Thesis submitted by Lisa Welling in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Psychology at the University of Aberdeen 2008
This thesis describes a series of studies that investigated sources of individual differences in face preferences. Chapter 1 summarises previous work identifying the visual parameters that influence the attractiveness of faces (e.g., sexual dimorphism, symmetry, averageness, apparent health) and discusses sources of individual differences in face preferences (e.g., menstrual cycle phase, own attractiveness, visual adaptation). Chapters 2 and 3 report a series of 3 studies that investigated the role that changes in testosterone level might play in cyclic (Chapter 2) and diurnal (Chapter 3) variation in women’s preferences for masculinity in men’s faces. Chapters 4 and 5 examined the relationship between reported general (i.e., trait) sex drive and women’s masculinity preferences (Chapter 4) and current (i.e., state) sexual motivation and women’s masculinity preferences (Chapter 5). Chapter 6 describes a study that tested for a positive association between inter-individual variation in testosterone levels and women’s masculinity preferences. The final experimental chapter (Chapter 7) investigated whether aversions to facial cues of illness (e.g., pallor) reflect contagion avoidance behaviour by testing for an association between individual differences in perceived vulnerability to disease and aversions to facial cues of illness. The findings described in this thesis are evidence that testosterone level, sex drive/sexual motivation and perceived vulnerability to disease are sources of potentially adaptive variation in face preferences.
Chapter 1 - Facial Attractiveness: Visual Parameters and Theories
Abstract: Face preferences influence many different and important social outcomes, such as partner and associate choices, hiring decisions and voting behaviour. Here I review previous work identifying the visual parameters that influence the attractiveness of faces (e.g., sexual dimorphism, symmetry, averageness and apparent health) and discuss these in terms of two prominent theories of attraction: the evolutionary advantage account (which proposes that attractiveness judgements are psychological adaptations that identify high quality potential mates) and the perceptual bias account (which proposes that attractiveness judgements are a functionless by-product of the visual recognition system). Sources of individual differences in face preferences, such as the effects of menstrual cycle phase, own attractiveness and visual adaptation on face preferences, are also discussed.
Chapter 2 - Raised salivary testosterone in women is associated with increased attraction to masculine faces
Abstract: Women’s preferences for masculinity in men’s faces, voices and behavioural displays change during the menstrual cycle and are strongest around ovulation. While previous findings suggest that change in progesterone level is an important hormonal mechanism for such variation, it is likely that changes in the levels of other hormones will also contribute to cyclic variation in masculinity preferences. Here, women’s preferences for masculine faces at two points in the menstrual cycle where women differed in salivary testosterone, but not in salivary progesterone or oestrogen, were compared. Preferences for masculinity were strongest when women’s testosterone levels were relatively high. These findings complement those from previous studies that show systematic variation in masculinity preferences during the menstrual cycle and suggest that change in testosterone level may play an important role in cyclic shifts in women’s preferences for masculine traits.
Examples of masculinised (left column) and feminised (right column) male and female face images. Masculinised and feminised versions differ in sexual dimorphism of 2D face shape only and are matched in other regards (e.g., identity, skin colour and skin texture).
Increased attraction to masculinity in male and female faces is associated with raised testosterone level. Bars show the mean proportion of trials on which masculine faces were preferred and the standard error of the means.
Chapter 3 - Diurnal variation in women's face preferences
Abstract: The findings reported in Chapter 2 suggest that women’s preferences for masculine faces are enhanced on days of the menstrual cycle when testosterone levels are high. Since testosterone levels are also higher in the morning than in the evening, Chapter 3 reports two studies that tested for diurnal variation in women’s preferences for masculine male faces (Study 1: within-subjects design; Study 2: between-subjects design). In both studies, masculinity preferences were stronger in the morning than in the evening in women with natural menstrual cycles. By contrast, there was no diurnal shift in masculinity preference in women using hormonal contraceptives, potentially because hormonal contraceptives suppress endogenous testosterone production. While previous studies of cyclic variation in women’s face preferences have emphasised changes that occur over a time-course of days or weeks, these findings suggest that cyclic changes in women’s face preferences can also occur over a much shorter time-course (i.e., hours).
Findings from Study 1. Women not using hormonal contraceptives demonstrated stronger preferences for masculine faces in the AM test sessions than in the PM test sessions. No such diurnal shift in face preferences was evident in women who reported that they were currently using hormonal contraceptives. Bars show means and SEMs. 50% = chance (i.e., no preference for masculine or feminine faces).
Between-subjects comparisons of masculinity preferences in women with natural menstrual cycles and women using hormonal contraceptives (Study 2). Masculinity preferences were stronger in the AM group than in the PM group for women with natural menstrual cycles, but not for women using hormonal contraceptives.
Chapter 4 - Sex drive is positively associated with women’s preferences for sexual dimorphism in men’s and women’s faces
Abstract: Reported sex drive was recently found to be positively associated with heterosexual women’s attraction to both men and women (Lippa, 2006). This finding was interpreted as evidence that sex drive is a generalised energiser of women’s sexual behaviours and responses, rather than energising behaviour and responses towards potential mates only. The present studies show that reported sex drive is positively associated with heterosexual women’s preferences for sexual dimorphism in both men’s and women’s faces (Studies 1 and 2). These findings complement those reported by Lippa (2006), since both these studies and Lippa’s (2006) show that sex drive is positively associated with heterosexual women’s judgements of both men and women. These findings for associations between reported sex drive and women’s preferences for sexual dimorphism implicate sex drive as a possible source of individual differences in women’s face preferences and present novel converging evidence that sex drive is a generalised energiser of women’s sexual behaviours and responses.
Chapter 5 - Exposure to sexually attractive men increases women’s preferences for masculine faces
Abstract: This chapter (Chapter 5) reports a study showing that women’s preferences for masculinity in men’s faces are increased after viewing a slideshow of images of highly attractive men, but not after viewing a slideshow of relatively unattractive men. As masculinity is thought to be a cue of men’s heritable fitness and viewing images of highly attractive opposite-sex individuals increases sexual motivation, this may indicate that women increase their preferences for male cues of heritable fitness in circumstances where mating is likely to occur, making this context-sensitivity in women’s face preferences potentially adaptive. Interestingly, it was also found that viewing images of highly attractive men also increased women’s preferences for masculinity in female faces. This latter finding could either reflect increased derogation of attractive same-sex competitors or a low-cost functionless by-product of a mechanism for increasing preferences for cues of men’s heritable fitness when sexual motivation is high. Collectively, these findings demonstrate that recent visual experience with highly attractive opposite-sex individuals influences attractiveness judgements and present novel evidence for potentially adaptive context-sensitivity in attractiveness judgements.
Examples of masculinised (left) and feminised (right) versions of a male face image used in this study and the interface used to assess women’s preferences for masculinity.
The significant interaction between test phase and observation phase images. Viewing high sexual attractiveness images in the observation phase caused a significantly greater increase in masculinity preference than viewing low sexual attractiveness images.
Chapter 6 - Between-subjects variation in salivary testosterone is associated with individual differences in women’s preferences for masculine men
Abstract: Findings from Chapters 2 and 3 suggest that within-subjects variation in testosterone levels are associated with changes in women’s masculinity preferences. Chapter 6 reports a study that found a positive association between between-subjects variation in salivary testosterone and between-subjects variation in women’s preferences for masculine men. This association complements findings from Chapters 2 and 3 and presents further evidence that testosterone level plays an important role in systematic variation in women’s masculinity preferences. While Chapters 2 and 3 presented evidence that within-subjects variation in women’s testosterone levels are associated with within-subjects changes in preferences, this chapter (Chapter 6) implicates testosterone level in between-subjects variation in masculinity preferences.
Chapter 7 - Perceived vulnerability to disease is positively related to the strength of preferences for apparent health in faces
Abstract: People who are particularly vulnerable to disease may reduce their likelihood of contracting illnesses during social interactions by having particularly strong aversions to individuals who appear ill. Consistent with this proposal, this chapter (Chapter 7) shows that men and women who perceive themselves to be particularly vulnerable to disease have stronger preferences for apparent health in dynamic faces than individuals who perceive themselves to be relatively less vulnerable to disease. This relationship was independent of possible effects of general disgust sensitivity. Furthermore, perceived vulnerability to disease was not related to preferences for other facial cues that are attractive, but do not necessarily signal an individual’s current health (i.e., perceiver-directed smiles). Collectively, these findings reveal a relatively domain-specific association between perceived vulnerability to disease and the strength of aversions to facial cues associated with illness and are further evidence that variation in attractiveness judgements is not arbitrary, but reflects potentially adaptive individual differences in face preferences.
First (left image of each pair) and last (right image of each pair) frames from videoclips used in this study. Sequences of images were manufactured using prototype-based transformations. In these sequences, a face image either smiled at the viewer (left column pairs) or away from the viewer (right column pairs). The face images in these sequences were also manipulated to appear particularly healthy (first row) or unhealthy (second row).
Chapter 8 - Thesis discussion
Abstract: This final chapter will first summarize the key findings from each of the previous experimental chapters. These findings suggest that variation in testosterone levels (Chapters 2, 3 and 6), sex drive/sexual motivation (Chapters 4 and 5) and perceived vulnerability to disease (Chapter 7) are sources of variation in face preferences. The remainder of this chapter will highlight possible directions for future research on individual differences in face preferences, paying close attention to issues that arise from my research.