Bucharest, Department of Anatomy, Animal Physiology and Biophysics 18 8 Department of Psychology, Boğaziçi University, Istanbul, Turkey. 19 20 *corresponding author: [email protected] 21 22 Keywords 23 sex-typicality, attractiveness, sexual selection, human evolution, facial morphology 24 25 Abstract 26 27 Sexual selection, including mate choice and intrasexual competition, is responsible for the evolution 28 of some of the most elaborated and sexually dimorphic traits in animals. Although there is sexual 29 dimorphism in the shape of human faces, it is not clear whether this is similarly due to mate choice, 30 or whether mate choice affects only part of the facial shape difference between men and women. 31 Here we explore these questions by investigating patterns of both facial shape and facial preference 32 across a diverse set of human populations. We find evidence that human populations vary 33 substantially and unexpectedly in both the range and pattern of facial sexually dimorphic traits. In 34 particular, European and South American populations display larger levels of facial sexual 35 dimorphism than African populations. Neither cross-cultural differences in facial shape variation, sex 36 differences in body height, nor differing preferences for facial sex-typicality across countries, explain 37 the observed patterns of facial dimorphism. Altogether, the association between morphological sex- 38 typicality and attractiveness is moderate for women and weak (or absent) for men. Analysis that 39 distinguishes between allometric and non-allometric components reveals that non-allometric sex- 40 typicality is preferred in women’s faces but not in faces of men. This might be due to different 41 regimes of ongoing sexual selection acting on men, such as stronger intersexual selection for body 42 height and more intense intrasexual physical competition, compared with women. 43 44 45 Introduction 46 47 Sexual dimorphism is among the most striking of phenomena across various species, our own species 48 being no exception. Human sexual dimorphism is associated with many biological and psychological 49 characteristics including sexual maturity, reproductive potential, mating success, general health, 50 immune responses, sociosexuality, perceived age, and personality attributions 1–6. Since Darwin, 51 numerous proposals have been suggested to explain sexual dimorphism, the most popular of which 52 is sexual selection. 53 The most extensively studied dimorphic trait in humans is facial dimorphism. While some 54 sex-typical facial traits are mediated by prenatal exposure to sex hormones 7, dimorphism reaches 55 full expression after puberty, due to the influence of androgens and estrogens 8,9. High levels of 56 testosterone are primarily responsible for development of masculine facial morphologies, while 57 development of female secondary sexual traits is attributed to a high estrogen-to-testosterone ratio. 58 10–15 59 Evidence from several vertebrate taxa indicates that testosterone may increase disease 60 susceptibility due to negative effects on immune responsiveness 16. Testosterone-dependent traits, 61 such as male secondary sexual characters, are thus posited to signal health and physical fitness 62 because only high-quality individuals can develop them 17–19. We might therefore expect systematic 63 preferences for males possessing testosterone dependent traits (but see 1,20–22. In humans, there is 64 some evidence that taller and physically stronger men with more masculine faces and voices are 65 relatively healthy 23,24. 66 For women, preference for higher levels of testosterone and concomitant masculinity might 67 bring not only benefits of increased social dominance and immunological competence in a partner, 68 but also potential costs in a resulting committed relationship, including risk of aggression and low 69 paternal investment 25–27. Studies in Western cultures have reported varying effects, with some 70 showing that women prefer facial masculinity in men 28–30 and others the opposite, that is, a 71 preference for relative facial femininity in male faces 31–33. Male facial masculinity appears to be a 72 trait preferred more strongly in some contexts or by some individuals, and reasons for such 73 differences are not entirely clear. One explanation is that masculinity preferences might be 74 influenced by temporal context, such that women seeking short-term partners prefer relatively 75 masculine traits compared with those seeking long-term partners 34. However, other evidence 76 suggests the opposite, that masculinity preferences are stronger when seeking longer-term, co- 77 parenting partners compared with short-term, sexual partners 35. Self-assessed individual 78 differences, such as one’s own condition, may also affect preferences 36. On the other hand, the 79 significance of context-dependent factors was recently challenged by a twin study showing that 80 genetic differences explain vastly more variation in women’s preferences 37. 81 In contrast, men’s preferences for female facial sex-typicality show a much more systematic 82 pattern. Although there is some research showing environment-related variation in facial femininity 83 preferences 38, more feminine female faces are perceived as more attractive unanimously across 84 cultures 12,39–41. This may be because it could reveal reproductive potential (i.e. fecundity), as women 85 with more feminine faces may have higher levels of estrogen 42. 86 There are two contrasting views on cross-cultural variation in preferences of human sexual 87 dimorphism. The first is that masculine traits signal characteristics that are adaptive and thus should 88 be preferred in harsh environments (e.g., with resource scarcity, high disease prevalence, and 89 pathogen load)27. For instance, Jamaican women exhibited a greater preference for masculine over 90 feminine male faces compared to British women 43. Furthermore, it has been indirectly suggested 91 that masculine traits in women may be adaptive, for its possible advantages in heavy labor and social 92 competition, in communities living in harsh environmental conditions 44–46, including rural and 93 traditional (pastoral and semi-nomadic, hunter-gatherer) societies. On the other hand, Scott et al. 20 94 suggested that preference for sexually dimorphic traits is an evolutionarily novel feature which 95 emerged in urban Western societies. Thus, according to this second view, sex-typical traits should be 96 preferred only in developed societies with resource richness, and there should be a null association 97 between sexual dimorphism and attractiveness in traditional societies exposed to a relatively harsh 98 environment. 99 In this study, we address these questions by generating a large dataset (N = 1,317) of facial 100 portraits to explore how facial dimorphism varies across eight human populations (Brazil, Cameroon, 101 Colombia, Czech Republic, Namibia, Romania, Turkey, and United Kingdom). Although there is 102 anatomical and anthropological evidence that dimorphism in the craniofacial complex varies across 103 various human populations 21,47–50, our study more directly addresses patterns of face shape and its 104 perception, investigating whether the observed patterns of sexual dimorphism are explained by 105 overall face shape variation, local differences in stature between sexes, variation due to size 106 (allometry), and mate preferences for faces. Specifically, we address the following theoretical 107 predictions: 108 (i) That null preferences for masculine male faces in small-scale traditional societies is 109 explained by encountering fewer different faces and by having fewer social interactions with 110 potential mates over a lifespan 20. The conjoint phenomenon might be the existence of lower 111 variability of facial morphologies in less populated rural societies compared to higher facial variation 112 in large-scale urban societies. Higher morphological variation may be a necessary precursor to a 113 higher degree of sexual dimorphism. We therefore expect (a) that populations with lower levels of 114 facial shape variation will be less sexually dimorphic and (b) that rural (non-urban) societies will show 115 a lower degree of facial sexual dimorphism compared with large-scale, developed urban societies. 116 (ii) Facial morphology is affected by overall body size 51–54, which is also sexually dimorphic 117 and thus influences the perception of various social traits. Taller and heavier men are not only 118 perceived as more masculine 55 but also possess a more masculinized facial structure (24).