Bernhard Fink

Darwinian aesthetics: sexual selection and the biology of beauty

Current theoretical and empirical findings suggest that mate preferences are mainly cued on visual, vocal and chemical cues that reveal health including developmental health. Beautiful and irresistible features have evolved numerous times in plants and animals due to sexual selection, and such preferences and beauty standards provide evidence for the claim that human beauty and obsession with bodily beauty are mirrored in analogous traits and tendencies throughout the plant and animal kingdoms. Human beauty standards reflect our evolutionary distant and recent past and emphasize the role of health assessment in mate choice as reflected by analyses of the attractiveness of visual characters of the face and the body, but also of vocal and olfactory signals. Although beauty standards may vary between cultures and between times, we show in this review that the underlying selection pressures, which shaped the standards, are the same. Moreover we show that it is not the content of the standards that show evidence of convergence ‐ it is the rules or how we construct beauty ideals that have universalities across cultures. These findings have implications for medical, social and biological sciences.

Second to fourth digit ratio, testosterone and perceived male dominance.

Previous studies have shown that male faces with extreme features associated with testosterone are perceived as dominant and masculine. Women have been reported to prefer more masculinized male faces as they may consider testosterone markers to be an ‘honest’ indication of good health, and such considerations may underlie their aesthetic preferences. However, pronounced testosterone facial markers are also associated with dominance, and several negative personality traits. This suggests that female aesthetic preferences may be an adaptive compromise between positive attributes associated with higher than average testosterone, and negative attributes associated with more extreme masculinization. This current study attempts to clarify the role of hormone markers in female perceptions of dominance, masculinity and attractiveness, in male facial images. Recent evidence suggests that the relative length of the 2nd to 4th finger (2D : 4D ratio) is a pointer to prenatal testosterone levels and may thus serve as a window to the prenatal hormonal environment. We measured 2D : 4D in a sample of male college students and took salivary samples to analyse circulating levels of testosterone. Women rated facial images of these males for dominance, masculinity and attractiveness. Our results show that male 2D : 4D was significantly negatively related to perceived dominance and masculinity but not attractiveness. Circulating testosterone levels were not related to dominance, masculinity or attractiveness. These findings suggest that: (i) high prenatal levels of testosterone serve to ‘organize’ male facial features to subsequently reflect dominance and masculine characteristics presumably activated during puberty; and (ii) attractiveness is not directly related to testosterone levels. We conclude that facial dominance and masculinity reflect a males perceived status rather than his physical attraction to women.

Human (Homo sapiens) facial attractiveness in relation to skin texture and color.

The notion that surface texture may provide important information about the geometry of visible surfaces has attracted considerable attention for a long time. The present study shows that skin texture plays a significant role in the judgment of female facial beauty. Following research in clinical dermatology, the authors developed a computer program that implemented an algorithm based on co-occurrence matrices for the analysis of facial skin texture. Homogeneity and contrast features as well as color parameters were extracted out of stimulus faces. Attractiveness ratings of the images made by male participants relate positively to parameters of skin homogeneity. The authors propose that skin texture is a cue to fertility and health. In contrast to some previous studies, the authors found that dark skin, not light skin, was rated as most attractive.

Digit ratio (2D:4D), dominance, reproductive success, asymmetry, and sociosexuality in the BBC Internet Study

Digit ratio (2D:4D) may be a correlate of prenatal sex steroids, and has been linked to traits, which are influenced by fetal testosterone and estrogen. Here we consider such links in a large Internet study of sex differences (the BBC Internet Study) in which finger lengths were self‐measured. Consistent with lab‐based findings the 2D:4D in this study shows sexual dimorphism, ethnic differences and higher dimorphism of right 2D:4D than left, thereby indicating that 2D:4D does measure real between‐participant variation. High error in self‐measurement of fingers reduces effect sizes. However, the large sample size gives assurance that significant effects are likely to be real. We controlled for ethnicity and sexual orientation by considering White heterosexuals only (153,429 participants). Sexual dimorphism was confirmed in 2D:4D and for the difference of right–left 2D:4D. After Bonferroni correction we found highly significant relationships with low effect sizes as follows. In males and females there were negative associations between 2D:4D and dominance. In males there were negative associations between 2D:4D and family size and factors associated with reproductive success. For females these associations were positive. For asymmetry we found U‐shaped relationships with 2D:4D in both males and females. We found no relationship between 2D:4D and promiscuity (sociosexuality). In total, we considered 48 relationships and found 29 to be significant. We compare our findings with a similar study reported by Putz et al. (2004), which found only 2 out of 57 correlations to be significant and discuss possible reasons for the discrepancies between the studies. Am. J. Hum. Biol., 2008.

Second to fourth digit ratio and face shape

The average human male face differs from the average female face in size and shape of the jaws, cheek-bones, lips, eyes and nose. It is possible that this dimorphism is determined by sex steroids such as testosterone (T) and oestrogen (E), and several studies on the perception of such characteristics have been based on this assumption, but those studies focussed mainly on the relationship of male faces with circulating hormone levels; the corresponding biology of the female face remains mainly speculative. This paper is concerned with the relative importance of prenatal T and E levels (assessed via the 2D : 4D finger length ratio, a proxy for the ratio of T/E) and sex in the determination of facial form as characterized by 64 landmark points on facial photographs of 106 Austrians of college age. We found that (i) prenatal sex steroid ratios (in terms of 2D : 4D) and actual chromosomal sex dimorphism operate differently on faces, (ii) 2D : 4D affects male and female face shape by similar patterns, but (iii) is three times more intense in men than in women. There was no evidence that these effects were confounded by allometry or facial asymmetry. Our results suggest that studies on the perception of facial characteristics need to consider differential effects of prenatal hormone exposure and actual chromosomal gender in order to understand how characteristics have come to be rated ‘masculine’ or ‘feminine’ and the consequences of these perceptions in terms of mate preferences.

Second to fourth digit ratio, body mass index, waist-to-hip ratio, and waist-to-chest ratio: their relationships in heterosexual men and women

Background: Prenatal sex steroids have been broadly discussed in terms of their possible effect on brain differentiation, whereas pubertal/adult sex hormones are thought to be the main regulators of sexually dimorphic physical features in males and females. Assessing prenatal steroid exposure has previously been difficult but evidence now suggests that finger length ratio may provide a ‘window’ into prenatal hormone exposure. The length of the second digit (the index finger) relative to the length of the fourth digit (the ring finger) is sexually dimorphic as males have a lower second to fourth digit ratio (2D:4D). The sexual dimorphism is determined as early as the 14th week of fetal life, and remains unchanged at puberty. There is evidence that sex differences in 2D:4D arise from in utero concentrations of sex steroids, with a low 2D:4D (male typical ratio) being positively related to prenatal testosterone, while a high 2D:4D (female typical ratio) is positively associated with prenatal oestrogen. Aim: The studied aimed to determine whether, and to what extent, adult sexually dimorphic physical traits, which are largely determined at puberty, relate to traits that are largely determined in utero. This work examined the relationship between three sexually dimorphic traits—body mass index (BMI), waist-to-hip ratio (WHR) and waist-to-chest ratio (WCR)—and digit ratio. Participants and methods: BMI, WHR and WCR were assessed in 30 heterosexual male and 50 heterosexual female participants by recording their body height, weight, and measuring their waist, hip and chest circumference. Digit lengths of the second and fourth fingers were measured from photocopies of the ventral surface of the hand and by actual finger measurements. Results: Digit ratio was found to be significantly lower in men than in women. Significant negative correlations were found between female’s left and right hand 2D:4D, waist and hip circumference, and WCR. In males, BMI was found to be positively related to digit ratio but remained significant only for left hand 2D:4D. Generally, the relationships were stronger for females than for males. Although not all relationships were found to be significant, they were in accord with our predictions. Conclusion: In addition to an activational effect of sex hormones at puberty, the present data suggest an early organizational effect of sex hormones through the association between indices of female body shape, male BMI, and human finger length patterns.

Amygdala activation at 3T in response to human and avatar facial expressions of emotions.

Facial expressions of emotions are important in nonverbal communication. Although numerous neural structures have been identified to be involved in emotional face processing, the amygdala is thought to be a core moderator. While previous studies have relied on facial images of humans, the present study is concerned with the effect of computer-generated (avatar) emotional faces on amygdala activation. Moreover, elicited activation patterns in response to viewing avatar faces are compared with the neuronal responses to human facial expressions of emotions. Twelve healthy subjects (five females) performed facial emotion recognition tasks with optimized 3T event-related fMRI. Robust amygdala activation was apparent in response to both human and avatar emotional faces, but the response was significantly stronger to human faces in face-sensitive structures, i.e. fusiform gyri. We suggest that avatars could be a useful tool in neuroimaging studies of facial expression processing because they elicit amygdala activation similarly to human faces, yet have the advantage of being highly manipulable and fully controllable. However, the finding of differences between human and avatar faces in face-sensitive regions indicates the presence of mechanisms by which human brains can differentiate between them. This mechanism merits further investigation.

Geometric Morphometrics of Male Facial Shape in Relation to Physical Strength and Perceived Attractiveness, Dominance, and Masculinity

Evolutionary psychologists claim that women have adaptive preferences for specific male physical traits. Physical strength may be one of those traits, because recent research suggests that women rate faces of physically strong men as more masculine, dominant, and attractive. Yet, previous research has been limited in its ability to statistically map specific male facial shapes and features to corresponding physical measures (e.g., strength) and ratings (e.g., attractiveness).

The effects of skin colour distribution and topography cues on the perception of female facial age and health

According to evolutionary psychology, the preference for some facial characteristics reflects adaptations for mate choice because they signal aspects of mate quality. Although morphological features such as facial symmetry and sexually dimorphic traits have been studied extensively in recent years, little is known about skin condition in this context. The preferences for young and healthy looking skin could offer an explanation as to why women place such an importance on the condition of their skin and its refinement through e.g., cosmetic products. Recent research showed that facial skin colour distribution significantly influences the perception of age and attractiveness of female faces, independent of skin surface topography cues. However, the relative effect of skin colour distribution and topography cues on age and health perception remains to be investigated. We present data showing that both skin colour distribution and skin surface topography cues not only significantly influence the perception of female facial age and health but also convey differential information with regard to the strength of these effects. Our data indicate that skin surface topography cues account for a large proportion of variation in facial age perception, whereas skin colour distribution seems to be a stronger health cue.

Digit Ratio (2D:4D): A Biomarker for Prenatal Sex Steroids and Adult Sex Steroids in Challenge Situations.

Digit ratio (2D:4D) denotes the relative length of the second and fourth digits. This ratio is considered to be a biomarker of the balance between fetal testosterone (T) and estrogen (E) in a narrow window of early ontogeny. Evidence for this assertion is derived from direct and indirect measures of prenatal hormonal exposure (in experimental animals, via amniotic fluid samples and in the study of sex-typical traits) in relation to 2D:4D. In contrast, the relationships between 2D:4D and levels of sex steroids in adults are less clear, as many correlational studies of 2D:4D and adult sex steroids have concluded that this association is statistically non-significant. Here, we suggest that in order to understand the link between 2D:4D and sex hormones, one must consider both fetal organizing and adult activating effects of T and E. In particular, we hypothesize that 2D:4D correlates with organizing effects on the endocrine system that moderate activating effects in adulthood. We argue that this is particularly evident in “challenging” conditions such as aggressive and sexual encounters, in which individuals show increased levels of T. We discuss this refinement of the 2D:4D paradigm in relation to the links between 2D:4D and sports performance, and aggression.