PS4029/30 Perspectives on social attributions Lecture 3

1.Symmetric faces are attractive: recap 2.Simple perceptual bias account 3.Prototype-based perceptual bias account 4. Problems for perceptual bias accounts Lecture 3: Perceptual bias accounts of symmetry preferences

Methods for symmetry manipulation 1. [good method - Perrett et al. 1999] Warping face shape (natural proportions) 1. Symmetric faces are attractive: recap (methods) attractive, symmetric less attractive, less symmetric

Methods for symmetry manipulation 2. [bad method - Samuels et al. 1994] LL/RR Chimeric faces (unnatural proportions) 1. Symmetric faces are attractive: recap (methods)

Symmetric faces have been found to be attractive in diverse cultures [Rhodes et al: Asia, Perrett et al: UK] It has been suggested that this stability in (or high agreement for) symmetric faces occurs because symmetric faces have a biological basis 1. Symmetric faces are attractive: recap (x-cultural comparison)

Although most researchers agree preferences for symmetric faces have a biological basis, there is enormous debate about the nature of this basis 1. Perceptual bias account (trick of the visual system) 2. Evolutionary advantage view (symmetry preferred because it signals health, fertility) 1. Symmetric faces are attractive: recap (biological basis)

preference for bilateral symmetry may occur simply because the human visual system is particularly sensitive to bilateral symmetry ocular musculature is bilaterally symmetrical and the left and right visual field is processed in different hemispheres (allowing L-R matching and easy symmetry detection) (Mach, 1887; Herbert & Humphrey, 1996) 2. Simple perceptual bias account of symmetry preferences

Evidence for Simple Perceptual Bias Symmetry preferences seen in: –objects –decorative art OriginalSymmetrical Gombrich, 1984 Rensch, 1963 OriginalSymmetrical These effects suggest there may be nothing ‘special’ about facial symmetry that is attractive - consistent with view preferences for symmetric faces are a ‘trick’ of the visual system

For each class of stimuli it is possible that the visual system develops an internal prototype. Such a prototype is made up of an average of the characteristics of all the different stimuli of that type that have been seen. 3. Prototype-based perceptual bias accounts

3 face average asymmetric 10 face average more prototypical and more symmetric When novel stimuli are seen they are compared to this symmetric prototype This figure shows that increasing prototypicality increases symmetry - random deviations from symmetric are averaged out

In other words: symmetric faces may be attractive because they more closely resemble prototypical faces than asymmetric faces do Evidence: from neural networks and chicken preferences 3. Prototype-based perceptual bias accounts

Neural Networks & Perceptual Bias Computer programs trained on stimuli for recognition Show that recognition training can create preference (recognition) for symmetry Johnstone, 1994, Nature Enquist & Arak, 1994, Nature Training Set = Novel symmetric stimuli preferred (most reaction)

Prototypes and Perceptual Bias Train chickens to discriminate between rewarding and non- rewarding stimuli Stimuli were two asymmetric crosses which were mirror images of each other Jansson et al., 2002, Anim Behav Both associated with reward

Train chickens to discriminate between rewarding and non- rewarding stimuli On subsequent testing chickens preferred novel symmetric cross to either asymmetric cross So symmetry preference can arise as by-product of visual system & experience Jansson et al., 2002, Anim Behav PECK! Prototypes and Perceptual Bias Novel symmetric cross

 Individual differences in symmetry preferences  If everyone’s visual / recognition systems work in the same way (at least among healthy adults), and symmetry preference is a by- product of the way this system works, then why do some people find symmetric faces more attractive than other people do?  Jones et al. (2001) 4. Problem for perceptual bias account I

Attractive women like symmetric male faces more than relatively unattractive women do Little et al (2001) 4. Own attractiveness and symmetry preferences

Opposite-sex bias in symmetry preferences if symmetry is attractive only because of the way the way the visual system works, symmetry preferences should be the same regardless of the sex of the faces presented (as both sexes of face thought to be processed in an identical manner by the visual recognition system) 4. Problem for perceptual bias account II

Female judges (Little et al., 2001) NB - symmetry attractive in BOTH male and female faces but MORE attractive in opposite-sex than own-sex faces

Next week: Evolutionary advantage view of symmetry preferences This view posits that facial symmetry signals qualities such as health that are desirable in mates. Evidence that symmetry preferences are stronger for mate choice relevant stimuli (i.e. faces) than for other objects (e.g. inverted faces) will be discussed. Evidence that symmetric individuals also possess attractive voices and body odours will also be discussed as implying symmetry signals an underlying attractive quality. Evidence that symmetry signals health and fertility will also be discussed