Revision lecture 2008

Exam formant

Three questions in Section A

Three questions in Section B

You must complete one question from each section

Revision lecture outline

1. Attractiveness & health

2. Self-resemblance as a cue of kinship

3. Why are symmetric faces attractive?

1. Attractiveness & healthEvolutionary Advantage account of attractivenessProposes that attractiveness judgments reflect adaptations that promote choice of healthy partners (Thornhill & Gangestad, 1999)

i.e. facial attractiveness signals aspects of health (e.g. fertility, low number of past health problems, ‘healthy’ genetic profile)

BUT - many researchers have challenged this proposal, noting that there is little evidence to support this view (e.g. Enquist et al. 2002; Kalick et al. 1999; Valentine et al. 2004)

While it is true that early studies of the link between attractiveness and actual health were not encouraging, more recent studies (with improved measures of health) present a different picture

1. Attractiveness & healthKalick et al. (1999)

Tested for a positive correlation between incidence of past health problems (assessed from medical records) and attractiveness

No relationship observedBUT - some problems with this study

1. Interpreting null findings is typically problematic

2. Face stimuli were low resolution B&W photographs (and some later studies suggest skin quality may play important role in attractiveness-health relationship, e.g. Roberts et al.)

3. Subsequent studies with same image-set found relationships between some attractive facial cues (e.g. averageness) and health measures

1. Attractiveness & fertilityLaw Smith et al. 2006 (women)

High levels of oestrogen and progesterone are associated with fertility among women and are positively related to women’s facial attractiveness (and attractive facial cues like perceived health + femininity)

Penton-Voak et al. 2003 (women)Low (i.e. feminine) waist-hip ratio is associated with fertility among women and is associated with attractive facial appearance

Both findings support the view that attractiveness in women signals reproductive health

1. Attractiveness & fertilityRoberts et al. 2004 (women)

Late follicular phase of menstrual cycle (i.e. around ovulation) is most fertile phase

Women’s faces more attractive around ovulation than at other times

Soler et al. 2003 (men)Facial attractiveness in men is associated with good semen quality (i.e. higher sperm count and better sperm mobility)

Both findings support link between attractiveness and fertility

1. Attractiveness & ‘good genes’Roberts et al. 2005 (men)

Heterozygosity at the MHC complex (genes that code for immunity to infectious diseases) associated with strong immune system

Heterozygotes judged more attractive than homozygotes

Heterozygotes have healthier-looking facial skin than homozygotes

Although Thornhill et al. (2003) found no link between MHC heterozygosity and men’s facial attractiveness, they did not control for age of men or ethnicity

1. Attractiveness & healthConclusions

Although there is little evidence that facial attractiveness is associated with (low) frequency of past health problems, recent findings for links between attractiveness and more objective/rigorous measures of fertility (e.g. measured hormone levels, semen quality) and measures of immune system strength (MHC heterozygosity) present compelling evidence that facial attractiveness is a cue to various aspects of health

2. Self-resemblance as a cue of kinship

Two biological theories predict that self-resemblance will influence attitudes to faces:

1. Inclusive fitness theory: By helping kin you help your genes pass onto subsequent generations

2. Inbreeding avoidance: By avoiding sex with kin you prevent deleterious effects of inbreeding on offspring

[an alternative psychological account is ‘mere exposure’ - self-resembling faces are familiar and therefore ‘liked’]

2. Self-resemblance as a cue of kinship

Trusting behavior (DeBruine, 2002)

Tested for effects of self-resemblance of other same-sex players in an economic ‘trust’ game

People more likely to behave in trusting fashion towards self-resembling players than other-resembling players

Supports key prediction of inclusive fitness theory (trust kin more than non-kin)

2. Self-resemblance as a cue of kinship

Attractiveness in own- and other-sex faces (DeBruine, 2004)

Tested for effects of self-resemblance on preferences for own- and other-sex faces

Self-resemblance increased attractiveness of own-sex faces (promoting affiliation with own-sex kin)

Self-resemblance did not increase attractiveness of other-sex faces to the same extent (reducing likelihood of inbreeding)

2. Self-resemblance as a cue of kinship

‘Trustworthy not lustworthy’ (DeBruine, 2005)

Previous findings suggested that self-resemblance in other-sex faces increases trusting but not attractiveness

Self-resembling other-sex faces are 1) perceived as trustworthy, 2) unattractive for short-term relationships (e.g. one-night stands)

and 3) ‘neutral’ in terms of attractiveness for long-term relationships

Again, suggests that self-resemblance is a cue of kinship - trust kin but don’t sleep with them!

2. Self-resemblance as a cue of kinship

Children’s faces (DeBruine, 2004)

DeBruine found self-resemblance increased positive attitudes for judgments of children’s faces (again, positive attitudes to kin)

Children’s faces are obviously not potential mates, so findings consistent with claim that self-resemblance preferred in faces of

individuals who are not potential mates (or when faces judged out with mating context)

Various studies by Platek found the above effect more pronounced in men than women (no sex difference in DeBruine)

2. Self-resemblance as a cue of kinship

Attitudes to self-resemblance

Increased preference for self-resembling faces when raised progesterone level prepares body for pregnancy (DeBruine, Jones & Perrett, 2005)

That the effect is most pronounced for female faces suggests the effect reflects increased preference for sources of support and care than mechanism for avoiding inbreeding

Indeed, change in preference for self-similar faces related to progesterone level NOT conception risk

2. Self-resemblance as a cue of kinship

Conclusions

People appear to use self-resemblance as a cue of kinship when judging others

Consistent with inclusive fitness theory, self-resemblance increases positive attitudes when ‘target’ is not a potential mate (e.g. children and same-sex individuals) or when other-sex faces are judged in a non-mating context (e.g. increases perceived trustworthiness of other-sex faces)

Consistent with inbreeding avoidance explanation of attitudes to self-resembling faces, self-resemblance decreases attractiveness of potential mates when judged for an explicitly sexual relationship (e.g. a one night stand)

That attitudes to self-resemblance are sensitive to the context (i.e. the ‘question’ asked) and face-type (child, own-sex, other-sex) in these ways supports the view that self-resemblance acts as a cue of kinship and is difficult to explain in terms of attitudes to familiar stimuli (i.e. hard to explain in terms of ‘mere exposure’ effects)

Symmetry is attractive (Perrett et al 1999)When the symmetry of faces is increased using computer graphic methods, this increases it’s attractiveness -

why?

Evolutionary advantage account (Thornhill & Gangestad 1999)Symmetry is attractive because it signals health + fertility

Simple perceptual bias account (Mach 1887)Symmetry is attractive because symmetric because the human visual system is particularly sensitive to

bilateral symmetry

Prototype-based perceptual bias accounts (Enquist et al 2002)Symmetric faces are attractive because they resemble prototypical mental representations of faces

ConclusionsAlthough there is evidence perceptual bias can cause symmetry preferences, perceptual bias accounts

cannot explain human’s symmetry preferences

3. Why are symmetric faces attractive?

Evidence for Perceptual Bias

Symmetry preferences seen in:– objects

– decorative art

Original Symmetrical

Gombrich, 1984

Rensch, 1963

Original Symmetrical

These effects suggest there may be nothing ‘special’ about facial symmetry that is attractive - consistent with viewpreferences for symmetric faces are a ‘trick’of the visual system

Neural Networks & Perceptual Bias

• Computer programs trained on stimuli for recognition

• Show that recognition training can create preference (recognition) for symmetry

Johnstone, 1994, NatureEnquist & Arak, 1994, NatureTraining 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

Attractivewomen like

symmetric male faces more than

relatively unattractive women do

Little et al (2001)

Perceptual bias accounts cannot accommodate individual differences in

symmetry preferences

Perceptual bias accounts cannot accommodate sex-specific symmetry preferences

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

Perceptual bias accounts cannot accommodate greater symmetry preferences in mate choice

relevant stimuli

Little & Jones, 2003, Proc Royal Soc

Symmetry and attractiveness in other modalities

• Symmetric individuals have attractive voices• (Hughes et al., 2002)

• Symmetric individuals have attractive body odours• Rikowski & Grammer (1998)

These effects suggest symmetry signals an attractive underlying quality

Evidence for perceptual bias accountsSymmetry preferred in artEvidence for prototype preference (in chickens and neural net.)

Problems for perceptual bias accountIndividual differences in symmetry preferencesOpposite-sex face advantageUpright face advantage

Also - symmetry attractive independent of prototypicality (Rhodes et al.)Also - symmetry predicts attractiveness in other (non-visual) modalities

ConclusionsThere is evidence perceptual bias can cause symmetry preferencesBUTPerceptual bias accounts cannot explain human’s symmetry preferences

Good luck in the exam

happy holidays