SEX DIFFERENCES IN SMILING AND OTHER PHOTOGRAPHED TRAITS: A THEORETICAL ASSESSMENT

Smile dimensions affect self-perceived smile attractiveness

Facial expressions play a leading role in human interactions because they provide signaling information of emotion and create social perceptions of an individuals’ physical and personality traits. Smiling increases socially perceived attractiveness and is considered a signal of trustworthiness and intelligence. Despite the ample information regarding the social importance of an attractive smile, little is known about the association between smile characteristics and self-assessed smile attractiveness. Here we investigate the effect of smile dimensions on ratings of self-perceived smile attractiveness, in a group of 613 young adults using 3D facial imaging. We show a significant effect of proportional smile width (ratio of smile width to facial width) on self-perceived smile attractiveness. In fact, for every 10% increase in proportional smile width, self-perceived attractiveness ratings increased by 10.26%. In the present sample, this association was primarily evident in females. Our results indicate that objective characteristics of the smile influence self-perception of smile attractiveness. The increased strength of the effect in females provides support to the notion that females are overall more aware of their smile and the impact it has on their public image.

Sexual dimorphism in the facial morphology of adult humans: A three-dimensional analysis

The objective of this study was to demonstrate sexual dimorphism in the entire three-dimensional facial surface form of adult humans. The sample consisted of female and male groups (n=200; age range, 18-35 years). Three-dimensional images of each participant's face at rest were recorded. A total of 185 variables that described facial surface configuration features were extracted from each image. The variables were compared between the groups using t-tests, and those exhibiting P-values <0.0001 were entered into a stepwise discriminant function analysis for sex determination. Wire mesh fitting was also performed on each image to examine the facial surface morphology. The mean node coordinates of the fitted mesh were compared between the groups using t-tests. Sixty-seven of the 185 variables differed significantly between the groups, and 11 qualified for inclusion in the stepwise analysis. The female group exhibited a greater vertical height of the eye fissure, shorter postero-anterior height of the nasal tip, vertically greater supraorbital ridge, shorter lower face height relative to the total upper anterior face height, more prominent cheeks in the infraorbital region, less prominent cheeks in the buccal region, shorter vertical height of the subnasal region, a smaller nasal hump, and a smaller alar. The discriminant function analysis was 96.5% accurate overall. The wire mesh fitting results showed that the eyes, forehead, and chin were in vertically higher positions in the female group than in the male group. The cheeks and nose were more protuberant in the female group and male group, respectively.

The adaptive value associated with expressing and perceiving angry-male and happy-female faces

Facial expressions are valuable for conveying and understanding the inner thoughts and feelings of the expressor. However, the adaptive value associated with a specific expression on a male face is different from a female face. The present review uses a functional-evolutionary analysis to elucidate the evolutionary advantage in the expression and perception of angry-male and happy-female faces over angry-female and happy-male faces. For the expressors, it is more advantageous for men to show angry facial expression as it signals dominance, averts aggression and deters mate poaching; it is more advantageous for women to display happy facial expression as it signals their willingness for childcare, tending and befriending. For the perceivers, those sensitive to angry men avoid being physically harmed while those sensitive to happy women gain social support. Extant evidence suggests that facial structure and cognitive mechanisms evolved to express and perceive angry-male and happy-female faces more efficiently compared to angry-female and happy-male faces.

Masculine voices signal men's threat potential in forager and industrial societies

Humans and many non-human primates exhibit large sexual dimorphisms in vocalizations and vocal anatomy. In humans, same-sex competitors and potential mates attend to acoustic features of male vocalizations, but vocal masculinity especially increases perceptions of physical prowess. Yet, the information content of male vocalizations remains obscure. We therefore examined relationships between sexually dimorphic acoustic properties and men's threat potential. We first introduce a new measure of the structure of vocal formant frequencies, 'formant position' (Pf), which we show is more sexually dimorphic and more strongly related to height than is the most widely used measure of formant structure, 'formant dispersion', in both a US sample and a sample of Hadza foragers from Tanzania. We also show large sexual dimorphisms in the mean fundamental frequency (F0) and the within-utterance standard deviation in F0 (F0-s.d.) in both samples. We then explore relationships between these acoustic parameters and men's body size, strength, testosterone and physical aggressiveness. Each acoustic parameter was related to at least one measure of male threat potential. The most dimorphic parameters, F0 and Pf, were most strongly related to body size in both samples. In the US sample, F0 predicted testosterone levels, Pf predicted upper body strength and F0-s.d. predicted physical aggressiveness.