Feinberg; Symmetry and sexual dimorphism in human faces: Symmetry and masculinity in human faces have been proposed to be cues to the quality of the owner. Accordingly, symmetry is generally found attractive in male and female faces, and femininity is attractive in female faces. Women's preferences for male facial masculinity vary in ways that may maximize genetic benefits to women's offspring. Here we examine same- and opposite-sex preferences for both traits Study 1 and intercorrelations between preferences for symmetry and sexual dimorphism in faces Study 1 and Study 2 using computer-manipulated faces.
For symmetry, we found that male and female judges preferred symmetric faces more when judging faces of the opposite-sex than when judging same-sex faces.
A similar pattern was seen for sexual dimorphism i. This suggests that women are more concerned with female femininity than are men. We also found that in women, preferences for symmetry were positively correlated with preferences for masculinity in male faces and that in men preferences for symmetry were positively correlated with preferences for femininity in female faces.
These latter findings suggest that symmetry and sexual dimorphism advertise a common quality in faces or that preferences for these facial cues are dependent on a common quality in the judges.
Collectively, our findings support the view that preferences for symmetry and sexual dimorphism are related to mechanisms involved in sexual selection and mate choice rather than functionless by-products of other perceptual mechanisms.
Several researchers have proposed that symmetry and sexual dimorphism masculine appearance in men and feminine appearance in women in human faces may be cues to heritable fitness benefits and therefore relate to attractiveness see e.
Symmetry has long been proposed to be associated with male and female genotypic quality Jasienska et al. For many traits any deviation from perfect symmetry can be considered a reflection of imperfect development. It has then been suggested that only high-quality individuals can maintain symmetric development under environmental and genetic stress, and therefore, symmetry can serve as an indicator of phenotypic quality as well as genotypic quality e.
Both studies of real faces Grammer and Thornhill ; Mealey et al. Masculine facial traits large jaws and prominent brows in males are thought to be testosterone dependent and therefore may represent an honest immunocompetence handicap signaling quality Folstad and Karterindeed masculine-faced men do report having lower incidence of disease Thornhill and Gangestadand so should be found attractive by members of the opposite sex e.
There is some evidence that masculine male faces are found attractive e. In females, estrogen-dependent characteristics of the female body correlate with health and reproductive fitness Jasienska et al.
Increasing the sexual dimorphism of female faces should therefore enhance attractiveness as estrogen also affects facial growth Enlowand indeed, there is considerable evidence that feminine female faces and faces of women with high estrogen Law-Smith et al.
Studies measuring facial features from photographs of women Cunningham ; Jones and Hill ; Grammer and Thornhill and studies of manipulating facial composites Perrett et al. It is plausible that sexual dimorphism in both males and females is related to intrasexual selection or competition within a sex for mates.
Association between sexual dimorphism and quality would indicate that masculine men and feminine women are better able to compete with others of their own sex. For example, high-quality sexually dimorphic individuals may be better able to physically fight off competitors or be able to travel further in the of mates than lower quality, less sexually dimorphic members of the same sex.
Indeed, photographs of military cadets that were rated most dominant looking tended to achieve the highest rank later in their military careers Mueller and Mazur It has been shown that when An example of a sexually dimorphic behavior noted in humans would be 3 different morphing techniques that as masculinity increases in male faces, they are perceived as more dominant DeBruine et al.
It is possible that masculinity in male faces is related to competition between males and not just to attractiveness to females. One alternative to adaptive hypotheses for preference for symmetry and masculine male or feminine female facial features is that a preference for these traits reflects sensory bias in perception. This explanation for face preferences is often referred to as the perceptual bias view e. Certainly, preferences for symmetry have been observed for stimuli not related to mate choice such as everyday objects Rensch and decorative art Gombrich Computer-based neural networks trained to recognize asymmetric stimuli stimuli with high fluctuating asymmetry respond most strongly to novel symmetric stimuli, which are the average of training stimuli Johnstone Preferences for symmetry can arise in a similar manner in bird species as well.
The stimuli were 2 asymmetric crosses that were mirror images of each other. On subsequent testing, chickens preferred a novel symmetric cross to either asymmetric cross despite the fact that it was never associated with reward, confirming that it is possible for symmetry preference to arise as a by-product of the visual system via perceptual experience.
Preferences for masculinity in male and femininity in female faces may also arise in a similar way. Enquist and Arak used computer neural networks to examine the mechanisms involved in signal recognition.
They used these neural networks to model the evolution of female preferences for long-tailed conspecifics.
Simulated female birds were trained to recognize different patterns that represented males. Again, there is some evidence that the visual systems of real birds behave as predicted by computer modeling.
Chickens trained to discriminate between human male and female faces show just such an effect—after training, chickens respond most strongly to faces with exaggerated sexual dimorphism, more so than they respond to the original rewarded average male and female stimuli Ghirlanda et al.
Of course, there is no reason to assume that an inherent preference can be solely attributed to sensory bias. Whereas many studies have examined the link between measures of quality and measured sexual dimorphism and symmetry, competing hypothesis from an evolutionary and perceptual bias view can also be usefully examined using perceptual tests. We examined 2 aspects of the perception of symmetry and sexual dimorphism, 1 preferences in same- and opposite-sex faces and 2 intercorrelation of preferences for symmetry and sexual dimorphism.
Greater preferences for opposite-sex faces might be predicted if preferences "An example of a sexually dimorphic behavior noted in humans would be" adaptations to mate choice, and this notion has received some support for symmetry preferences Jones et al. On one hand, if sexual dimorphism is an advertisement of quality and important for mate choice, we might expect more extreme preferences for opposite-sex faces.
On the other hand, if it is more involved in intrasexual competition, judges may assume that extremes of sexual dimorphism are attractive in the same sex.
Finally, if sexual dimorphism and symmetry are advertisements of the same measure of quality, we would expect preferences for these traits An example of a sexually dimorphic behavior noted in humans would be covary as individual differences increasing preference for one would be likely to increase preference for the other.
If both traits advertise different aspects of quality, we might also expect a relationship as factors in the perceiver that cause individual differences in attention to different aspects of quality could also drive covariation.
Participants were students who responded to an e-mail link to an Internet study and were selected for reporting to be heterosexual. To measure preferences for sexually dimorphic features, we used 20 pairs of composite face images 10 male pairs and 10 female pairs.
Each pair comprised one masculinized and one feminized version of the same face see Figure 1 for example images. Original images were 50 young adult Caucasian male and 50 female photographs taken under standard lighting conditions and with a neutral expression. The composite images were made by creating an average image made up of 5 randomly assigned individual facial photographs this technique has been used to create composite images in previous studies, see, e.
Faces were transformed on a sexual dimorphism dimension using the linear difference between a composite of all 50 adult males and a composite of all 50 young adult females following the technique reported in Perrett et al. This meant that each face was transformed along the sexual dimorphism axis by the same amount, either increasing masculinity or increasing femininity, and that faces retained their identities and perceived sex female faces remained female in appearance and male faces remained male in appearance.
Composite images were made perfectly symmetric so that transforms did not manipulate symmetry. Examples of feminized left and masculinized right female and male faces participants viewed full color versions. To measure symmetry preferences, we used 30 stimulus pairs that have been used in previous studies Perrett et al.
Each pair was made up of one original and one symmetric image. All images were manipulated to match the position of the left and the right eyes. To generate the symmetric images, original images were warped so that the position of the features on either side of the face was symmetrical.
Images maintained original textural cues and were symmetric in shape alone. See Perrett et al. An example of an original and symmetrical face can be seen in Figure 2. The symmetry manipulation was independent of sexual dimorphism. Examples of original top and symmetric bottom versions of male and female faces participants viewed full color versions. Participants were administered a short questionnaire assessing age, sex, and sexual orientation, followed by the face tests.
Order of rating of same- and opposite-sex faces was randomly determined for each participant.
The 10 pairs of masculine and feminine faces and the 15 pairs of symmetric and asymmetric faces for each sex were presented together. Faces were shown as pairs with both order and side of presentation randomized. Participants were asked to choose the face from the pair that they found most attractive. Clicking on one of these 8 buttons moved participants on to the next face trial.