Symmetry and Mate Choice: Evolutionary Advantage and Perceptual Bias Perspectives

1. PS4029/30 Perspectives on social attributions Lecture 4

2. Lecture 3: Evolutionary advantage accounts of symmetry preferences Accounts of sym preferences (recap) Does symmetry signal mate quality? Is symmetry particularly attractive in mate choice relevant stimuli? Is symmetry attractive independent of prototypicality? Symmetric individuals are not only visually attractive

3. 1. Perceptual bias accounts (recap) Symmetry is attractive because : - symmetric stimuli of any kind are processed more easily by the visual system than relatively asymmetric stimuli - as a byproduct of the tendency for symmetric objects to be prototypical

4. 1. Evolutionary advantage view Symmetry is attractive because of advantages associated with choosing a symmetric mate

5. Non-Human Species Preferences for Symmetry • Female barn swallows prefer males with symmetrical tail feathers • Female Zebra finches prefer symmetrical males Møller, 1994, Anim Behav Swaddle & Cuthill, 1994, Nature

6. 2. Does symmetry signal mate quality? An important component of the evolutionary advantage view is that symmetry may signal qualities that are desirable in a mate (e.g. health, fertility) [e.g. Thornhill & Gangestad, 1999] This a hugely controversial issue but there have been some positive relationships reported between symmetry and ‘mate quality’

7. Symmetry and Quality • Controversial area (Enquist et al., 2002) • Non-humans • Symmetry associated with ejaculate quality in 3 ungulates (hoofed animals) • Antler symmetry positively related to immune measures in reindeer • Humans • Body symmetry positively related to sperm number per ejaculate and sperm speed • Breast symmetry positively correlated with fecundity Gomendio et al., 2000, Proc Royal Soc Lagesen & Folstad, 1998, Behav Ecol Sociobio Manning et al., 1998, Evo Human Behav Møller et al., 1995, Ethol. Sociobio

8. Symmetry and Quality II Humans • facial symmetry positively related to intelligence (Zebrowitz et al., 2000) • Body symmetry positively related to athletic ability (Manning et al., 1998) • Facial symmetry positively related to masculinity (another putative cue to immunesystem strength, Gangestad & Thornhill, 2003) [we’ll return to the issue of masculine facial proportions as a signal of immunity to infection in lecture 7]

9. 3. Is symmetry particularly attractive in mate choice relevant stimuli? Evolutionary advantage view emphasises importance of symmetry for ‘mate preferences/choices’ Perceptual bias views don’t

10. The Mona Lisa

11. The Thatcher Illusion Eyes and mouth inverted

12. Symmetry Preferences in Upright and Inverted Faces • Inversion disrupts normal face processing - inverted faces are processed more like visual objects than faces • In regard to symmetry preference: • A simple perceptual bias view suggests symmetry preference should be constant across orientation about a vertical axis • Evolutionary Advantage view suggests symmetry particularly attractive in upright faces Murray, Yong, & Rhodes, 2000, Psych Sci

13. Symmetry Preference in Upright and Inverted Opposite-Sex Faces Little & Jones, 2003, Proc Royal Soc

14. 4. Is symmetry attractive independent of prototypicality? Central to the perceptual bias account is the notion that symmetry preferences are an artifact of preferences for prototypicality Evolutionary bias makes no such claim

15. Symmetry in Familiar Faces Symmetric Asymmetric • Even though symmetric version is less familiar (and less prototypical), symmetry is still preferred - Little and Jones, 2003)

16. Symmetry and prototypicality Ratings of the attractiveness of faces and ratings of their distinctiveness (inverse of averageness) are -vely correlated Ratings of the attractiveness of faces and symmetry are +vely correlated Ratings of the attractiveness of faces and symmetry are +vely correlated when effects of distinctiveness are controlled using a partial correlation design • Symmetry attractive independent of averageness (Rhodes et al., 1999)

17. 5. Symmetric individuals are not only visually attractive Perceptual bias account proposes that symmetry preferences are a byproduct of the visual system Evolutionary advantage view proposes that symmetry is attractive because it signals an important underlying quality (e.g. health) Does symmetry predict attractiveness when you can’t see it? Perceptual bias: NO (visual system needs symmetry as input) Evolutionary advantage: YES (underlying quality also reflected in other ways)

18. Prototype faces tend towards very high symmetry Composites of symmetric faces more attractive than composites of asymmetric faces (Penton-Voak et al., 2001) Supports evolutionary advantage view (must be correlates of symmetry that are causing effect below because symmetry not visible) Judged: Unattractive Unhealthy Unfit Judged: Attractive Healthy Fit Composite of asymmetric faces Composite of symmetric faces

19. Symmetry and attractiveness in other modalities • Symmetric individuals have attractive voices • (Hughes et al., 2002) • Symmetric individuals have attractive body odours • [NB - in men this effect only emerges when female judges are in late follicular, fertile phase of menstrual cycle] • Rikowski & Grammer (1998)

20. Lecture 4 Key Themes 1. That symmetry advertises qualities that are attractive in potential mates Is consistent with Evolutionary Advantage view 2. That symmetry is more attractive in mate-choice relevant stimuli than Other classes of stimuli Is consistent with Evolutionary Advantage view (and also problematic for perceptual bias accounts) 3. That symmetry preferences occur for familiar faces and are attractive independent of prototypicality is problematic for the perceptual bias view 4. That symmetry predicts attractive in non-visual stimuli is problematic for the perceptual bias view and suggests attractiveness of symmetry reflects an attractive underlying quality

21. Next week: Does facial attractiveness signal health Many theories of attraction (e.g. evolutionary advantage view) propose that facial attractiveness signals health Next week we’ll review the evidence for this claim.