Facial cues to physical strength are not always associated with facial masculinity: Comparative study of Europeans and Southern Siberians.

OBJECTIVES: The aim of our study was to compare morphometric patterns of facial sexual dimorphism with strength-face relationship in members of two distinct populations of European and Central Asian origin: Russians and Tuvans. METHODS: Handgrip strength (HGS) measures and facial photographs were collected from Russian (n = 233) and Tuvan (n = 187) men and women. We digitized 70 landmarks and semilandmarks on full-face and 54 landmarks and semilandmarks on profile photos. This was done to capture variation in facial morphology. After that, we performed the shape regressions of landmarks' coordinates upon sex and HGS. Results were visualized in forms of thin-plate deformation grids and geometric morphometric morphs. RESULTS: In both populations, HGS was associated significantly with male facial shape only. In Russian men, strength-related changes of facial shape were almost completely in direction of increase in male-typicality. This was especially evident for the relative lower facial width, which was higher in men compared to women, as well as in stronger men compared to weaker ones. On the contrary, in Tuvans the lower face was relatively narrower in men than in women. However, the facial shape of strong Tuvan men was also associated with relatively wider lower face. Our results indicate that the effect of strength on facial shape is relatively independent of facial sexual dimorphism. CONCLUSIONS: Our findings clearly demonstrate that physical strength is associated with the shape of the lower part of male faces even in populations with a mismatched direction of lower face sexual dimorphism.

Unravelling the many facets of human cooperation in an experimental study.

Humans readily cooperate, even with strangers and without prospects of reciprocation. Despite thousands of studies, this finding is not well understood. Most studies focussed on a single aspect of cooperation and were conducted under anonymous conditions. However, cooperation is a multi-faceted phenomenon, involving generosity, readiness to share, fairness, trust, trustworthiness, and willingness to take cooperative risks. Here, we report findings of an experiment where subjects had to make decisions in ten situations representing different aspects of cooperation, both under anonymous and 'personalised' conditions. In an anonymous setting, we found considerable individual variation in each decision situation, while individuals were consistent both within and across situations. Prosocial tendencies such as generosity, trust, and trustworthiness were positively correlated, constituting a 'cooperativeness syndrome', but the tendency to punish non-cooperative individuals is not part of this syndrome. In a personalised setting, information on the appearance of the interaction partner systematically affected cooperation-related behaviour. Subjects were more cooperative toward interaction partners whose facial photographs were judged 'generous', 'trustworthy', 'not greedy', 'happy', 'attractive', and 'not angry' by a separate panel. However, individuals eliciting more cooperation were not more cooperative themselves in our experiment. Our study shows that a multi-faceted approach can reveal general behavioural tendencies underlying cooperation, but it also uncovers new puzzling features of human cooperation.

Perception of Emergent Leaders' Faces and Evolution of Social Cheating: Cross-Cultural Experiments.

The aim of the present study was to investigate whether neutral faces of individuals with different propensities for leadership may convey information about their personal qualities, and are there impacts of sex, population and social environment on the facial perception. This study is based on a previous experiment ( Rostovtseva et al., 2022), where emergent leadership in the context of male group cooperation was investigated in Buryats (Mongolian population of Siberia). In the previous study three behavioural types of participants were revealed: non-leaders, prosocial leaders and leaders-cheaters, each having a set of distinguishing personality, communicative, and cooperative features. In the current study, three composite portraits representing different leadership qualities of Buryat men from the prior experiment were created. The composites were then scored on a number of traits by male and female Russian and Buryat independent raters (N = 435). The results revealed that ratings on masculinity, physical strength, dominance, competitiveness, and perceived leadership were positively correlated, while perceived trustworthiness was negatively associated with these traits. However, the composite portraits of actual leaders generally were scored as more trustworthy, masculine, and physically strong, with the prosocial leaders' portrait being perceived as healthier than others. Surprisingly, the composite of leaders-cheaters was scored as the most trustworthy and generous, and the least competitive than others. No significant effects of raters' sex, origin, or degree of familiarity with Mongolian appearance were revealed. We conclude that static facial morphology contributes to appearing trustworthy, which may allow exploitation of others.

Facial and body sexual dimorphism are not interconnected in the Maasai.

BACKGROUND: In this paper, we investigate facial sexual dimorphism and its' association with body dimorphism in Maasai, the traditional seminomadic population of Tanzania. We discuss findings on other human populations and possible factors affecting the developmental processes in Maasai. METHODS: Full-face anthropological photographs were obtained from 305 Maasai (185 men, 120 women) aged 17-90 years. Facial shape was assessed combining geometric morphometrics and classical facial indices. Body parameters were measured directly using precise anthropological instruments. RESULTS: Sexual dimorphism in Maasai faces was low, sex explained 1.8% of the total shape variance. However, male faces were relatively narrower and vertically prolonged, with slightly wider noses, narrower-set and lower eyebrows, wider mouths, and higher forehead hairline. The most sexually dimorphic regions of the face were the lower jaw and the nose. Facial width-to-height ratio (fWHR), measured in six known variants, revealed no significant sexual dimorphism. The allometric effects on facial traits were mostly related to the face growth, rather than the growth of the whole body (body height). Significant body dimorphism was demonstrated, men being significantly higher, with larger wrist diameter and hand grip strength, and women having higher BMI, hips circumferences, upper arm circumferences, triceps skinfolds. Facial and body sexual dimorphisms were not associated. CONCLUSIONS: Facial sex differences in Maasai are very low, while on the contrary, the body sexual dimorphism is high. There were practically no associations between facial and body measures. These findings are interpreted in the light of trade-offs between environmental, cultural, and sexual selection pressures.

Effects of sex and sex-related facial traits on trust and trustworthiness: An experimental study.

The ability to trust others, including strangers, is a prerequisite for human cooperation. Economically it is not rational to trust strangers, as trust can be easily exploited. Still, generally, the level of trust toward strangers is relatively high. Trust is closely related to trustworthiness: when trusting others, one expects them to reciprocate. Some individuals elicit more trust than others. Apparently, humans use subtle cues for judging the trustworthiness of their interaction partners. Here, we report on an experiment that investigates trust and trustworthiness in a population of 176 mainly Dutch students. The aims of our study were: (1) to investigate how the sex of interaction partners and their facial appearance (femininity/masculinity) affect the degree of trust and trustworthiness, compared to fully anonymous conditions; (2) to test whether individuals who elicit trust in their interaction partners are trustworthy themselves. Each subject of our experiment played five one-shot Trust Games: one with an anonymous interaction partner, and four "personalized" games after seeing a 20 s silent video of their interaction partner (twice same-sex, and twice opposite-sex). The degree of facial sexual dimorphism was investigated with geometric morphometrics based on full-face photographs. Our results revealed that, despite the already high level of trust in the anonymous setting, the personalization of interactions had a clear effect on behavior. Females elicited more trust in partners of both sexes. Interestingly, females with more feminine faces elicited less trust in both male and female partners, while males with more masculine facial shape were more trusted by females, but less trusted by males. Neither sex nor facial femininity/masculinity predicted trustworthiness. Our results demonstrate that (1) sex and sex-related facial traits of interaction partners have a clear effect on eliciting trust in strangers. However, (2) these cues are not reliable predictors of actual trustworthiness.

Sex, population origin, age and average digit length as predictors of digit ratio in three large world populations.

Recently, a number of authors have claimed that sexual dimorphism in the second-to-fourth digit ratio (2D:4D) is simply dependent on digit length and is an artifact of allometry. The goal of our study is to verify the validity of these assumptions. The study sample comprised 7,582 individuals (3,802 men and 3,780 women) from three large world populations: Europeans (n = 3043), East Africans (n = 2844), and Central Asians (n = 1695). The lengths of the second and fourth digits on both hands were measured. Digit ratios were computed according to standard procedures. Analyses were conducted separately for each hand for the whole sample and in succession for the three large populations. Additionally, we separately tested four age cohorts (≤ 13, 14-18, 19-30, and 31 ≥ years) to test the effect of developmental allometry. The second and fourth digits showed strong positive linear relationships on both hands, and demonstrated an increase with age; digit length in women from the youngest age cohort was longer or equal to that of men, and shorter than men in older age cohorts. However, the 2D:4D magnitude and its sexual dimorphism remained stable throughout the ontogeny. To test for an allometric effect on 2D:4D, the average digit lengths were calculated. Both sex and population origin were permanent reliable predictors of 2D:4D, whereas average digit length was not. Height was applied as another measure of allometric effect on the limited sample (≤ 30 years) from the European population, along with sex and age. No allometric effect was observed in this case. We conclude that sex differences in 2D:4D are not an artifact of allometry.

Sexual dimorphism in facial shape of modern Buryats of Southern Siberia.

OBJECTIVES: The aim of the present study was to investigate sexual dimorphism in the full facial shape of modern Buryats-people of Southern Siberia of Mongolian origin. METHODS: For this purpose, we have used geometric morphometrics based on standardized full-face frontal photographs. This allowed us to assess and visualize differences in facial shapes between Buryat men (n = 98) and women (n = 89). To specify the facial areas, where the differences occurred, we have complemented our analysis with standard anthropometric facial parameters based on approximations to the craniofacial and mandibular landmarks and soft-tissue morphology of specific facial areas. RESULTS: Our results revealed that Buryat women have a set of sexually dimorphic features similar to those reported earlier for other Asian populations (a relatively wider and vertically shorter lower face, more round visible areas of the eyes, relatively narrower noses, smaller mouths, larger [in vertical dimension] foreheads, and relatively thinner upper lips, when compared to Buryat males). At the same time, Buryat women had a specific characteristic, distinguishing them from other world populations-a significantly higher upper face width-to-height ratio (fWHR) compared to males. This indicates that the high fWHR is not a universally male feature in humans, which raises a question of underlying developmental mechanisms. CONCLUSIONS: Our results clearly demonstrate that some elements of sexually dimorphic facial shapes may differ across populations with different genetic and ecological backgrounds, and suggest that universal mechanisms of sex-specific facial morphogenesis still need to be clarified in the future.

Sex differences in cooperativeness-An experiment with Buryats in Southern Siberia.

We report on an experimental study that was set up to reveal differences in the tendencies of men and women to cooperate in same-sex interactions. Former studies on this subject were mostly conducted in industrialized modern societies. In contrast, we tested the cooperation tendency among Buryats, a people from Southern Siberia of Mongolian origin. All subjects participated in (1) one iterated Public Goods Game in a group of four individuals of the same sex and (2) four one-shot Prisoner's Dilemma games with different partners of the same sex. The interactions were in a face-to-face setting, but any intentional communication during the experiments was prohibited. We found that Buryat men were more cooperative than Buryat women in both types of same-sex interactions. In particular, the fraction of men employing a strategy of unconditional cooperation in the iterated Public Goods Game was much higher (36%) than the fraction of unconditional cooperators among women (21%). In general, the behavior of men was less context dependent than the behavior of women. In both sexes, individuals who were more cooperative in one type of game tended to be more cooperative in the other type of game. Although direct communication was prohibited, the interaction partners in the Prisoner's Dilemma games employed the same strategy much more frequently than expected by chance. We conclude that, even among strangers, the exchange of subtle signals is sufficient to coordinate strategic decisions.

Oxytocin receptor gene polymorphism (rs53576) and digit ratio associates with aggression: comparison in seven ethnic groups.

BACKGROUND: The specific role of the oxytocin receptor (OXTR) gene polymorphisms in emotional support seeking, related to social norms and culturally normative behavior, has been discussed in several studies. Evidence on the association between aggression and OXTR polymorphisms has also been reported. The goal of the current study was to analyze the effect of the OXTR rs53576 polymorphism, prenatal testosterone effect (second-to-fourth digit ratio, or 2D:4D), and culture on aggression assessed with the Buss-Perry Aggression Questionnaire (BPAQ). METHODS: The data were collected in Russia and Tanzania and included seven ethnic groups of European, Asian, and African origin. The total sample included 1705 adults (837 males, 868 females). All the subjects were evaluated with the BPAQ. As a measure of prenatal androgenization, the second and fourth digits were measured directly from hand, and the digit ratios were calculated. All the participants provided buccal samples, from which genomic DNA was extracted, and the OXTR gene rs53576 polymorphism was genotyped. Statistical analysis was performed using SPSS version 23.0; the alpha level for all analyses was set at 0.05. RESULTS: The ethnic group factor was the most significant predictor of ratings on BPAQ (medium effect size for physical aggression, anger and hostility scales, and low for verbal aggression). To study the effect of sex, the OXTR polymorphism, and prenatal androgenization, we conducted the z-score transformation for BPAQ scales and 2D:4D for each ethnic group and pooled these data into new z-score variables. According to the GLM analysis after leveling the effects of culture (z-transformation), all four scales of BPAQ demonstrated association with sex (main effects), with men scoring higher on physical and verbal aggression and women scoring higher on anger and hostility. Anger and hostility scales were also associated with OXTR polymorphism and 2D:4D of the right hand. The lowest levels of anger and hostility were observed in individuals with the AA genotype, especially in men. CONCLUSIONS: Our data suggest that both oxytocin (OXTR gene polymorphism) and fetal testosterone (2D:4D) may significantly affect emotional (anger) and cognitive (hostility) aggression in humans, given the leveling the role of culture.

Second-to-fourth digit ratio and facial shape in Buryats of Southern Siberia.

BACKGROUND: The 2nd-to-4th digit ratio (2D:4D) is a putative predictor of a prenatal exposure to sex hormones. 2D:4D is sexually dimorphic (males < females). Studies, linking digit ratio and full facial shapes among Europeans, show that a low 2D:4D is associated with a set of male-specific facial features. Buryats - Mongolian people from Southern Siberia - demonstrate a different pattern of facial sexual dimorphism than Europeans (narrower and more vertically elongated faces in men as opposed to women). AIM: The aim of the present study was to investigate the association between facial shape and the 2D:4D in comparison to the pattern of facial sexual dimorphism in Buryats. SUBJECTS: Buryats: 88 men and 80 women aged 20 ± 2 years. OUTCOME MEASURES: To assess relationship between facial shape and 2D:4D we used a geometric morphometric approach based on standardized full-face frontal photographs and direct measurements of the digit lengths among right-handed individuals. RESULTS: The results revealed that 2D:4D was associated with facial morphology in Buryat men, and to a lesser extent in women. Narrower faces, elongated in the vertical direction, and a narrower lower facial outline, were characteristic of Buryat men with low 2D:4D ratios, which corresponded to the male-like facial shapes in Buryats. CONCLUSIONS: In Europeans, such facial features were reported for men with a high 2D:4D, which corresponded more to female-like European facial shapes. Hence, our results show that sex-specific morphogenesis in humans is multidirectional, and that digit ratio is capable of predicting sex-specific facial traits even in populations with differing sexually-dimorphic morphology.