Vojtěch Fiala

It has been suggested that sexually dimorphic facial traits are related to competitive behavior and aggression, but empirical evidence remains inconsistent and is based on a small number of populations and culturally homogeneous samples. We have combined geometric morphometric analyses with hierarchical Bayesian modeling of sixteen culturally and geographically diverse but understudied ethnic groups in order to test whether self-reported aggression covaries with facial sexual shape dimorphism within and between populations. Across populations, we found no consistent association between the individual level of facial sexual dimorphism and self-reported aggression in either sex. Although populations did differ in mean aggression levels, mean self-reported aggression covaried neither with the magnitude of sexual dimorphism nor with overall facial masculinization. Finally, sex differences in self-reported aggression did not reflect morphological sex differences. These findings challenge the generalizability of claims that aggression is encoded in the sexually dimorphic facial structure.

Primate eyes vary strikingly in pigmentation, yet the drivers of said variation are strongly debated. Recent revisions of the Cooperative Eye Hypothesis (CEH) propose that the human eye’s sclerae evolved to enhance gaze communication specifically under challenging conditions of visibility. We tested this idea under ecologically realistic conditions by presenting observers with a live model wearing contact lenses that simulated either a human-like or a chimpanzee-like eye. At a university lab, observers judged gaze direction at different viewing distances and lighting levels. Contrary to expectations, chimpanzee-like outperformed human-like eyes in dim lighting and close-viewing conditions. Human-like eyes yielded the highest accuracy under bright, far-viewing conditions, consistent with a long-distance signaling advantage. Our results demonstrate that ecological visual constraints shape the potential informativeness of distinct ocular configurations. We hypothesize that species-typical eye appearances may be tuned to their species-typical visual ecology.

Research focusing on first impression formation based on facial stimuli lacks a conclusion on whether there is a cross-cultural agreement and how deeply it has proliferated across distant populations. Social media may play an important role in the level of cross-cultural agreement as they provide us with overwhelming numbers of visual stimuli, including faces. Sharing social media aesthetics, their users may utilise facial cues congruently. We asked participants from seven distant, ethnically variable countries from five continents to rate facial attractiveness, trustworthiness and dominance of a single ethnically invariant facial sample ( N = 195, 106 women, M_Age = 23.23), also accounting for their self-reported social media use intensity and socioeconomic background. We expected the agreement between cultures to be better for participants who reported a higher intensity of social media use. Instead, we observed substantial cross-cultural agreement, especially for attractiveness and trustworthiness, regardless of the self-reported social media use intensity. However, the samples of participants from similar cultural backgrounds (same countries) agreed more. We also see substantial agreement in facial cue utilisation. In line with previous research, the distinctiveness of facial shape affects perceived attractiveness congruently across cultures. Despite the relatively small age range, age positively affects ascribed dominance.

The Internet both substantially expands and potentially biases our “visual diet” concerning faces. Using an evolutionary psychology framework, we compared the facial characteristics of popular content creators on online social media with control faces from the same populations. We analysed facial configurations with landmark-based geometric morphometrics and Bayesian models, conducting the first such comparison on a large sample. Across all four samples (Colombia: N = 291, 153 creators; Czechia: N = 360, 165 creators; India: N = 201, 63 creators; Turkey: N = 407, 145 creators), creators showed similar traits, including pronounced sexual dimorphism and evenly distributed features. The analysis also consistently showed significant differences between the creators and controls. We then extended the dataset with faces generated by a text-to-image model to investigate Internet typicality, for which we found only limited support. These results indicate systematic differences between the faces of popular creators and those of the general population. Conclusion: These results indicate systematic differences between the faces of popular creators and those of the general population.

Evolutionary psychology views the human eye as special. In particular, it claims that the light peri-iridial tissues surrounding a relatively darker iris form a combination that sets us apart from other primates. From this perspective, much less attention has been paid to how eye colouration varies between humans, although evidence indicates that variations in peri-iridial and iridial colouration influence both perceived facial attractiveness and sex-typicality. To determine what aspects of eye colouration influence the perception of faces, we have measured the colour of peri-iridial eye tissues (‘the white of the eye’) and the iris in nine samples from seven distant cultures ( N = 1033) across three continents. The faces were rated on facial attractiveness and sex-typicality by raters from the corresponding populations. Accounting for the effects of skin lightness, age, and facial shape, we ran a Bayesian multilevel model to estimate global and sample-specific effects of colouration of the iris and peri-iridial tissues on perceived sex-typicality and facial attractiveness. This exploratory, cross-sectional study revealed an overall preference for slightly darker peri-iridial tissues in women, whereby this association was mediated by perceived sex-typicality. Our findings challenge the notion that the light-eyed phenotype is universally preferred by human raters. Instead, they suggest a preference for a moderate phenotype, perhaps because very light peri-iridial tissues are typical of faces which are generally perceived as less feminine. Women with bluer irises were generally perceived as more attractive but findings related to other colour channels and iris features were inconsistent and varied across samples. Significance statement: The morphological variation of human eyes is an understudied phenomenon. While attention has been paid to the alleged uniqueness of human eyes (compared to other primates), little is known about how variations in eye colouration influence human perception of faces. Our study included over 1000 individuals from seven culturally distinct regions, mapped human eye variation, and tested how eye colouration influences perceived attractiveness and sex-typicality. In humans, variation in eye colouration is relatively large and differs across populations. Our findings suggest that it affects the perception of faces. Paradoxically, darker peri-iridial regions (scleras) slightly enhance the perception of femininity and female attractiveness, which challenges the idea that lighter eyes are universally preferred. Moreover, blue/light irises were in some contexts linked to a higher attractiveness. These results further refine our understanding of the role of eye colouration in perceived attractiveness.

By Vojtěch Fiala, Petr Tureček & 7 more. ...

Body mass is the most important predictor of success in fighting between conspecifics, with the heavier individual winning most fights. For this reason, the opponents in combat sports are usually weight-matched. The exception is the heavyweight division, which has no weight limit. Elite heavyweight athletes may cluster around an unknown global heavyweight optimum or compete at an individually specific optimal weight. We used comprehensive data (taken from boxrec.com) on bout outcomes for active male professional heavyweight boxers and Bayesian logistic regressions to explore the association between anthropometric predictors and the probability of winning. We identified the population-level optimum weight for heavyweight boxers at around 110 kg. The effects of age and height on the probability of winning were, however, much stronger than the effect of weight. Changing division during a boxer's professional career also affected performance: heavyweight athletes who during their professional careers also fought in lower weight classes were 19.9% [31.1–5.6%] less likely to win a bout in the heavyweight class. The study points to the importance of anthropometric measures for long-term success in professional heavyweight boxing and suggests a cut-off point, at which on average the marginal benefits of being a heavier boxer drop to 0.

Dear author, Congratulations on the manuscript. It reads well, introduces interesting data, and mostly manages to present the problem, results, and discussion in an easy-to-follow way. In my opinion, this is an outcome of a large-scale, thorough...