Ghirlanda, Jansson & Enquist (2002) conducted a novel study to examine the origins of the preference for attractive, sexually dimorphic faces in humans.
The researchers constructed seven male and female faces on a spectrum from moderately to strongly sexually dimorphic, obtained by 'averaging' a set of 35 facial photographs of individuals of each sex. A group of university students (N = 7 females and N = 7 males) were then requested to rate the faces in a random order, on a decile (1-10) scale for sexual attractiveness, answering the question 'how desirable would it be to go out on a date with this individual'.
Then the researchers utilized six chickens as experimental subjects (Gallus gallus domesticus; four being female chickens), which were rewarded with food for pecking at the faces of the humans that were of the sex opposite of their own (e.g hens male faces, cocks female faces) and trained to do so over a course of a few weeks. The researchers note that 'no reinforcement was given on test trials', so the chickens were only trained to peck faces of the correct sex, they weren't guided to pick at any particular target.
To test the behavior that the chickens had learned, the researchers counted the times the chicken picked at a given face within a certain time interval. Interestingly, it was discovered that human and chicken preferences for opposite sex faces were very highly correlated when varying the degree of sexual dimorphism of the presented faces (r² = 0.98); that is to say nearly identical. That means the chickens were nearly equally as likely to peck at the highly dimorphic faces as the human subjects were to prefer them as potential romantic partners. This result even generalized to when the researchers added faces with even more exaggerated dimorphism not present during training, i.e. both humans and chickens agreed on the more exaggerated faces being more attractive.
This provides evidence that any kind of sexual dimorphism tends to be exaggerated by sexual selection as it will appear are more attractive to the opposite sex (though within bounds, too much of a desirable trait also tends to reduce fitness, which is related to goldilocks principle).
Discussion:
The findings provide support for the hypothesis that human preferences for sexually dimorphic faces are innate and hardwired in our 'lizard brain', that is, there is a deep-seated desire for such aesthetic features, and this desire even predates the evolution of modern humans, with the last common ancestor of humans and chickens thought to have been a reptilian creature that lived more than 310 million years ago. The result strongly suggests that the preference for extremely masculine and feminine faces is not a cultural construct, but it inevitably emerges in biological brains.
Quotes:
The researchers constructed seven male and female faces on a spectrum from moderately to strongly sexually dimorphic, obtained by 'averaging' a set of 35 facial photographs of individuals of each sex. A group of university students (N = 7 females and N = 7 males) were then requested to rate the faces in a random order, on a decile (1-10) scale for sexual attractiveness, answering the question 'how desirable would it be to go out on a date with this individual'.
Then the researchers utilized six chickens as experimental subjects (Gallus gallus domesticus; four being female chickens), which were rewarded with food for pecking at the faces of the humans that were of the sex opposite of their own (e.g hens male faces, cocks female faces) and trained to do so over a course of a few weeks. The researchers note that 'no reinforcement was given on test trials', so the chickens were only trained to peck faces of the correct sex, they weren't guided to pick at any particular target.
To test the behavior that the chickens had learned, the researchers counted the times the chicken picked at a given face within a certain time interval. Interestingly, it was discovered that human and chicken preferences for opposite sex faces were very highly correlated when varying the degree of sexual dimorphism of the presented faces (r² = 0.98); that is to say nearly identical. That means the chickens were nearly equally as likely to peck at the highly dimorphic faces as the human subjects were to prefer them as potential romantic partners. This result even generalized to when the researchers added faces with even more exaggerated dimorphism not present during training, i.e. both humans and chickens agreed on the more exaggerated faces being more attractive.
This provides evidence that any kind of sexual dimorphism tends to be exaggerated by sexual selection as it will appear are more attractive to the opposite sex (though within bounds, too much of a desirable trait also tends to reduce fitness, which is related to goldilocks principle).
Discussion:
The findings provide support for the hypothesis that human preferences for sexually dimorphic faces are innate and hardwired in our 'lizard brain', that is, there is a deep-seated desire for such aesthetic features, and this desire even predates the evolution of modern humans, with the last common ancestor of humans and chickens thought to have been a reptilian creature that lived more than 310 million years ago. The result strongly suggests that the preference for extremely masculine and feminine faces is not a cultural construct, but it inevitably emerges in biological brains.
Quotes:
- Human and chicken behavior was almost identical (correlation between the two gradients: r² = 0.98). Moreover, chicken and human data for each face never differed significantly.
- We cannot of course be sure that chickens and humans processed the face images in exactly the same way. This leaves open the possibility that, while chickens use some general mechanism, humans possess instead a specially evolved mechanism for processing faces.
- Ours is of course a preliminary study. We believe, however, that it shows the potentials of the comparative study of preferences. This method is not only relevant to the study of human faces, it can be applied to any communication system to evaluate whether its evolution has favored information transfer or rather is a product of receiver biases.
- Ghirlanda S, Jansson L, Enquist M. 2002. Chickens prefer beautiful humans. Human Nature. 13(3): 383–389. [Abstract]
